The Most Mind-Blowing Aspect of Circular Motion

In this video we take an in depth look at what happens when a ball is being swung around in circular motion on the end of a string and you then release the string. This phenomenon turns out to be quite surprising!
Support this channel: If you like this video please share it with others who you think would find it interesting. And by all means subscribe!
This project was supported, in part, by Dickinson College.
Special thanks to Aaron Titus and Jeff Regester for being such a big help at High Point University. A big thanks also to Noah Lape for helping with almost every aspect of this video, and for producing such a nice slinky simulation! Lastly, thanks to my Dickinson colleagues for helpful discussions and to Jonathan Barrick for being willing to make me anything at any time!
This project was inspired by a paper written by Aaron and Jeff, along with their colleagues and students. The paper was published in the American Journal of Physics and is available here: doi.org/10.1119/1.4960475; arXiv version: arxiv.org/abs/1508.04037.
Although not very math-y, I went ahead and entered this video into #SoME3
Multiple people have asked where I got my shirt. I got it at TulsaTieDye on Etsy: www.etsy.com/shop/TulsaTieDye...
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Vincent Rubinetti:
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Chill Abstract (Intention) by ComaStudio, downloaded from Pixabay pixabay.com/music/upbeat-chil...

[@pataplan]

In a way, this is a trick question. I think most people just think of the string being released from the center as being essentially the same problem as the ball detaching from the end of the string. If the problem were the latter, the ball detaching from the string, the answer would indeed be "b." The reason it's "a" is because no string is an infinitely rigid body, thus of course it would take a non-instantaneous amount of time (I imagine no faster than the speed of sound in the material the string is made of) for the ball to experience a change in centripetal force coming from the other end of the string. A question arises, what's the maximum angle that the ball can continue to subtend after the string is released? I'm guessing it's equal to the length of the string in the ideal case (that is to say 1 radian) but have no idea what it would be with the best real world material.

[@AllThingsPhysicsYouTube]

I don’t think this qualifies as a trick question, but I do deliberately allow people to misinterpret the question. I love the question you posed and don’t have an answer. But one radian certainly seems reasonable. Interestingly, I believe the time for a slinky to collapse is essentially constant, regardless of how much it’s stretched. So this would suggest the same angle, even if a heavier ball was attached to the slinky. This, I’m guessing the angle that the slinky undergoes may very well be the maximum angle you are seeking. I may have to go watch the video again to see what this angle is!

[@pataplan]

@@AllThingsPhysicsKZbin My apologies, I didn't mean to imply that you were being tricky, just that it's quite easy to deceive ourselves with an imprecise base mental model. I loved this video. Thank you.

[@pataplan]

@@AllThingsPhysicsKZbin also, you surmise contstant angle irrespective of weight, which seems reasonable, but what of higher angular velocities?

[@AllThingsPhysicsYouTube]

@@pataplan I took no offense. I agree that it's easy to deceive ourselves, and that's part of the point of this video. I ask a very specific question and most people will interpret the question differently than it's asked. We don't always hear things the way they are stated!

[@AllThingsPhysicsYouTube]

@@pataplan The constant angle is just for a specific setup. If you change the length of the slinky, or the mass of the ball, or the angular velocity of the table, then this angle will likely change. But it will remain the same as you move around the circle.

[@Tim3.14]

The moral for physics teachers is “don’t forget to specify a massless string”😁

[@chrismerklin8460]

Yes! The whole trickery as I saw it was the difference in perspective - theory vs. reality. So, what if the ball was more massive compared to a very low mass, non-elastic string? This would be much closer to the theoretical perspective. Anyways, the exploration and explanation of the reality perspective was really great. Thanks!

[@Zalex612]

My answer was (B) both when he asked the question and after he explained that it was (A) simply because I imagined the string being cut at the ball and not near the center... I would like to point out that I was only technically right by happenstance. Excellent video and a great thought experiment!

[@tuberroot1112]

The moral for physics teachers is "say what you mean". If you just say ball on a string, we assume the simplified case of a point mass on an inelastic, massless chord. If your "correct answer" depends upon the mass and elasticity of the chord, you are obviously being deliberately misleading by omitting that information. The rest was well treated.

[@suryahitam3588]

Surely not massless, but non-elastic. The ball keeps on moving in a circle because it does not "know" that the string has been released. With a non-elastic string this information should be propagated instantly, or at least at the speed of light.

[@mensaswede4028]

That was my instant question when the problem was posed at the beginning of the video…. What is the mass of the string? Sure the ball continues in a circular motion, but only for a tiny tiny faction of a second. At the end of the day, I don’t like the initial question because it purposely leaves out key information to trick viewers into answering incorrectly.

[@qtube2007]

and this is why all those Applied Math questions always stated " a non-elastic string" where they would assume the reaction to be instantaneous.

[@haroldwestrich3312]

Interesting perspective - I once did an experiment similar to this but I used a 9" nail with feathers on it like an arrow and spun it at high speed by hand at about ten feet of line. I had set up a knife so that when I wanted to "Release" the nail, I would drop down a little at the knees and let the string be cut by the knife near the nail - worked great and it really was traveling at high velocity and I believe that the tiny speck of fishing line left beyond the knife was so minuscule that it had very nearly ZERO effect on the "STRAIGHT" trajectory of the nail - Your ball that continues on the circular path is interesting looking but, in effect, is simply NOT yet truly released from the force holding in in the circular pattern. The sliding puck was a similar case because - although the puck lost enough friction to slide; it was still, partially, being restricted by friction.

[@Ghryst]

correct, he frames the question disingenuously .. aka, clickbait

[@hansrens2453]

That's very good clickbait. He claims that the ball is, shortly, continuing the circular pat. And yes, when the total system of the ball PLUS it's fixture are released AT THE CENTER POINT of the circular movement, the ball itself won't immediately move in a straight line. Fysics tells us, correctly, and repeatedly measured, that, when the ball, moving in circular motion (mark that the circular motion is measured in a system that is NOT rotating with the ball) So a ball moving on a circle, held on the circle by anything, and IS RELEASED FROM THE ANYTHING, the ball IMMEDIATELY stops changing its direction. That is: from the very picosecond when the ball is freed from its centripetal force, it continues moving but in a straight line, tangent to the circle in the point where the ball was released. In this vidéo, the ball stays linked to its SLINKY. It is not released from the 'slinky'. So that system leaving the circular motion is ball + fixture. So Release the slinky Slinky+ball have a centre of gravity, that before the release was moving on a circle, and suddenly, immediately, without any delay, starts moving in a straight line . I don't know WHO did that type of experiments. I only cannot imagine that some physicist and/or engineer didn't measure this. If David had let go of his sling, he would have hit Goliath with the leather line instead of the stone. But he released the stone. Somehow detaching the stone from the sling. At the right moment when the tangent was pointing to Goliath's head. Bingo. The stone, in a straight line, hit the giant and we read the happy ending of the story. If David had released the sling, not the stone, that (loaded) sling would have flown away. It's centre of mass going in the straight line. Leaving its circle (smaller than the circle where the ball was circling) on the known tangent. Why does he pretend that would be different for his contaption? Where in this video does he measure the path of the centre of mass of what is released? Maybe The standard problem is solved, so it isn't a problem any more. If the ball

[@ryanqvincent144]

Agreed that the issue is the 'friction' between the puck and the table. It allows an unbalanced Centrifugal / Centripetal forces because of the friction! The assumption that the 'friction' is equivalent to instantly releasing the puck from the tether to the center is completely false. Imagine this test done on an 'air table' with the puck circling a center point on the table with a rigid arm. Also, there is mechanism to release the puck instantly from the bar at any moment. Now, the puck is forced to move in a circle because of the bar. i.e. their are no unbalanced centrifugal / centripetal forces. What will happen when the puck is released? The centrifugal force ends instantly as does the centripetal force. What is left is the horizontal motion. It really will be straight line motion at a tangent to the circle.

[@Error6503]

This is why is in all physics exams I took the questions started with "Assume you have a system with friction-less couplings in a vacuum and a perfectly uniform spherical object connected by a rigid rod to an infinitesimally small single point" because once you have to take account of material tension, air resistance and even object widths then the question gets increasing more problematic to answer correctly.

[@AllThingsPhysicsYouTube]

LOL! Yes, you do need to be a bit careful!

[@labbeaj]

Assume... Why do we need to assume? Assumptions are almost like beliefs. Neither are true.

[@mhzprayer]

Beliefs are not true? Well, they are not false either. Believing or not has no bearing on whether something is true.

[@yanair2091]

@@labbeaj Because if we didn't assume, we'd know nothing. We could in principle improve our instruments near perfection, but even then, there lurks Heisenberg's Uncertainty Principle.

[@BariumCobaltNitrog3n]

@@labbeaj Truth is different than fact. Truth is relative to the speaker, but facts are absolute. A belief is a truth. Truth may or may not be factual.

[@prentishancockgardening]

Excellent video. It boils down to definition. When released from the centre, then you're no longer talking about a ball because the object is a [ball + string]. You'd have to release the ball at the radial end of the string to remove the string from the object, and then you will get answer B. It would have been good if you replaced the string with a metal rod with a release mechanism at the end to show the trajectory of the ball on its own.

[@vinceturner3863]

Agreed, I think this would be more interesting. Presumably the direction of the ball would be tangential at the point of release, but would the ball have some spin and would the mechanism of release cause problems?

[@curteaton8162]

I suggested a hollow ridged piece of wood or plastic pipe with the wire passing down the middle to a releasing device inside the ball.

[@datamatters8]

Or how about using a thread (or fishing line) and then take a very very sharp knife and insert from above into the plane of rotation near the ball. As you say with the slinky or the rubber tube the radial force on the ball is delayed by the tension propagation time (just like change in the gravity field on a planet if the sun vanished). The visual effect is interesting though. To say that people get the answer wrong is a bit disingenuous because the physics question is typically for an idealized world where the tension prop time is infinite. But I suppose the goal is to create a bit of drama.

[@isaacharris2200]

The circular motion after release is because there is still tension for a small time even after release, use the release at circle, not at centre then result would be different

[@nowandrew4442]

The question is more correctly posed as a linguistic one, rather than a physical sciences one. It boils down to how you define "release", because technically speaking the ball doesn't leave the [ball + string] system until the tension wave frees the ball - at which point it does precisely move tangentially to the point of "release". This is not a circular motion effect. It's a [ball + tensioned string]-system effect. Dropping a ball or spinning a ball are simply different ways of arranging the [ball+string] system.

[@frankmueller25]

Sorry, but this video is a bit misleading. He considers the elastic property of the string but not the mass. I like that he identifies the center of mass. He mentions that it is air friction that causes the slinky or ball not to be perpendicular to the tangent of the circular path the ball travels at the ball. I assure you that it is not the only (and probably not the most) cause for that phenomenon.

[@davidseed2939]

coriolis force applies where the orbiting object is in contact with the rotating reference frame. if a ball is attached to a string ans the string breaks, there is no such contact and the object is released tangentially. This is how a sling shot works. A satellite realease system has also been devised on this principle. The “rotating disc with sliding puck” system is different because, although the puck is slipping, there is still a frictional force, because the puck is still in contact with the disk.

[@rv6amark]

Wave propagation is a fascinating thing, and is involved in surprising processes. It appears that it is involved in the swinging of a ball on a string (or slinky) in a circle...at least upon release. If you looked carefully, you could actually see a small reflected wave traveling back up the string when the tension wave reached the ball. The impedance of the ball is quite high compared to the string, resulting in the reflected wave. Same thing happens with sound waves, radio waves, and even water waves. Fascinating stuff.

[@vincecox8376]

Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. This is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..

[@PolskiTesla2]

drive.google.com/file/d/15qCtZTSe-GuGbEbuc7NmGh5Pc9T9rhN1/view?usp=drivesdk

[@keywestjimmy]

@@vincecox8376You may want to correct your English to promote a fringe hypotheses. "Non" not none. "A lot" not allot. "Spread" not spred. "Huge" not hugh.

[@vincecox8376]

Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. The anti gravity is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..

[@lukebradley3193]

Agreed, really fascinating. So you have the force propagated out as a slow moving signal, which doesn't update and tell the ball what to do in accordance with physics until it arrives, that's one picture. But then there is this second picture which is more local, about the center of mass and equilibrium and it just works out to match the first picture. Really mind blowing video to me.

[@lake5044]

Since we're not ignoring small details, the ball also has to rotate. We can explain it either as to conserve angular momentum since it will no longer be rotating, or because points in the ball have different speeds since they are at different distances from the center of the rotation.

[@dougaltolan3017]

Amply demonstrated by the puck on the rotating table.

[@davidtownsend8875]

The ball might also be subject to libration (pendulum action) in one plane or another. Libration would cease but the ball's rotation would be slightly different depending on just when the "pull" stopped. I think.

[@szymonbaranowski8184]

they would need to try it with magnet "gravity" but they would probably cheat different way then as well

[@vibratingstring]

This is annoying. Slinkies and elastic chords are not the system you asked about in the beginning Thank God I listened while doing something useful with my hands and arms and a hammer

[@onlyeyeno]

@@vibratingstring Please note that I'm not out to start an argument. But I think that part of the point of this video is that "in the real world" every "system" has this "kind of delay". And that the length of the delay only differs in magnitude but never "goes away" completely. And the "shortest" delay You can get can be calculated by measuring the distance from the "origin of the holding force" to the "released(point)object", and dividing that distance by the speed of light. And I think this can(should) be said to apply even to "ideal systems", it's just that in most cases it's usually explicitly stated (in one way or another) that the "delay" and other real world factors that have "minor effects should be neglected" (e.g. air resistance, loads not being "points", etc.). But that doesn't mean that these "minor forces" don't exist in the real world, rather just that they can be ignored in strictly hypothetical cases. Best regards. t

[@adrianstephens56]

Thank you. I'm a retiree with a PhD in physics. You gave me something new to think about. I had a teacher when I was a student, Prof Brian Pippard (en.wikipedia.org/wiki/Brian_Pippard), who loved to demonstrate simple physical systems that gave unexpected results, such as spinning potatoes. Or how to use a glass of milk, a laser and a pencil to measure the astigmatism in your eye. I class this video as being in that mold.

[@AllThingsPhysicsYouTube]

Glad you enjoyed it! I hope you'll consider subscribing and sharing the video with others!

[@selectfew2884]

As a young boy, I made and practiced throwing the bolas. I'm wondering if some of these same properties affected my throws? Had a lot of fun all the same.

[@coolcat23]

@adrianstephens56 I don't see the misleading point about "circular motion" being in the same class as your experiments, since the latter demonstrate genuine and interesting effects, whereas the idea of this video is to mislead people with an abstract diagram -- signalling the usual abstracting away from nasty real-world effects, such as air-resistance, non-instant releases, wobbly centrers of rotations, etc. -- to essentially claim that the correct intuition most people have about circular motion is wrong. The intuition is not wrong, all this video does is to point out that the idealisation of an instant release propagation does not exist in the real world. That's a remark on the property of materials and (a point not made in this video) the limited communication speed between cause and effect, it is not a remark on "circular motion". I like your experiments.

[@RuthvenMurgatroyd]

@@coolcat23 Yes, but this is a physics video not a mathematics video so it's not so bad that it does that. I agree though, it doesn't sound as interesting as what the original poster mentioned imho.

[@user-zu1ix3yq2w]

@@coolcat23 I agree mostly but the propagation is very easy to overlook/miss. And I'd say in the real world, lacking this intuition could be a problem, as explained at the end of the video.

[@tanvirahmed7985]

Man, just blown my mind. I didn't know that from my college. Also graphic quality as well as briefing is Extremely good. Keep up the good work

[@AllThingsPhysicsYouTube]

Thanks a lot! I hope you will consider subscribing and sharing the video with others!

[@DarrenDignam]

As others have said. These kinds of questions assume idealised scenarios. Almost like the string vanishes from existence (like the sun in the final example). So B is the 'correct' answer. Great expansion of the concept. The falling slinky alone is a great challenge to the assumptions! Love this video

[@gordonelliott7870]

Ah, very thoughtful response. One principle of classical physics calculations is that one can use the coordinate system that gives the ease of calculation that is desired, and the result will be the same from other "correct" analyses in different frames when translated back to that frame. In this case, we also need to consider the assumptions of the problem as Darren notes. In physics class we would usually suggest the string was infinitesimal or insignificant mass. If not, then that needed to to be specified as part of the problem description. If assuming infinitesimal mass frictionless string, then the Cartesian frame is the easiest frame to use. Then what is meant by "release"? If instantaneously discontinuing the force or tension on the string, rather than letting the string slip slowly out of one's fingers, then very simple indeed and "b)" is clearly the answer because ball's motion is -Y direction and no forces to modify that. The drawings idealize over the actual motion of a person swinging the ball, because that itself would most likely be a non-circular motion because the person is not moving in a circular hand motion and all the interactions involved having stabilized. So the drawing belies the actual video example. If we are following the drawing, are we not using simplified assumptions like circular motion, so why not also infinitesimal string mass and instantaneous release? The video segment with the puck on a rotating disk is sort of a non-sequitur (though interesting). The point there is that after beginning to slip -- a matter of non-linear frictional forces that are reduced when the tendency to "stick" is overcome, are nevertheless forces imparted by the rotating disk upon which the puck is moving, which explain the partial spiral motion. Now a similar case might be rigid rod with significant mass replacing the string, but circular motion and instantaneous release. In this case the center of mass of the rod/ball system is useful, and also probably Cartesian frame. Then the rate of rotation will be constant at the moment of release, and the center of mass should follow path "b)" but of course not the ball but the CM. So a translating frame following the CM is useful, wherein the motion will be strictly circular, but then translate to the still frame to see the compound motions. Then extending what I said, the Slinky (tm) tends to demonstrate a transmission line effect (traveling wave). So at point of release, the change in forces are only apparent at the center part of the slinky system. In that manner the tension forces on more outer portions remain in effect until the traveling wave reaches that portion. Here a CM frame would again be useful, and would indeed the CM should follow the -Y path (as 'b') but the object is not rigid so the motions within that frame will include the traveling wave effect and be quite complex. Then in the end of course any real string is not only not massless, nor frictionless in air, but also has a modulus of elasticity so that it actually can be modeled along with its mass density as the same as the Slinky, but the wave travels must faster. Thus in the end, "a" is the answer for any physical string -- but the time frame before looking more like CM frame above would be very very short. AND of course -- the CM frame is not completely accurate because the air, presumably stationary to the reference frame, puts force on the ensemble and slows the overall frame and effects the parts.

[@mokiloke]

It just has to release, like that spinlaunch system.

[@stuntpenguin]

​@@gordonelliott7870I think repeating the puck-on-a-surface situation, but immediately removing the centripetal force by halting the turntable with a hard stop would come close to simulating the instantaneous release of a string. The puck will, for a short time, continue to move in the same direction it was going when the table was braked down to 0 very very quickly.

[@myleswillis]

I'd just like say that the production on this video was outstanding. The clear writing, your eloquent narration, the beautiful graphics and slow motion edits to demonstrate your points. I think if you can pick the right topics you will have a million subs before too long.

[@AllThingsPhysicsYouTube]

Wow, thank you! I hope you’re right! And I’ve got a fantastic list of topics just waiting to be made into videos! Stay tuned!

[@Anon54387]

@@AllThingsPhysicsKZbin Isn't the net force on the ball zero in both cases? Both when it is hanging vertically and when it is being rotated on the turntable the speed isn't changing. It's a constant radial distance from the center when in motion around the circle, and that doesn't change until the inner end of the spring reaches that radius and goes beyond it.

[@Anon54387]

@@AllThingsPhysicsKZbin My first impression was that it would be the usual sum of orthogonal vectors ie of the tangential and radial vectors.

[@joelwexler]

@@AllThingsPhysicsKZbin More slinky material! I came home with a box of them my junior year at Drexel. But we didn't have cameras in 1978. It must be so great in school today. This was great, thanks.

[@AllThingsPhysicsYouTube]

@@joelwexler You don't need to be in school...you can continue to learn/figure things out for yourself right now! Never stop learning!

[@craig7350]

Its always nice to know an event as catastrophic as the sun disappearing could take place and I would remain blissfully unaware of it.

[@rogergeyer9851]

But only if 8 minutes is longer than the rest of your life, so there's that. But I was rather shocked when I learned that all forces, including gravity, only propagate at the speed of light. We didn't discuss that in a year of introductory physics in college (which is all I had). So while gravity is nonlocal, it isn't simultaneously nonlocal (I ran into this as an adult (layman) casually trying to understand how gravity could be nonlocal AND have particles as the mechanism to control the force.

[@varunkashyapv8383]

More than that, I was thinking, what if the sun disappears, as explained for 8 mins, I will be oblivious as the earth keeps on rotating... but say, if the sun re-appears after 5 mins, what will happen? Will earth continue to rotate? There won't be any issues with earth's motion because of sun's disappearance for full 5mins?

[@philouzlouis2042]

Really interesting video and presentation, Stil, I"m a bit puzzled by some things into the video. 1°) I have noticed that the tiny upper disc when you increase the speed of the downside sustaining rotating disc; not only at a certain speed start to glide radialy; but also spins on itself (seen on the larger disc but also after passing over the edge). 2°) When you think about the problem, you may see it: -As letting the rope and the weight go at a given time of the rotation; as such the tension into the string is proportional to the weight and to the number of rotations per second (rpm); then you may consider the string to be a spring elastically proportional to the tension; when releasing the rope, the weight continues its circular motion until the contraction wave (tension) from the rope hits it; then the weight and rope follow the radial trajectory. But this reaction is so fast that it must not be visibleon footage. --As a stone from a spinning sling, and this usually go straight radialy to its target when set free from the sling. Regards, PHZ (PHILOU Zrealone from the Science Madness forum)

[@theloganator13]

You have an incredible talent for making physics videos that are engaging for physics novices and experts! I can't wait to see what else you do with this channel. 😊 A note for physics teachers: standardized tests (AP Physics, MCATS, etc) will expect your students to give the straight line answer (B). This is typically because instead of asking about the string being released, they ask about the string being CUT, and the assumption is that it is cut at the location of the ball, so there is no delay while a wave propagates. If your students are comfortable with the material, be sure to point out this important difference.

[@AllThingsPhysicsYouTube]

Thank you for the kind words, and for the important disclaimer regarding tests. I think it's fair to say that ALL test questions on this topic assume that the force goes to zero instantaneously, for example, but cutting the string right next to the mass. It is definitely important that students be aware of the difference!

[@joeo3377]

I think it is more subtle than that; really, in introductory physics we basically just assume that changes propagate instantaneously, rather than doing the messy business of taking propagation time into account. I think it is probably a good idea to point out that we are making this assumption, and how it simplifies what we are doing without substantially changing the physics involved. Ultimately, the question is trying to ask what happens to something in circular motion when it no longer feels a centripetal force. For the example of a ball on a string, the ball no longer feels the centripetal force only after the information has had time to propagate along the string, but that is close enough to being the same moment the string is released as to not matter in most instances.

[@csours]

@@joeo3377 kind of low how in real life everything has inductance and capacitance

[@adb012]

@@AllThingsPhysicsKZbin ... The wave propagation speed in ideal strings is infinite because a) they are infinitely longitudinally rigid (Hook spring modulus k is infinite) and they have zero mass so the acceleration for any force applied is infinite which in turn leads to infinite speed.

[@AllThingsPhysicsYouTube]

@@csours Indeed, and that is messy business!

[@dhavalmysore]

Good presentation and the slow motion video with the slinky spinning the ball to illustrate the effect of finite time required to propagate the information was very impressive. I was thinking of the "what if the sun disappears" case as an example but you mentioned it at the end. That being said, the question in the beginning was tricky in the sense that most people would assume you are using "fully rigid" spring. Could have started off by clearly stating that the string is elastic, or by not even posing this question but just saying that this video demonstrates the effect of elasticity or finite speed of propagation of information on circular motion. That alone in itself is incredible in itself, as you have demonstrated in the rest of the video. This tricky question at the beginning made it hard to take you seriously in the beginning, especially when it was immediately followed by the example of an object on a rotating turntable wherein the cause of the centripetal force is frictional force and the behavior you showed (it slipping and it following a curved path) was for a phenomenon not directly related to this topic, which you didn't even get into in the video! Could have avoided the sensationalism, just my opinion.

[@maxedgar]

Thank you so much for this video. I was pondering this exact question a few months ago and had also followed the logic of the slinky drop. My final question in fact was what would happen to earth’s trajectory if the sun was to suddenly cease to exist. Great demonstration of the physics of it all.

[@salamander554]

You were pondering this question? Alright then

[@samking4179]

That was cool. Got here by wondering about a golf club being let go from a turning body, the player. Initially wondering about inertia creation and release of mass/object. This demo was more than I bargained for. Thanks!

[@AllThingsPhysicsYouTube]

Glad you liked it. I hope you’ll consider subscribing and sharing the video with others!

[@WayneKitching]

As an Electrical engineer, I only did physics in my first year of University (freshman year, but I don't live in the USA). Our physics, statics and dynamics problems always had disclaimers like a "light rope," "ignoring air resistance," "rigid bodies," etc. Therefore my answer was also B.

[@justgivemethetruth]

I think it's B too.

[@raynic1173]

Where he gets you is his question , "what happen to the ball when I release "the string." and on this he's right, but as in his example of the 12.6 cm deviation that only equates to 1.26%, almost hardly worth mentioning unless you're slinging rockets to space via a rope or calculating the theoretical release point when David binged Goliath in the head and won the day.....lol. Mostly your right "b" is the right answer.

[@rogergeyer9851]

Your answer was correct. This is about semantics games, and the circular motion is about the SYSTEM, not the ball. You really can't DO elementary physics problems correctly without such disclaimers. I remember being VERY PISSED OFF during my first physics test, where my (excellent) physics teacher gave a problem that was very hard to think about given the reality of friction, in the context of a simple Sum of forces in two dimensions problem. After literally 40 hours over the weekend of working problems to be prepared, I almost confused myself and messed up the problem (he always gave unique problems unlike those in the book / lectures to ensure people knew how to THINK vs. memorize). I did calm down, and tell myself to trust the principles and techniques I had learned well and studied, and worked the problem correctly. Then, later in his office, we had a chat about clearly stating such assumptions, which he agreed with.

[@grkhetan]

Excellent video. I did not think of the string tension to have a material impact. Impact of this issue was really striking with the slinky. Thanks for making this video - learned something!

[@vincecox8376]

Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. This is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..

[@herbertlong3981]

@@vincecox8376 What the heck has this to do with the material in this video? Really, you are all over the place with this same cut/paste comment Yadda yadda Kind of obsessive, methinx

[@codypendant6745]

Circular acceleration (and logic) I really appreciated this video. You really made me think. Looks like you spent quite a bit of time and some coin to make this video happen. This video was eye-opening. I have watched this video several times now and am still amazed. This is like "Zeno's Postulate". You know the one. He poses a seeming conundrum based on half the distance to the finish line(fictitious premise). From there he expounds on that matter instead of the real focus: WIN THE DAMN RACE! Any Kindergartner would simply run full tilt across the finish line and smile real big. The premise that spends the most time in front of the student will take up the most of his/her cognitive skills. Bravo to you. You had many of us doubting reality. However, I only briefly wavered off my initial choice of option (b.). :) So, I spent several hours watching, listening and writing up a rebuttal, but I won't be sending it. I looked below and read some of the comments and realized you had set us up intentionally for a misunderstanding. Good on you for challenging what we know and can deduce from observation against what we are 'told' or 'lead' to believe. You even went deep by quoting Newton and putting up several formulae as a smoke screen to what you were about to play on us. My only concern was that at no time in the video did you let us in on the pranks, so I thought you were serious, and flawed. My bad. LoL. I simply didn't understand that it was a valid and viably elaborate joke. I suppose when you mentioned "wind resistance" on the puck and then launched into mathematically proving the initial motion of the puck, I should have caught on, but I passed right over it the first time through. I love learning from these types of lessons and here is what I deduced from your video. 1. The puck: "Wind resistance". Good one. NOT "0" but not significant. Still sort of true LOL. Using the reference frame of the camera rotating with the disc was also very clever at shifting our attention away from the forces acting on the puck. (friction, inertia, centrifugal force, Coriolis) The big question that opened my eyes when I asked it of myself was: How fast is the puck moving in contrast with those points on the disc that were outside the circle of the puck? About 1/2 the speed at half the circumference. Per circumference formula 2πR. If I did my calculations correctly. 1, a. Established that the puck is moving at about 1/2 the speed of the rim of the disc. Once the puck starts sliding, it cannot be circling half as fast as the outer portions of the disc, while simultaneously circling at speed with those outer portions, so physics demands it cannot follow a straight line off the disc, hence the left turn (from reference inside the disc frame). 1, b. Filming from the floor of the 'system', reveals the puck following a 'French curve' type arc to the right(our left) as it is accelerating from friction with the rotation of the ever-increasing radius (faster moving at RPM) of the disc under it. At the same time, the datum line on the disc is seen rotating away, ahead of the puck in a widening gap. This demonstrates that the puck was initially moving slower than the rim of the disc. This is all just physical interaction. 1, c. Filming the puck while rotating with the disc merely reveals how cool it looks from within the rotating frame, and that it looks like it makes a left turn all on its own. The floor outside the disc is conveniently not in frame for visual reference and confirmation of motion. Once understood that the puck is moving slower than the outer rim of the disc, the left turn becomes a scalable indicator of the speed differences between puck and disc. 1, d. Although sliding grip has reduced friction between puck and disc, the remnant friction/grip still imparts some acceleration on the puck in the same direction the disc is spinning. We've all seen this play before, but in a different paradigm and with other actors: Remember that disk sander you held that was influencing, or being influenced by, the piece it is sanding? So, as the puck slides off, it gets a boost in speed from the rotating disc, which increases its departure speed from the disc and continuously changes its direction. 2. You invoke Newton when your game was set up to dispute him. Is that clever, or cruel? I mean, it was eye-opening but not Newtonian in structure nor outcome. Nonetheless, I'm happy to have been honored to observe this strange and amazing phenomenon. You DID give us the true information while exhibiting the Slinkys dropping along side the balls, and then while tethered to the ball. But just like a skilled magician, you distracted our attention away from the correct frame of reference even while showing us the truth. A little guile goes a long way to make us re-think what we think we know. I just may be hooked on your channel now. 3. "Releasing" the ball while keeping it still tethered to some mass that imparted observable force on the ball was a bit disingenuous. All very interesting just the same. 4. The Slinky release was the most interesting. Several observable phenomena were revealed. 4, a. The Slinky never catches the ball. Ball is moving away as the Slinky dives for where the ball was upon release. 4, b. The Slinky changes direction along it's length even as it contracts under tension. 4, c. The Slinky keeps rotating at the same rate it had when moored to the turn-table. 4, d. The ball is eventually pulled away from the circle and indeed, gets its forward motion checked by the Slinky. Proving the ball was under tremendous outside force, and never actually released. 5. In your challenge questions: I find that both (a.) and (b.) can be correct depending on the parameters, premises, and frames of reference established and accepted by the observer. See below. (c.) could never be true. Your demonstrations showed that. Refer to both times you filmed the puck sliding off the disc, each from a different frame of reference. Only if a rail were mounted on the disc to force the puck to dive straight off it could (c.) become true, and then, only from within the rotating frame. Option (a.) is true ONLY if outside forces are allowed to remain in effect for various periods of time. Option (b.) being true from Newton's frame of reference of the balanced 'system', with no external forces allowed to persist. Thank you again for this grand mind bender. How did I do? lol.

[@obiwanduglobi6359]

Got me with that one. When the connection between the ball and the string is released, the correct answer is (b)... But the experiments were great anyway!

[@AnthonyCelata]

Wow. Instant sub. You rock man! Please don't stop making videos! I absolutely love this kind of educational content!!!!

[@AllThingsPhysicsYouTube]

Thanks so much…it’s nice to hear a positive comment once in a while!

[@noeckel]

Another way to answer the question at the outset: At the moment the slinky is released, the system comprised of the ball and slinky COMBINED will have a center-of-mass trajectory that is straight. The motion of the ball plus slinky is not shown in the video, but it will be a tumbling motion around the uniformly moving center of mass. The system (ball plus slinky) is free of external forces (ignoring gravity) from the instant it's released. On the other hand, the ball feels a force from the slinky. The mass of the slinky is essential because it provides the inertia that allows the end of the slinky connected to the ball to maintain the tension that it had before it was released. That tension is of course the centripetal force that kept the ball on a circle before the release, and it indeed continues to force the ball onto a circle for some time. The tension at the outer end can change only if the deformation of the slinky slightly further inward changes, and this can in turn only happen if the tension further inward from that portion of the slinky changes, etc. The change in deformation at every location is an acceleration that happens with a delay dictated by the inertia of that portion of the slinky (Newton's Second Law). As always when inertia and tension of a medium compete, the outcome is a wave.

[@loganocallahan4074]

I agree. You can really see this if you track the approximate COM from 12:52 in the video.

[@anyfriendofkevinbaconisafr177]

This was just a grifter making click bait. The slinky is NOT analogous to a string at all. This should be obvious.

[@Robinlarsson83]

​@@anyfriendofkevinbaconisafr177you are 100% wrong. Of course the slinky is analogous to the string, and the fact that you don't understand that, makes it obvious that you don't know all that much..

[@JimC]

"The motion of the ball plus slinky is not shown in the video, but it will be a tumbling motion around the uniformly moving center of mass." You can see it very briefly at 7:52.

[@vincecox8376]

Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. The anti gravity is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..

[@user-le6lt1jz9m]

Thank you very much for making this video! I was entirely skeptical at the beginning, but you had won me over by the end of the Slinky section. I hope that I remember your video when it comes time for me to teach circular motion. It can be so easy to accidently get locked into the ideal models of intro physics that one forgets the beauty contained in the ``nitty-gritty" details. :)

[@vincecox8376]

Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. This is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..

[@ImStillJohnny]

There are a lot of comments complaining about the question being misleading, but I feel like those people are way too concerned about being right and how that affects their own egos. In reality, the video isn't about if you already know the answer, it's about learning something new, or thinking about things in a new way, and it does a fantastic job of it. It's very well communicated, a very solid length for this topic, and has a very good mix of theoretical and experimental sections. Great job and I'm looking forward to more videos!

[@AllThingsPhysicsYouTube]

Thank you so much for this comment. It mirrors my own thoughts quite well!

[@GOLaun]

Question: is there a length and a material with the correct tension that would allow a ball to do a complete rotation after release?

[@AllThingsPhysicsYouTube]

I have no idea, but that would be really awesome! I suspect the answer is no, and that we’ve done about as good as you can do with the slinky.

[@AfricanLionBat]

I watch a lot of science channels and this was such a unique video. I havent really watched anything on classical physics and forgot how interesting it can be.

[@theBestInvertebrate]

I would have liked to see a ball releised on the end of a string after the demonstration with the elastic, I get that it isn't really nessisary for understanding but would have "closed the loop" if you will in terms of explaining the topic. Very cool video.

[@AllThingsPhysicsYouTube]

Yes, unfortunately it would not show anything. The wave propagation at (approximately) 2,000 m/s, coupled with the lower frame rates we had to use because the camera was so far away, just made it impossible. I thought including that footage was simply not worth it.

[@LuckyDrD]

@@AllThingsPhysicsKZbin But if you release the ball at the end, rather than release the tether, why would it continue on a curve path? Isn’t the curved path resulting from the tension equilibrium that then dissipates outwardly whereas by releasing from the ball end, the dissipation would occur centripetally and then you would see the ball take a tangential path?

[@user-zu1ix3yq2w]

@@AllThingsPhysicsKZbin move the camera closer and time the release. You get a short section but you might still be able to trace a curve (depending ofc)

[@prome3us550]

Really loved the way you built up from slinky to string! Would be interesting to see the path traced by the COG of the entire released system, which may actually continue on a tangent from it's position at the time of release.. Thanks!

[@AllThingsPhysicsYouTube]

Yes, that is exactly what the COM will do!

[@nowandrew4442]

That's not correct, again. 🙄 the COM travels rapidly up the string towards the ball after release. But since the string is still attached to the ball, we already know what happens to the ball+string system, as it's what this video is about. But if you are to now claim that the COM immediately moves tangentially from the circle at the moment of release, then your whole premise of this video is incorrect! If a much-more-massive string is used, the COG never reaches the ball, but stays along the string somewhere. Therefore if the ball+string COG system as a whole moves tangentially immediately, the net effect on the ball is **also** to move away from the circle. This would be very clear with a massive string. With a low-mass string it just **appears** that the ball doesn't stray from its circular path until the tension wave releases it from its tether; but actually it's merely that the COG change relative to the tension wave is not humanly perceptible. It has after already moved off the circumference. Using a heavier string would demonstrate this clearly.

[@AllThingsPhysicsYouTube]

@@nowandrew4442 I think you are mistaken. Consider the ball and string as the system. As soon as the string is released, then there are no forces acting on the ball/string system. Hence, the CM of the ball string system will move in a straight line with a constant velocity (assuming no air resistance). This line will be tangent to the circle that the ball/string CM was originally moving along, not tangent to the ball's trajectory. The ball continues to move along the circle because the string is moving toward the ball. These two motions are what keep the CM of the ball/string system moving in a straight. line.

[@prome3us550]

@@nowandrew4442 that's a great way to look at it, yes. It's not so much the mass of the string but the rigidity of it that is the core of the unintuitive motion which makes the experimental result so interesting though. (edit - the question was where does the *ball* go, not the COM) Other than that bit you seem to agree that a rigid system if viewed as a discrete point (hence at the COM) would continue straight on once released? This is what makes engineering the fun branch of science, sometimes life sneaks in a bit of "elasticity" to keep our minds engaged!

[@nowandrew4442]

@ creator But then A) would be incorrect. If the COG moves **immediately** tangentially, then the tension is not the key actor. This is not the case. There is a necessary interplay between the tension wave and the COG. It can't be perfectly tangential at the moment of release. The COG *also* moves tangentially **only once the tension wave passes it** - not immediately after release. Because the COG is tethered just like the ball is. We don't see the effect of the COG release because the COG is too close to the ball, when it's light-string+ball. Use a heavier string though, and we would see the ball's movement not be tangential as the COG is already dragging it in its (the COG's) tangential path after tension wave propagation. So the true story is: it is not the ball that awaits tension propagation, but the COG. When the tension wave reaches the COG, the COG moves tangentially. If the ball is sufficiently massive compared to the string, the COG is in the ball and those movements appear to be the same. But they are not.

[@shankarh6915]

Given that the ball is not being swung by an infinitely rigid rod/object, it will take a “while” for the ball to realize that the string has been let go of.. so it does follow that the instantaneous direction of the motion of the ball right after the string is cut/released will still be along the circle… fascinating video, lovely to see the slinky experiment

[@AllThingsPhysicsYouTube]

Thanks. Please consider subscribing and sharing the video with others!

[@thibaut5345]

Where are the people? Your work deserves 8.1 billions views

[@AllThingsPhysicsYouTube]

Ha...I agree! Where are the people? Please considering subscribing and sharing the video with anyone you think might enjoy it!

[@stevelc777]

The words "immediately on release of the string" do not mean the same as "when the ball is let go". The ball is still under the force of the tension of the string all the while until the propagated wave reaches the ball. It is at this point that the ball is "let go" and then it turns out that B was indeed the correct answer, at this point in time.

[@b92555]

Two questions come to mind. # 1 is the original question , “ when the string is released …” my question is “when the ball is released …” the first question refers to the system of ball and string while the resultant prediction only asks about the ball . The trick to the un expected result is one of language. A better question would have been: what is the difference in trajectory when the ball is released from the string vs. release of the string from the center. Then a question about the energy released from the elastic deformation of the string vs. the difference in center of gravity of each system. A depiction of the trajectory of the center of gravity of the string ball system would be interesting to see as well. This is a fun video to watch that invites additional exploration. Thanks for the effort, these videos are not easy to produce. Edit: I should have read the previous comments before commenting sorry for any redundancy.😁

[@jms.1]

Very nice!. As others have said it shows the importance of those words and phrases used to make elementary problems tractable: '...light, inelastic string...', '...a point mass...`, '...a rigid rod...`, `...rolls without slipping...` or whatever. Good to see videos like this that show how removing these kinds of assumptions has measurable and often surprising effects.

[@vincecox8376]

Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. The anti gravity is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..

[@thatonedude1820]

Very well demonstrated. I really appreciated the fact that you reiterated the importance of this by giving examples of how it could actually effect the outcome of systems.

[@ExtantFrodo2]

The makers of "Spinlaunch" should watch this.

[@ExtantFrodo2]

@@Lt.-Dans-Legs No one can know everything. Even when their life depends on it. Your ridicule is unwarranted.

[@vincecox8376]

Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. The anti gravity is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..

[@halrogoff-lx7fs]

This is wonderful. I like that the "string" continues rotating about its center. And the connection to the gravity conundrum about the disappearance of the sun. Thank you

[@vincecox8376]

Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. This is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..

[@37rainman]

@@vincecox8376 Err ...... Who here is talking about antigravity, or ANYTHING you mention in this comment. Try to make comments about the material in the vid. No one is impressed

[@jklmnop8607]

The puck on the rotating table is governed by one other force, not mentioned in the early part of the demonstration. It is held down onto the table by friction. This changes the whole game because it can't possibly behave like a weight on a string. 5:16 In high school physics, we saw the same experiment performed where the rotating weight is an air puck on a steel table. Essentially frictionless. So when testing was quickly severed by a flame, the weight indeed left orbit in a tangential direction.

[@michaelporter1]

I wasn't crazy about the puck description, either. It seems to ignore the friction that still exists once the puck is moving (kinetic friction), and if you account for that the puck isn't a good analog for the ball on the string. I think the video would be better without it.

[@undercoveragent9889]

@@michaelporter1 And swinging a ball around his head produced and elliptical path, _not_ a circular one. False premise leads to false conclusion. Ah well; so much for post-truth physics, huh?

[@drewkelly1955]

This one got me! At first I was ready to be mad at clickbait, but you've completely won me over. I've never thought about tension waves in this physical situation before, and tying it in with the falling slinky was a wonderful way to do that. Thanks!

[@AllThingsPhysicsYouTube]

Glad I won you over! I hope you’ll consider subscribing and sharing the video with others!

[@adoatero5129]

I didn't like the misleading start, and it partly spoiled the rest for me, although the rest is very well made and interesting. That's one of the problems of the today's world: to get other people interested people feel that they have to "shock" them somehow, and to achieve that they resort to misleading, by exaggeration, confusion or something else. The start of this video utilizes the popular (and annoying) myth-busting format, where you first present some kind of popular "myth" (which typically doesn't exist in the presented format), and then surprise people by "busting" it. When the area of your video is science, you shouldn't do that. If honesty is not a good reason enough to you, then perhaps preserving your credibility is.

[@rogergeyer9851]

It was more semantics than physics, which IMO, should NOT be what science is all about. At a minimum, it should have been stated MUCH more clearly / accurately re the motion of the SYSTEM (tether plus ball).

[@ThenameisMarsh]

definitely a bit of a trick question. shows the difference between "releasing the string" versus "detaching the ball from the string". Under most circumstances the mass and momentum of the string would be considered negligible, but if its not then these two scenarios are completely different for the unintuitive reasons demonstrated in this video.

[@AllThingsPhysicsYouTube]

Hmmm...not sure I'd call it a "trick" question, given that I provided experimental evidence in three different systems, but no doubt I am focusing on a bit of a technicality. And technically, this doesn't really have anything to do with the mass/momentum of the string because the same thing would happen if there was a ball connected to the other end of the string. It is simply due to the finite travel time of the wave speed (which, admittedly, probably depends on the mass density of the string).

[@ThenameisMarsh]

@@AllThingsPhysicsKZbin Sorry, I wasn't being precise in my language. In most simplified physics problem it would be the tension wave of the string not being considered, not the mass and momentum. That was how I should have worded it. And my first point still stands. This phenomenon would not be present if the ball were to be detached from the string/slinky at their connection point, rather than the string/slinky being released from the center of the circle with the ball still attached.

[@AllThingsPhysicsYouTube]

@@ThenameisMarsh You're absolutely right that this phenomenon would not be present if the string was cut right where the ball is connected. And I've seen the question posed exactly that way. But that's not very realistic; it's certainly not the way most people would actually do the experiment.

[@johnsihler4034]

It's not a trick question. It is a challenge question meant to illustrate systems are complex. I first encountered this in college: the Earth does not travel in an exact elliptical path around the sun. The moon has mass, it and the Earth wobble during orbit. The center of the system is what does not wobble. @@AllThingsPhysicsKZbin

[@zhelyo_physics]

I very much enjoyed this! Excellent work! Fantastic incorporation of Manim animations too, I know how long these can take to code!

[@AllThingsPhysicsYouTube]

Glad you enjoyed it! You're right, this was a TON of work, but a lot of fun! Please consider subscribing and sharing the video with others!

[@jrouquie]

The wave propagating along the string transmit the information that the string has been detached. As long as the information has not reached the ball, the ball must continue on the exact same path as if the string was still attached at the center. Congratulation on finding a medium with such a slow signal velocity.

[@agranero6]

Very stimulating, thanks. That is why in many statements of this problem is said that the string "disappears" or "disconnects at the ball side" (a relatively simple mechanism to do) so students focus on the main question. Even with a rigid bar this would happen just the sound velocity in the bar would be so fast that we could not see it (Ok there are come complications like the mass and inertia moment of the set ball-bar though). That is why some say physics is the art of ignore all the complications and focus on the essential (or like a professor I had used to say anytime you see a potential well approximate it by a parabola because we only want to solve for simple harmonic oscillator, we add complications later aka here there is more light). Imagine if Galileo in his experiences did not extrapolate the friction out: he would still be stuck in Aristotelian Physics. That is why we only tell that to students after they are mature enough to factor all the complexities.

[@robloggia]

My first thought when seeing the slinky the high speed camera experiment was. Oh, gravitational waves suddenly make more sense. Amazing how much changing the camera angle helped!

[@PraniGopu]

If I grasped it right, the ball continues to move in circular motion for a brief amount of time due to the tension of the string (or whatever is attached to it, like a slinky or noodle) exerted on the ball (until the tension wave reaches the ball). So, if we instead had something like a slingshot, where the ball is released detached from the string (rather than attached to the string), it will then go in a tangent and not in circular motion?

[@AllThingsPhysicsYouTube]

Absolutely. You grasped it correctly!

[@PraniGopu]

@@AllThingsPhysicsKZbin Thanks for confirming 😁 And thanks for the fascinating video!

[@AllThingsPhysicsYouTube]

@@PraniGopu You are most welcome. Please consider sharing with others and subscribing, it really helps!

[@PraniGopu]

​​@@AllThingsPhysicsKZbinI did :)

[@steveschwartzm.d.7362]

Thank you! That was incredibly interesting. And the explanation was remarkable. Thank you again.

[@AllThingsPhysicsYouTube]

You’re welcome! I hope you’ll consider subscribing and sharing the video with others who might like it.

[@massimomoro5895]

Actually the straight trajectory tangent to the circular one is NOT an approssimation. Just as you showed with the movement of a falling slinky, you should only base your calculations on the center of mass of the system. In all of the real examples the center of mass left the circular trajectory in a straight line. If you were only considering the trajectory of the ball, you should release it without the string attached. This way the position of the center of mass will coincide with the geometric center of the sphere, therefore leading to the expected result of the ball continuing in a straight line.

[@AllThingsPhysicsYouTube]

You are exactly right. As I mentioned in my response to @jadegecko the center of mass of the slinky/ball system will move in a straight line once released, but it won't be tangent to the outer circle, it will be tangent to the circle of the trajectory of the center of mass. So the straight line trajectory of the ball tangent to the outer circle IS an approximation to what actually happens.

[@oo88oo]

This is like saying Galileo’s x ~ t^2 finding was actually wrong because of air friction… Bottom line: Answer b was and is the correct one.

[@myronsmith8343]

I agree. Good thing that David didn't buy into A) or C)

[@TexGuvnah]

@@oo88oo Exactly. This isn't a "surprising result", it's a pedantic trick question. If you're trying to demonstrate physical laws about circular motion of "a body", you use a string because it's necessary to apply a force to "the body" for the demonstration. He uses that necessity to smuggle in a concealed fact about the "body" that we assume we're supposed to be considering. He made the "force" part of the "mass of the system". Congratulations. Newton's Laws of Motion haven't been broken and B is still correct.

[@andrerodon3921]

My thoughts exactly - this was a "gotcha" video that leaves me disinclined to watch others from this channel.

[@davelaneve2446]

We were presented with this question by our university physics professor back in the early 90s. But in our scenario, the ball was released from the end of the string. So we determined that the ball would continue in an arced path between B and C, due to the centrifugal force and angular momentum.

[@JH-jx1hs]

And this is the truth of it. The premise of this demonstration was disguised in the introduction to the problem to mislead. It was obvious to me that your solution was correct, but it was not offered as a "solution". This allowed them to introduce the elasticity and reframe it as "briefly" and "subtle" during the discussion. Not impressed.

[@viliml2763]

Angular momentum is conserved by linear motion, and the centrifugal force is zero after the ball is released.

[@nemdenemam9753]

I don't think you can talk about centrifugal force in an inertial reference frame which I assume is the one we take when we try to explain this. Also I think angular momentum is perfectly conserved on a straight path as well so I don't understand your reasoning. If your answer were correct, what would provide a non-tangential force to change the direction of the movement of the center of mass? The only way I could see rotation playing any role is if there were some aerodynamic effects of lower pressure on one side ,thanks to the initial rotation, and airdrag

[@marcfruchtman9473]

Very Interesting! Thank you for making the experiment. I wonder what happens if there is no "string", just a ball being released, perhaps mechanically.

[@pecker1859]

Great Video. I would like to see a similar analysis of throwing a spinning Frisbee at the point of release that makes it go exactly where intended depending on the release point.

[@vincecox8376]

Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. This is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..

[@pecker1859]

@@vincecox8376 ?

[@WoolyCow]

as usual, great vid! that was a really interesting concept and your presentation was both intuitive, and thorough :D so glad to see you back!

[@AllThingsPhysicsYouTube]

Thank you. It feels really good to be back, and hopefully I'm back for good now!

[@BirdbrainEngineer]

A cosmological analogy to this is that if the Sun was to suddenly poof out of existence, then the Earth would in-fact continue orbiting around the sun for about another 8 minutes. Essentially, in any physical system, there is a speed of causality of some sort - The information about "the ball was released" *always* takes some time to move from point A (the place of release) to point B (the ball). For the Earth rotating around the Sun, that speed is the speed of light - the ultimate speed of causality. This video is a cautionary tale to always make sure that the mathematics makes physical sense if you apply mathematics to physics.

[@AllThingsPhysicsYouTube]

Heh. I guess you didn't watch the video all the way through when you made this comment. I talk about this at the end of the video. Please watch again! 😊

[@BirdbrainEngineer]

@@AllThingsPhysicsKZbin Haha, I clicked off *literally* the second before that clip.

[@AllThingsPhysicsYouTube]

@@BirdbrainEngineer Then you probably missed the giant slinky as well! 😂

[@BirdbrainEngineer]

@@AllThingsPhysicsKZbin I did see it when I was checking the end again :P

[@YAHaqabNatsariym2701]

But again the sun rotates around the earth. The earth is stationary. NASA even accepts this but lies to the public.

[@dsthorp]

These days, it is comforting to hear exact language. Thank you.

[@28aminoacids]

Really learned something new. I almost freaked out when you striked out option b. But you convinced me.

[@bagnon]

The tension after the slinky or string is released acts on the ball and pulls it towards the center in a similar way that the acceleration was in the same direction while moving, so tangential motion + acceleration to the center = curved motion.

[@37rainman]

The slinky does exactly what he illustrated. And it is easy to visualize why. The ball does not. Simply tie a wright to a thread and experiment experiment experiment. The ball, upon release obviously takes a tangential straight path. The two experiments are not even related. Gullibility doesnt look good on you

[@DrDeuteron]

Great video. If you look at in a frame moving at w/2, then the centrifugal force goes down by a factor of 4, but the Coriolis force makes up for it. That's why you need the 2w in it.

[@malvoliosf]

I am wondering if the propagation speed of the tension wave in a material is related to the speed of sound in that material. Also, I am wondering if the tendency of the string and the ball to spin about each other comes from this same time-delay: the end of the string near the center is traveling along the tangent at the time it was released; the ball and the other end of the string are traveling along the tangent at the time at the time the elasticity wave got to them.

[@AllThingsPhysicsYouTube]

It is the speed of sound!

[@darby5987]

I had to think about this for a minute or two. As a thought experiment I scaled it up to a 1 km noodle/slinky. Then the answer was clear. Special Relativity - information cannot be transmitted at greater than the speed of light. For the ball to take either straight path (tangent or parallel) the signal "I've cut you loose from the center" would have to be instantaneously transmitted to the ball 1 km away. Therefore b) and c) can't be correct. The answer must be a). The noodle or slinky is released from the center. The signal starts propagating along the noodle/slinky and takes a finite interval of time to arrive at the ball. In the mean time the ball has no idea it has been cut loose. It continues moving in the same circular motion until the signal arrives.

[@48306jw]

I’m not sure you’ve considered all the variables in this equation. Whether it’s a slinky or a rubber band, once released, they both want to fly away from the center, but the other end is also trying to retract back into the center of its own mass causing a moment of delay at the ball. If you do the same experiment but with a ridged bar or wire, something with theoretically no stretch, and release it at the ball thereby removing any influence of the tether, you might find a different result.

[@ImieNazwiskoOK]

The last example is quite good at showing the effect if someone knows it. And the whole question reminds me once again of thought experiment of "using a stick to nudge a spacecraft around Jupiter", with conclusion that all objects don't move as one but at the speed that a wave propagates through them.

[@eeee69]

Really awesome video, 12:20 I was struck with the realization of "phonon!" and wave propagations etc and it's really cool you could show that. I'm a materials science undergrad (senior)

[@AllThingsPhysicsYouTube]

Glad you liked the video! I hope you'll consider subscribing and sharing the video with others. If you're a materials science major, I've got a couple of other videos coming up that I think you're going to love!

[@BrianStDenis-pj1tq]

It might be interesting to use your equipment and try the experiment with a small aero-drag at the end of the string (in the center). When you release, this would provide a constant (but small) drag on the released object, maybe you could get it to follow the curve farther around.

[@rogergeyer9851]

Someone could also carry the ball in a circle. The whole thing is about a SYSTEM (ball AND something else), though posed as the ball. Semantic trickery vs. actual physics.

[@lucasgroves137]

Thank you for the stunning video, and for all your rigorous work. I'd like to add my two cents on how we think about this problem. Our rigor must extend to our intent when we say "release of the ball." Instead of thinking of the ball as continuing in circular motion AFTER ITS RELEASE, we must recognize that release of the ball does not occur-at all-until the tension wave reaches it, some time after release of the tension element. Once it becomes apparent that release of the tension element does not equate to release of the ball, we should not maintain that the ball continues in circular motion AFTER IT IS RELEASED, because during that period, the ball still has not been released-the same centripetal force acts on it as before we released the tension element! Thank you for a superb demo, including the beautifully clear treatment of Coriolis force, etc, on the plastic puck. And for showing us the magnificent theatre at High Point Uni!

[@vincecox8376]

Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. The anti gravity is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..

[@dreupen]

I always cut my "massless" string at the ball. While I think your presentation of the question is disingenuous, I do love the video and how it sheds "light" on the finer details. Don't forget the mass (or more appropriately, the moment of inertia) of the string, this will have a similar effect without retardation (change your slinky to a solid rod). I stand by the answer "b", but admittedly to the question of motion after breaking/cutting the string at the ball. What I really like about this video is that it make one think about the real life impacts of the fictitious assumptions/simplifications we employ and quickly forget about.

[@AllThingsPhysicsYouTube]

Well, to me the term disingenuous has a rather negative connotation, as if I was trying to pull a fast one. And I wasn’t, really. Yes, I knew people would misinterpret the question, but the point is not whether (a), (b), or (c) is the right answer, the point is to watch the ball continue in circular motion after the string is released! Anyway, I’m happy to hear that you still appreciated the video despite my choice to let people misinterpret the question.

[@blueobject]

Thank you. I also commented as he confuses everyone from the start and doesn't define things. But maybe that was just me vs 99%

[@KaiHenningsen]

@@AllThingsPhysicsKZbin Hmm. I'm thinking the negative connotation is not entirely unwarranted.

[@mrfranksan]

@@AllThingsPhysicsKZbin What if the "string" were completely inelastic, or rigid? Would the propagation be essentially instantaneous? I suppose the "string", being incompressible, would exert some additional effect, and perhaps how the result would play out would be influenced by the details of the release mechanism and the method of attachment of the "string" to the ball.

[@richardaraujo3492]

It was taught that way to me too, because it was meant to show what happens when the acceleration toward the center stops. If you leave the string, or slinky, attached it's a more complicated system. I don't think he was being disingenuous, just pointing out a problem with how the problem is presented by a lot of people. I've heard and read it presented both ways, my old physics teacher from high school presented it the correct way: what happens when the ball is disconnected, not the ball and string.

[@Observ45er]

A slightly different way to describe the Slinky drop wave. If you are well versed in waves, this is obvious. . You should understand that the Slinky has a longitudinal (up-down) tension wave that travels down the Slinky. It will do the same thing when stretched horizontally and one end is let go. . In the vertical case, Understand that each "turn" of the Slinky coil is held up by the tension-force from the one above and that each turn also has mass which, as always, takes time to accelerate in response to a force. The ratio of force to mass is like the characteristic (or surge) impedance of a transmission line that is determined by the line's inductance and capacitance. . It is a really neat demo for an experienced RF engineer. . This wave travels down the Slinky just like a radio wave travels along a transmission line, or a wave travels on water. Any given turn can not begin to accelerate until the force from above changes. There are several You Tube Videos of waves on Slinkys. .. .. .. .. ALSO. . . In the circular motion, the longitudinal wave also becomes a transverse (side-side) wave, so even though the sSlinky must now 'bend' sideways, it also takes time for that wave to propagate down the line. . . SO COOL!! . . . . . . Science works !!

[@CymruCreator]

It was good, I liked it :) The tension holds the ball in even though the string is let go.

[@AllThingsPhysicsYouTube]

That’s right!

[@37rainman]

@@AllThingsPhysicsKZbin Is this supposed to be some sort of joke to see how many gullible people you can muster? Or what actually is your point here? You strongly imply that upon release the ball will continue to take the same circular path, and around to the same point it started from, only the ball will be lower. Or you think that it will take sone sort of circular path but maybe not quite exactly the original path. Neither case is true, of course, and I didnt even need to test it. But, I did, i opened my tackle box, took out a spherical lead fishing weight, tied a very light thread to it and experimented, repeatedly. Upon release the ball flies in a straight line. A line tangential to the original circular path. It doesnt curve at al, much less in a circular path So looking at the comments here I guess your vid was a success -- it demod how many gullible yes-men there are in this world of ours But really, thanx for the vid. O look forward to others.

[@Grape99909]

Well done and thank you for sharing :) This will open the minds of many people to the relativistic nature of our universe. Everything that happens in our environment is a function of one’s perspective!

[@AllThingsPhysicsYouTube]

Absolutely!

[@KieranLeCam]

Great video! I was surprised at the results and tried to make intuitive sense of them, if it helps anyone: what makes the results feel weird, I think, is the idea that the ball is what we're accelerating, but it is the whole system, string or slinky included, that we're accelerating. This thought helps me much more easily understand why the tension wave needs to propagate to make a change to anything. When I let go I have only acted on the beginning of the slinky/string and the rest of the slinky/string is still being accelerated in circular motion, until it is told by the tension wave (which is actually a wave of change in direction of the acceleration!) to no longer maintain its stable previous circular motion.

[@AllThingsPhysicsYouTube]

We’ll said!

[@uwelinzbauer3973]

At first, I didn't want to watch and didn't want to believe. At school I learned: tangent line. But then I was surprised. Very interesting additional physical laws, that cause measurable effects, visualized with progressive equipment in advanced experiments. I learned several facts, I never thought about so far. Thanks and congratulations for this video!

[@j-me5820]

Thank you. This was a very well presented video. I learned a lot.

[@AllThingsPhysicsYouTube]

Glad it was helpful!

[@vincecox8376]

Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. The anti gravity is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..

[@JohnLRice]

Interesting, fascinating, fun, and well done video, Dave! 👏👏 So, what happens if the ball is released from the other end so that the elastic properties of the string or slinky have no effect on the ball's trajectory? 🤔

[@AllThingsPhysicsYouTube]

John-y-kins! Nice to hear from you! Hope you are doing well!

[@JohnLRice]

@@AllThingsPhysicsKZbin I'm doing OK Dave-y-kins 😄and I hope you and your family are as well! 🤗

[@capchuckpriceutyoub]

May a non-physicist attempt an answer? My intuition says if the attachment point to the ball is on the surface, release of the forces present due to the elastic property of the ball would have to propagate to the center of mass of the ball before circular motion ends. If the attachment point was through a hole in the ball to a point at the center of mass, there would be (given the perfect attachment point) no circular motion. But as I think further, even an attachment at the precise center of mass of the ball contains mechanical forces to be relieved, so perhaps that would keep the ball on an infinitesimal curve as that stabilizes. A physicist might chase that down to what, the Planck length? While an engineer would stop at “close enough” long before that.

[@JohnLRice]

@@capchuckpriceutyoub Thanks Chuck, that makes sense to me! 😎👍 And yeah, while physicists will go deep to determine EXACTLY what is taking place, engineers tend to shoot for achieving a reliable intended outcome within an acceptable range of variance, which is often degraded by budgetary concerns. ;-)

[@andrerouth4253]

Thank you David for this surprising result. The explanation became complex because of the slinky wave, the elasticity of the bungee, and the extensibility of the string. The experiment we performed for a science fair 60 odd years ago in Brighton, UK was much simpler: a lead fishing weight on some monofilament. The monofilament was designed to fail at the weight end as the rotational speed was increased. We only had cine film to record the event. Of course, we concluded that the path was indeed tangential because that is what theory told us would happen. Within experimental error, that is! Seriously, with a high speed camera and the fracturing monofilament, you would find the expected result BUT your video would have been a lot less interesting and thought provoking!

[@vincecox8376]

Please get tuned into facts, 1. anti gravity is a product of the center field of a magnet. 2. the center of a magnet when vibrated will repel water 3. If you vibrate granite rock with the center field of a magnet at the correct frequency it will turn like butter. that's just the beginning it will play a Hugh part of our country's future. This is only applicable to none iron type material such as glass or plastic. If you have a bar magnet simply tap the center near a trickle of water and watch the water move out of the way. Tap the center field on any none metalic surface and watch it loose weight. You can not use a sign wave type vibration it must be saw tooth . A sign wave signal contains the centerfield, that's what pushes AC and radio signals around the world. A sign wave has three elements on an oscilloscope you only see the two , On a scope you see the positive cycle and the negative cycle you never see the most significant part of the cycle and that is the center field that produces the energy to push the energy forward!!! There is allot to be learned about the magnetic center field .. Please help spred the news ..

[@pravinmarathe3838]

waht will happen if the string end connected to the ball is released? will it change the result? because from what i saw the deviation the ball pathe after release occurs when the string goes the other way crossing the past path of the bal

[@janurek3050]

You eaned my subscription. Great video, clear and easy to follow.

[@AllThingsPhysicsYouTube]

Awesome, thank you!

[@jugbrewer]

11:24 The "drag angle" can't just be due to air drag though right? The slinky isn't attached to the centre of the circle so it would have to experience some extra propulsive force in order to "catch up" to the angle of the arms on the turntable. It's being lead around in a circular path by it's connection point which itself is describing a circular path. I think air resistance would account for a curve in the rope, because tension is consistent throughout the rope but velocity and (therefore air resistance force) isn't.

[@AllThingsPhysicsYouTube]

No, the drag angle is caused by air drag alone. But you're right, if you look closely at the slinky you can see that it's slightly curved.

[@jugbrewer]

​@@AllThingsPhysicsKZbin But even in a vacuum, wouldn't there always have to be a tangential component to the tension in the rope? The connection point between the rope and the turntable is not situated at the centre of the mass' circular path. It's at some distance from the centre, and as the table spins that connection it is continuously changing direction and therefore has to be applying more than just a one dimensional force that runs along the radius of the slinky's orbit, right? Just like if you want to whip an object on a string around your head, your hand has to constantly change direction and in doing so applies some tangential force. If the force was simply radial, you could maintain the object's orbit with your hand perfectly stationary as long as you maintained a constant tension in the rope - but that won't work, the tension comes from the sweep of the hand around some circle with the object lagging behind. Please let me know if I'm missing something here.

[@AllThingsPhysicsYouTube]

@@jugbrewer Hmmm...I don't think there would be a tangential component in a vacuum. If tension is the only force acting on the mass, then if there were a tangential component the mass would then accelerated in the tangential direction. The only point of equilibrium is when the tangential force is zero. Now, it would take a little time to reach this equilibrium configuration after starting up the contraption, and it does assume there is some dissipation in the system. But I'm pretty sure that a real system in a a vacuum would end up with zero tangential component to tension.

[@jugbrewer]

​@@AllThingsPhysicsKZbin ​ Thanks for the reply! I think I see why I disagreed - you were describing a system with realistic air physics (the turntable has to do work to counter drag) but idealized slinky physics (the slinky converts 100% of its elastic potential energy into kinetic energy). In my mind the mass would never fully recover the energy lost from the slinky during the acceleration phase, but under the parameters you had in mind I can see that it would!

[@johngregor6743]

I think having had a 'd - none of the above' and then showing the center of mass moving off linearly from the right radial distance would have been a neat reveal. You could have come up with a pure 'ball' system as opposed to the various 'ball + massive string + stored energy' systems by replacing the string with a wire and an electromagnet at the end and a ball bearing. Cut the current and the ball bearing goes flying (modulo stored magnetic energy in the coil). Maybe a collab with the Slow Mo Guys would yield something interesting with those higher speed releases? Polarized light might reveal stress propagation in a clear plastic or glass string.

[@j.r7872]

Weird AL is good at teaching physics… Drink a shot every time he says “SLINKY”

[@AllThingsPhysicsYouTube]

SLINKY....SLINKY....SLINKY!

[@imaginaryangle]

This was an amazing demonstration, thank you! What I loved the most is how it builds intuitions about the different reference frames involved in this system to create a full understanding of their combined effect. Beautiful!

[@AllThingsPhysicsYouTube]

Thank you. I appreciate the kind words. Please consider sharing with others!

[@imaginaryangle]

@@AllThingsPhysicsKZbin Will do!

[@TheErichill]

We'd like to see a release mechanism on the ball end of the string. That would better match our early expectation of where you were going to take things.

[@veronicagorosito187]

Or a stick.

[@blackscreen8610]

i think it all drills down on how the effect travels throughout the system...

[@PerMortensen]

The thing that leads people (even physicists) to go for answer B I guess is the unspoken assumption that "the string being released" means the centripetal force is immediately gone. We are so used to working with idealized models (infinitel stiff rods, massless strings) in which case that information is indeed instantaneously transmitted and the answer is in fact B. Once you challenge that assumption it quickly becomes clear that the answer is A in a real world setting. I fell into the trap of saying B myself, though I knew that was gonna be wrong, because why else would you make a video about it. Quickly realized where you were going with it when you started talking about the slinky. Still though, you're right about how striking it is to actually see the effect! It looks so damn strange, even though you intellectually know that's how the motion must be. Terrific video!

[@AllThingsPhysicsYouTube]

Yes, we (physicists) often simplify things to get across the basics, which is obviously the way you need to teach the subject, but once you've got the basics I find it fun to think a little more deeply about things we often take for granted.

[@PerMortensen]

@@AllThingsPhysicsKZbin Absolutely :)

[@SeattlePioneer]

In the question posed as a test, we were given imprecise information and told to use it as if it were perfect. A ball swung on a string by a person is not going to describe a perfect circle, but we are expected to assume that it does act that way. Similarly, the wave traveling down the string is estimated to travel at the speed of a rifle bullet ---obviously far too fast for us to be able to measure and use in the problem. So just as we are expected to assume that the ball is describing a perfect circle, it is only reasonable to expect that we will assume the instantaneous release of the ball. So I suggest that for the purposes of this test, answer B is the correct answer. It's really dishonest to add a lot of other conditions to such a test after the fact, in my view.

[@vitalstatistic63]

So essentially, when the spring or string is released, the ball momentarily appears to defy gravity or centrifugal acceleration because the spring/string is still under tension towards the centre of mass, and is still pulling the ball towards that centre. Likewise, the stationary top or radial centre point of the spring, the now released attachment point, is also under the same tension towards the centre of mass and would be pulled towards it by spring tension as well as gravity or centrifugal force, and would travel faster away from the attachment point than expected. Both ends of the spring, as well as the ball, are being drawn towards the centre of mass of the spring, as that centre of mass moves under the force of gravity or centrifugal force. The spring is used in this example video to slow the effect down enough to be recorded, but the string works exactly the same way, just orders of magnitude faster. As others have said though, it depends on where the spring/string is released. If the string breaks at the ball end, then answer B would be correct and the ball will continue in a straight line at a tangent to the circle. If the string breaks at the non-ball end, then answer A is briefly correct, until the tension in the string is all released, then allowing the ball to move away at a tangent further around the circle. But this is something I had never considered before, so I've learned something today. Thanks for that.

[@hashtaghashtag9967]

I was sorta following along for most of the explaination on a "this doesnt seem right but i trust you cause you sound like you know what youre talking about" Basis, but with the gravity example at the very end it finally clicked as i have spent more time thinking about that sort of thing and suddenly the whole phenomenon seems intuitive!

[@AllThingsPhysicsYouTube]

Awesome!

[@nathanyim]

Great video -- you learn something new every day! I'm assuming that if the release point was located where the ball is attached to the string (instead of near the centre of the circular path), then we'd see the ball immediately follow the tangential direction. However, I'm curious about whether the ball would experience the Magnus effect at all soon after, which could curve its trajectory sideways. Since the ball is already experiencing some rotation while its still tethered (1 full rotation per revolution, similar to the coin rotation paradox), I guess I'm wondering if that rotation is preserved after the ball is released? Would it follow the same principle that you demonstrated in this video? Thanks!

[@AllThingsPhysicsYouTube]

Well, I'll be honest, I hadn't thought about that. But you are correct that the ball is rotating and I believe this rotation would continue. Thus, if we don't neglect air resistance then the Magnus force would certainly be acting, though the rotation rate is pretty small so this force would likewise be quite small.

[@gdahlm]

Yes a release at the ball would more closely approximate the tangent as not only would it more closely approximate the ridged body assumptions of these algorithms, it wouldn't have the string in the ensemble, which needs to be considered. While the Slinky's bottom doesn't move, the total mass of the spring is being accelerated as one would expect under gravity. IMHO some of this is a product of the limitations of the Newtonian approximations, Euler angles, and the cross product. In the GR case of the sun disappearing, the Earth is mostly following a geodesic, or the straight line on a curved surface. There would not be any perceived changes in angular momentum at all from the earth's reference frame. Just what is a straight line would change (ignoring tidal and other smaller effects) It is useful to replace fictitious force with apparent force when talking about effects like centrifugal force in Newtonian mechanics and for the apparent force of gravity under GR. These apparent forces are an artifact of the chosen frame of reference. Choosing the ball's perspective when attached to a string, and not the string ball assembly as a whole is the cause of the apparent paradox here. Spherical cows (oversimplified models) allow for practical models with simpler math. Euler angles, the cross product, and the fact that SO(3) is topologically a 3-torus really move your above question into the Grassman algebra realm for intuition IMHO

[@fjb4932]

" . . .you learn something new every day!" Sadly, many Do NOT ! ☆

[@BBQDad463]

Thank you for this video. It appears to me that the elastic connection (Slinky, silicone cord, and so on) has the effect of delaying the information about the time of release, i.e., the "time of potential release" until the moment the wave reaches the ball, which can be called the "time of true release." Therefore, in my mind, the ball travels in a straight line from the time of true release. Thus, B was the correct answer. If the ball had been released from the connecting object, instead of the connecting object being released from the center, then the ball would have begun to travel in a straight line immediately upon the "time of potential release," which would have been contemporaneous with the "time of true release," Nothing can travel faster than light, including the fact of the time of potential release of the connecting object from center. The revolving object (the ball) cannot begin to deviate from its circular path until it receives the information regarding the fact of release. Then, exactly then, and only then---at the time of true release---can the ball begin to travel in a straight line. Subscribed.

[@AllThingsPhysicsYouTube]

Yes, that's a good way of saying it! And I appreciate the sub; I'll try not to disappoint!

[@CarlosTriay-tl9bm]

Question: what would happen if the string separates from the ball at the level of the ball surface, instead of at the center of the rotating system?

[@toxiq5295]

Dude what a blast of a video, Awesome work

[@AllThingsPhysicsYouTube]

Thanks! I hope you’ll consider subscribing and sharing the video with others!

[@toxiq5295]

I did, this is so counterintuitive I love it

[@johnburgess2084]

Very fun video. Since the answer is so obvious, I knew this had to be a trick question if the answer is anything other than B. Usually, the questionee will neglect the mass of the string. As another commenter said, I would bet that the center of mass of the system would immediately depart in a straight line tangentially from the circumference. I would like to see the experiment performed somewhat differently. Support the ball so its COM defines the circular path, then release the COM. This could be approximated by attaching an arm to the rotating platform and hold the ball (a ferrous ball) by an electromagnet. Then turn off the magnet while spinning. I'd bet the ball will immediately depart on the tangent line. Except perhaps due to the fact that the ball still would not be supported EXACTLY at its center of mass.

[@markhumphrey8894]

Good idea about the magnet John. I thought "Why didn't he use a string on the machine to show the different responses at release.?" A Slinky has tension in the collapsing coil bringing the ball back on the line. The string just snaps or magnet just turns off and the ball is released on the tangent. To me it was like comparing apples to oranges. He starts the video with a string and ball then makes us think the released ball does not travel on the tangent. He then goes on to prove this hypothesis with using a Slinky and not a string. I'm surprised the Prof's at High Point University weren't saying "Hey, how come we aren't using a string or a magnet in this experiment?"

[@ozmaniac33]

Certainly an interesting situation, yet I agree with @pataplan the proposition was a little misleading or ambiguous. I too assumed the release point was at the outer end of the string, in which case the answer would be "b". Your results were surprising, yet after thinking about it, it does make sense but only because of the elastic tension in the tether, as it continues to provide a force on the ball until it is completely dissipated. I would like to see what would happen if the tether was rigid, say a carbon fiber rod or something with almost no tensile elasticity, release it from the hub as here, and then see what happens. I believe the ball then would continue in a still curved path, but no longer matching the radius of its former circle. In this case I imagine the center of mass of the ball/carbon fiber tether combo would instantly begin moving in a tangential trajectory, however the ball and tether would now begin rotating along this straight tangential trajectory, around the center of mass of the combined unit, and thus the ball itself would oscillate in its path around this center of mass. First video of yours I've seen, instantly subscribed and shall have to see what else you explore. This is the sort of stuff I'd stay in the classroom in high school during lunch time to argue/discuss with the other nerds! Love it!

[@Fogaata]

What about snaping/cutting the string before the wave gets to the ball? Would it keep going circular?

[@tomaszmasternak2225]

This is superb content. Thank you so much for sharing this!

[@AllThingsPhysicsYouTube]

Thanks so much! Please consider subscribing and sharing with others!

[@rafaelmaciasparra4540]

I'm REALLY curious: what would happen if instead of a string the ball were to be released from a solid arm? If you ever do that experiment, please let me know.

[@jaricci782]

I have only a rudimentary education physics, but one thing I know for certain is that circular motion is not a fundamental type of motion, it is the combination of an outward motion and an inward motion (the string "pulling back" the ball towards the center). So, if released by an arm, the motion would be straight. In the case of the string it stays circular because the segments of the string that have not been reached by the wave are still "pulling back" the ball.

[@samarjeetsingh5840]

I really find this phenomenon of circuler motion really unbelievable but this true . Iam very lucky to discover this with help of your video sir thanks for bringing this beautiful phenomenon of physics

[@AllThingsPhysicsYouTube]

Glad you liked it! Please consider sharing with anyone who you think would like it!

[@samarjeetsingh5840]

Yes sir I like would like to share this phenomenon with my friends and family members

[@smeeself]

Thoroughly enjoyable video. Thank you. 👍

[@AllThingsPhysicsYouTube]

Glad you enjoyed it

[@dennisk5818]

This was quite interesting. But, I've a question. If the object is released at the object and not from the center, will the object still continue to follow the path, then slowly break away from the point tangent to the original path? Also, will the object (regardless of it's mass) travel the same distance (in degrees), before breaking away from the tangent point?

[@AllThingsPhysicsYouTube]

No, if the string breaks right where the ball is attached then it will immediately move off tangent to the circle.

[@Jmnp08]

Excellent video. Please continue to do these. Very well done!

[@AllThingsPhysicsYouTube]

Thanks, will do!

[@dracus17]

As usual, impressive visuals and great explanations throughout. Keep it up!

[@AllThingsPhysicsYouTube]

Thanks, will do!

[@jamesthenabignumber]

This is wonderful! I have taught physics for a long time, and this isa. very accessible extension of the core physics of circular motion. Thank you for presenting it so well.

[@AllThingsPhysicsYouTube]

So glad you liked it. Please consider sharing with others who might be interested!

[@donaldorr8508]

impressive.... i wish i would have had this in skool. more please, thanks

[@AllThingsPhysicsYouTube]

More on the way! I promise!

[@Robinsonero]

People saying this is a trick question are just butthurt because they're not used to being wrong about math and physics. You framed the question as a physical ball on physical string and asked about the instant of release. At best there is ambiguity about where the release in the system is but your framing was absolutely consistent accurate. Others may have brought assumptions about perfect rigidity (that no string has) or reframed it in their minds about the ultimate trajectory, which is on them. The final nail is that on anstrophysival scale the same phenomenon holds for more or less the same reason. Thanks for the wonderful lesson.

[@AllThingsPhysicsYouTube]

“… people are just butthurt …”. 😂