Angular Momentum - Physics 101 / AP Physics 1 Review with Dianna Cowern

Lesson 15 (Angular Momentum) of Dianna's Intro Physics Class on Physics Girl. Never taken physics before? Want to learn the basics of physics? Need a review of AP Physics concepts before the exam? This course is for you!
More fun exploration of water spinning angular momentum demo from Steve Mould and Smarter Every Day: • Which Way Will the Wat...
Exercises in this video:
1. A box and a sphere are sliding and rolling, respectively, down a ramp. If they start from the same height, and we ignore friction on the box, which will reach the ground first?
What if it were a full soda can and an empty soda can?
2. A sphere of mass 4kg rolls down a ramp, starting with a height of 5 meters from the ground. How fast is the sphere rolling when it gets to the ground?
3. If every person on Earth ran around the equator in the same direction, what would their total angular momentum be? Use 65 kg as the average mass of the 7.8 billion people on Earth, and let them run at 5m/s.
4. Given 8 x 10³⁷ kg•m² as the Moment of Inertia of Earth, by how much would all those people running change the angular speed of the Earth?
Special thanks to Kari Byron for her wonderful shoutout at the end of the video! You can follow her on Twitter @KariByron and Instagram @therealkaribyron.
Credits:
Dianna Cowern - Executive Producer/Host/Writer
Jeff Brock - Lead Writer/Course Designer
Laura Chernikoff - Producer
Kaitlyn Today - Video Editor
Sophia Chen - Researcher/Writer
Erika K. Carlson - Researcher/Writer
Hope Butner - Production Assistant
Levi Butner - Videographer
Lauren Ivy - Set Design
Vanessa Hill - Consulting Producer
Aleeza McCant - Illustrator
Rachel Allen - Illustrator
Consultant - Kyle Kitzmiller
Lucy Brock, Samantha Ward - Curriculum Consultants
Cathy Cowern - Transcription

[@physicsgirl]

Since the question has come up in the comments: for the ramp problem, we're turning friction off for the square, but back ON for the sphere, so it can roll. Yes you are all correct, it would not roll without friction! :) 9:58 - "And then WITH friction, the spherical cow is going to roll."

[@danielvickery9244]

Would the spherical cow even roll if there where no friction? and if so why?

[@Djfmdotcom]

Totally random question: what was the record you used to demonstrate with?

[@pratyushkumarnanda4952]

Maam what if we make a hole form north pole to South Pole and throw a ball... What will happen ??? 😅😅

[@mr.aldave8308]

@@danielvickery9244 without friction it would just slide down.

[@En_theo]

Would a disk really spin forever if we let it spin in perfect conditions (in space, no external influence) ? I read something about gravitational waves slowing the disk down after a very long time.

[@iiserite_das_rahul_5603]

Probably my comment will get lost...... But I just want to say that I saw her videos which were 8 years old..... And the way she made progress is truly impressive...... From a few subscribers to 1.76M subscribers..... From 2 mins small experimental videos to 30 mins descriptive videos... This is how she made her progress..... You always deserve a 👍 from us..... Well keep it up .... Will wait for the next vdo .....✌️✌️✌️✌️✌️

[@Thomas..Anderson]

Well, if she would make videos about splashing make-up on faces she would have a lot more. It is a sad world we live in.

[@hakanyurtluk719]

Just a little correction: When water goes outside from the pipes and at the same time they are turning around, water will have a tangential speed to front and radial speed to the outward. Because of this reason, they will have a direction near to the front. However as it is seen in the video, they curl back. BUT! this is an illusion. Actually, they are not curled back. Just as an example let's think about not radial but the tangential speed. For a simplification we have two water particles. The first water particle will have a tangential speed when it leaves the pipe and goes forward. Then, the second water particle also goes forward but it will leave the pipe after the first one. Because pipes are turning, the second particle will leave the pipe from a further position. As the two particles just goes on a straight path, the second particle will have a path near to the front compared to first particle. As we think about all particles this way, first left particles looks behind and it seems like while the pipes are turning, water curls back. As a result, this is the reason of illusion why water just goes on a straight path but not curls back actually. And also in order to avoid from some misunderstood: The pipes that has water output to the middle works in the same way. Water particles follows straight lines and as the pipes are turning, they are seems to behave like they are approaching to front and turning outside. There is no words to explaining them with using angular momentum. It is already correct. As it is mentioned in the video, angular momentum is conserved. Also, as I mentioned here, water particles will follow the straight path and they will not curl anywhere, so it is normal that there is no reason to have change in angular momentum. So also just think about this way. Have a nice day :)

[@sarthakmalhotra7413]

Just a little correction he says with 3 big paragraphs!

[@hakanyurtluk719]

@@sarthakmalhotra7413 Hahaha you are totally right :D There is no words to say

[@sarthakmalhotra7413]

@@hakanyurtluk719 🤣🤣🤣

[@djleaf6945]

what about coriolis force

[@hakanyurtluk719]

​ @Dj Leaf Coriolis effect is also working in a similar way. It is also an illusion, thus it is not actually a force, but as it looks like something thrown to the center is curling ahead by the point of view of the people turning around, it seems like it is a force. The same logic I wrote above is valid here. It is simply applying when the thrown is going outward, it looks curling back, when the thrown is going inward, it looks curling ahead. Also let's think about particles again, assume pipe is turning CCW and we look from the pipe to jetting direction, when the first particle goes in a straight in the way to the center, we move to our right and the particle's velocity looks go to our right slightly, but it is still in the same straight direction according to the ground. The second particle is also thrown similarly as the first one, it goes straight to center but first's velocity is on more right according to the second's. The second one is on more right according to the third one, and so on. The first is our past and the third is our present. In continuity with particles, our past looks like turning right that the side which we are approaching to.

[@ElectroBOOM]

HELLO! Thanks for the great course! May I make a comment about the demo of 15:20 of weird water movement (if I understood your video correctly)? The reason water seems to move forward to the motion of the water pipe is not due to increased speed of water to maintain the same angular momentum. If that was true, then in around 16:12 where the water almost curls back to the same radius it would return to the same spot too. When water leaves the pipe, water droplets start moving in a straight line and won't continue rotating with the pipe. And where they end up in a few moments related to the nozzle makes you think they went ahead, while all of them were just moving straight. An animation would explain it more clearly! So what we see is an illusion of water moving ahead of the spin while it is not. HAVE FUN!

[@glennkrieger]

Dude, is this really you? I think if you plugged in the water hose somehow into an extension cord and then grabbed it...well, that would be familiar, huh?

[@GooogleGoglee]

I totally agree and honestly I think we should reproduce the experiment electrifying the water and see if the sparks follow the same direction of the water! 😄

[@LTV_inc]

Well you just flunked your first physics lesson. If you derive anything from this video on “momentum” and “acceleration” then you would know that the little masses of water molecules are at a certain speed at the radius they leave the pipe, pointed towards their axis of rotation. As Ms. Cowern so eloquently stated if their radius is smaller then they must travel faster to conserve that angular momentum so they actually pass the nozzle since they are traveling faster than their brethren that are closer. Which is visually elusive since our minds are wired to think that things farther away travel faster.....I could be wrong......but I’m not🥸

[@JohnSmith-fj3uf]

Electroboom apparently great minds watch the same videos. I wrote your business adress beginning with "Nat"...on 1/29/2020 at this point I don't expect you will make the video debunking grounding that I suggested. But. Would appreciate it if you acknowledged you saw it so I would not be tempted to waste effort looking for some other way to contact you. Here is an example of the comedy potential of debunking the health claims of barefoot grounders: One of the videos shows Chopra hugging an old pine tree and acting as if he can feel the free electrons healing him. The thick pine bark is dry and looks to be over a cm thick. A great insulator I suspect! Maybe a better one than shoes. Something you could confirm on video with little effort.

[@sundogaudio851]

@@JohnSmith-fj3uf but there is still the field effect...

[@RegularEarthlingEngineer]

I got a sci oly competition this Saturday and this is what I’m missing for the machines event thank you for uploading this!

[@cagataycakirtas8064]

I have a question, is this course going to continue after high school material is over or are you planning to start another course including calculus and higher-level subjects?

[@stillme4084]

Wow. That's a harsh comment. But I'm glad you are such a genius. For reals. Teach us.

[@rahmanbikmukhametov1253]

The “trick” with pipes and water has even more cool physics! Try to explain the curve of the water flow from the rotating frame of view - here how you come to the non-inertial reference frame and related inertial forces

[@K_ingh16]

Thank you diana for doing these science lessons, I don't need them anymore but you really help out many people with these videos.

[@cohblimeytrousers]

"Done being Humerus" 🦴 . Good one 🤣🤣

[@En_theo]

Thanks for pointing that out, English not being my first language, I totally missed the pun :)

[@FlyingSavannahs]

I caught it on my third view and English _is_ my primary language! This was quite a gem. It's too creative to diminish with my own pun retort as tempting as it is. Respect!

[@amritnalam9994]

Haha. Did she really mean it in that way though? 😂 Or literally meant humorous

[@En_theo]

@@amritnalam9994 Quantum Physics says both are possible until she answers... and even then, you'll never know if she lied.

[@FlyingSavannahs]

@@amritnalam9994 Oh, it was on purpose, I assure you. She did a fruit and vegetable series this last summer.

[@umanglunia2194]

These videos are so well paced. Love this series!!

[@deepakjoshi823]

The water pipe experiment is amazing! ❤❤❤👌👌👌👌

[@seanmostert4213]

I can’t thank you enough for sharing your wealth of knowledge and taking the time to simplify this for everyone

[@roraraptor]

Thank you for sharing your knowledge with the world like this! You’re a treasure! 💖

[@jameskidd7328]

Your such a breath of fresh air !!! I love your channel keep it up Girl!!!!

[@joloallexicepineda7347]

I just love how you present the lesson 😊 Whenever I watch your videos it put smile on my face

[@DEADPOOL-ti4cs]

10:52 there's no friction, why it would rotate.

[@pyroclasm1]

Is't that because the center of mass is not directly above the point of contact on an inclined surface?

[@michaelsommers2356]

It is assumed that there is enough static friction there to make the ball roll, but you are correct that if there were absolutely no friction, the ball would slide, not roll. Presumably this was not mentioned in order to concentrate on the main point of the lesson, and not get sidetracked.

[@michaelsommers2356]

@@pyroclasm1 That is why the ball rolls, but it would still need some friction to do so.

[@RitobanRoyChowdhury]

It does have friction. See the top comment

[@michaelsommers2356]

@party pack What are you talking about? What does water have to do with the rolling friction on an inclined plane?

[@RobertSmith-pw9io]

EXCELLENT again Professor! You bent my brain from the very beginning of this video! Thank you very much.

[@Suburp212]

Great format. Good to see you still make videos.

[@ayushkr.3944]

Thank you Dianna for your great physics videos❤️❤️🔥🔥..

[@tylerdean3489]

I have an AP physics test tomorrow, Im really thankful for this series as I can review a bunch of stuff that I haven't done in a while.

[@PTRMAN]

I love this stuff! Thanks so much for explaining it in a very easy-to-understand way....

[@migfed]

I really appreciate your effort and this is exactly the kind of stimulus that youngsters need, this is going to be key for millions of young minds to step forward and pick up a STEM career.

[@arulgupta1639]

best physics teacher ever🤩

[@nilishabharadwaj]

Thank you for existing, I love you.

[@aislinnswag9481]

this video was recommended to me out of nowhere. I'm a freshmen who is not even able to take physics. I'm now going to learn physics from you because I do want to take it and if I take it now I can ace it later. big brain moves.

[@kazimhussain3590]

Thanks Diana for helping me grasp my subject so much better

[@80taranjeet]

Hey diana, just don't stop these lessons, because I love your background😅( but your explanation is equally good)

[@timothyphillips4801]

You got me with Rodney Mullen (Yes,I am a skater),now I've been learning physics thanks to you.Sending you a virtual high five and a big thank you from Australia.Tim ♥️s Dianna.

[@dankuchar6821]

From a long time physics teacher, good job Diana!

[@ApolloMcDonnean]

Just found this channel. I wish there were more people like you. The world have surprisingly uneducated youth... Best of luck.

[@faisalbaloch5075]

A great concept, i really love the way you teach. Thanks a lot for. I want you to please upload a video on absolute gravitational energy.

[@dorianeric]

Great explanation and the sprinkler thing is so cool, but I think that technically there is a "more" correct solution to the problem with the velocity of the sphere vs cube cow, if there is no friction in both cases then there is no force to get the sphere cow rolling, just like when you throw a bowling ball it mostly just slides, so in the end they would slide down the ramp with the same speed. I think...

[@bilsanford7659]

great lesson! moment of inertia is always a tough concept and demonstrations are such a great way to supplement the mathematical modeling! excellent work my friend! :)

[@johnwalsh877]

I'm surprised to see all the rotational mechanics described without using vectors! I knew you could do it, but I always found that it starts to get messier to not use them when you start to tackle harder problems. But vectors are also a bit more unintuitive at first, especially when trying to describe rotation in a way that's similar to how people are taught linear motion, and I think you've done a really great job at bridging the two in a way that's fun, easy to digest and clear!

[@sruthiki4050]

I can see your audio, video, presentation-quality improving day by day, Kuddos diana.

[@TallinuTV]

"Spherical cow" said repeatedly with a straight face. XD

[@math_the_why_behind]

Thanks for yet another one of these videos! By the way, where did you get the cows? Thanks :)

[@geraldsnodd]

Thanks Diana ,you made physics fun.

[@VinnyK85]

Sphere vs Cube down a frictionless slope question: if there is no friction then the sphere would not rotate, so would they not both get to the bottom at the same time?

[@JS-lf4sm]

Hi Diana, is that not the Coriolis Acceleration leading to that strange water curve? Thanks!

[@powerprofile69]

I am your recent subscriber and I got addicted to your videos

[@Sarthak.2406]

Sweetest explanation ever😍😍🤩🤩 useful for exams !!

[@fredricprabu7815]

Well this video was amazing . I learnt a lot from this . Angular momentum topic was very interesting for students like me .

[@petero6471]

Just a funny observation. If you line up all the people on earth at the equator each human would only have 5mm space to stand. And most of them would need to know how to swim. But still a very nice idea to use this to explain Angular Momentum. Keep up the good work Dianna and crew.

[@wolvenar]

Not to be picky, but the sphere would not start rotating if there was no friction. This would then mean no difference between the two. Then again, no friction means there is essentially no way to hold back the objects on an incline plane, so the incline plane could not change the direction of the falling objects at all.

[@anirudhkashikar2300]

This is a excellent video 👌. Easy to understand. Thanks.

[@trxpicalsxph5864]

wow i just asked alexa who her favorite youtuber is and she said u! so i decided to check out ur account and its so cool! :D

[@andrewjones6693]

My 2 key takeaways were: 1) The skeleton laying on the floor was distracting! 2) Pulsar = Cosmic Lighthouse! 😊 These Physics lessons are great!

[@Villaboy78]

I like your spin on this topic 😀

[@vickash1072]

just the one video & instantly subscribed

[@charles_wipman]

Very pimp and i'm glad to see Kari Byron again, i was a fan of Mythbusters too and it's a show for everyone that i miss here, in the Spain's TV; happy new year!.

[@robinhooper7702]

You are awesome. Thanks Diane.

[@varunm7011]

I knew the water is going to jump ahead because I have already watched Steve mould's video. Great video btw

[@altair_cepheus]

I am a huge fan of yours! Do you ever do Q & A with your fans? How can I connect in a meaningful way with you, without taking too much of your time?

[@nishthasharma22]

Thank you, Diana!😊😊

[@pmuralimohanrao1000]

Good morning, Please clear the following doubt I am having since 42 years. Sound travels faster in solids than liquids or games but when we close a door why the volume of the sound from outside gets reduced instead of increasing. Second doubt: When a magnetic object is magnetized, north and south poles get separated so there should be a change in shape of the magnetized subject but it is thought that the shape doesn't change

[@caribbeanman3379]

Diana, I have a question: why does a balloon filled with air burst more explosively when punctured, than a balloon filled to the same size with water? My thinking is that the elasticity of the balloon more easily compresses the thinner air (because it's a gas) than the denser water with the result that the gas pressure on the walls of the balloon are stronger than the water pressure of an equally inflated water balloon and that's why the air balloon bursts more explosively. Is that it?

[@HadarCo]

1:02 - "And if you guessed it would jump ahead, idk how you knew" - Well, if you're subscribed long enough.... 😉 And I love it when you ask a tricky question, then we probably all think "Ok so the obvious answer is A. so the real answer must be B. ... But why...? 🤔" - And then the real answer is far cooler then we even thought 🤩 (Although if you learnt physics you know the answer, but it's still so cool to see it gets solved 😉)

[@ThePhysicsMathsWizard]

Nice lesson, well illustrated.

[@ansel_ale]

If we disregard friction in the spherical vs box cow, wouldn't the spherical cow simply fall without spinning? hence both reaching the same speed and finish at the same time, after all what causes the spinning is friction in the first place, unless I'm grossly mistaken.

[@pesveriyans8326]

Hi Dianna, I have doubt in the sphere and cube in ramp puzzle. If the friction is considered zero(0) then there will be no rotation in sphere ryt. Correct me if I'm wrong. Then the answer for this question will be both sphere and cube will reach the bottom at the same time. It is similar to free fall.....

[@prezlamen7906]

Read pinned comments first dude.

[@osmosisjones4912]

Water is great analogy for space or energy flow

[@antonioascone997]

Hi Diana, awesome video, I only have one doubt which is the following: if the plane has no friction, how does the spherical cow start rolling? I thought rolling needed friction to happen

[@emersonpropst2886]

You are correct. She says it quickly, but she mentions the cube cow has no friction, but the spherical cow has some.

[@phuclabik]

Well, Destin from Smarter Everyday has shown this experiment before. But it's fun to watch Dianna explaining it again.

[@GooogleGoglee]

11:30 in the example illustrated it is wrong to suppose that in absence of friction the sphere will start to roll, in fact it doesn't and it will reach the bottom together with the box.

[@RayKosby]

I thought the water pipes turned inward was a demonstration of the coriolis effect. Should I be envisioning the coriolis effect in terms of angular momentum?

[@JoeBob79569]

I think, perhaps, a way to understand the extra energy in the sphere rolling (versus sliding) down the slope would be to tell someone that they need to bring the sphere and the cube to the top of a cliff and then drop them off the edge, straight down. But, the catch is that the sphere must be spinning when it hits the ground. This might not seem like a big deal for the little cow sphere, but if it was a 1 ton sphere then you can bet that the person would remember having to put the extra effort into getting it spun up before dropping it.

[@DanielBenDavid]

Great series Dianna! Is it common to call all electromagnetic waves 'light'?

[@Shihab1979]

Thank you so much Miss Dayna.🌈

[@iampuzzleman282]

Please give me a heart. Dianna, you are why being human is so worthwhile.

[@samanvithagorinta2997]

Hi Dianna , I have a small question for you, (not related to the above video).. When you bring to near boiling temp of frozen milk in stainless steel vessel, it will give you burnt smell but won't if you do the same with milk at room temperature.. Why??

[@zzmark5477]

you made super cool video. I have a couple of questions to which confused me for long. 1) when did the quantity of matter (mass) come into people's mind? 2) how was it figured out ? by who? Newton or someone else? 3) I think there must be a relationship between an area of shape with some newton physics...but i can't figure out what is it....will you pls explain it a bit if you are spare? thank you

[@thrustprop67]

Hello Physics Girl I have a question maybe you would point me in the right direction . When an aircraft propeller spins on a craft called an airboat ( everglades swamp boat ) there is a centrifugal load on the prop blades . If the boat changes direction with the prop spinning and creating this centrifugal load there is a gyroscopic action where the spinning prop resists the change in direction of it's axis . What is that resistance called ??? I really would like to know,,,, please . Thanks Ed

[@PapaFlammy69]

sprinkler boi

[@sreedathpr6539]

Hey there Papa Flammy : )

[@PapaFlammy69]

@@sreedathpr6539 Hey :3

[@loveoflyricism2769]

I’m still searching for the words to describe how much I love Flammable Maths ❤️❤️❤️

[@meowwwww6350]

Papa! ! Guten tag!! I learnt german by taking u as inspiration

[@PapaFlammy69]

@@meowwwww6350

[@myessyallyahamericus8405]

The heat of the sun's atmosphere at mercury's orbital distance pushes it away slightly more than gravity pulls it which accounts for mercury's slightly odd orbit. I figured that one out a couple decades ago. Mercury got caught a little close for its density and hasnt worked into its kush spot just yet. And being the sun's gaining mass at a way faster ratio than mercury is able to keep up with its pushing it at a measurable level.

[@WhiteHenny]

If everyone ran around the equator they would very slightly change the speed of rotation of the earth, but when they stopped running the speed would go back to what it started at (again, due to conversation of angular momentum), right?

[@Micetticat]

It turns out you personally have enough strength to spin the Moon: you did it with Jupiter!

[@himanibhyan7569]

I 'm your very big fan... Thanks for your videos

[@BillMSmith]

I'm enjoying these as a refresher for my long ago physics studies. BTW, you're a much more engaging instructor than Mr. Kendrick. Question however. Having watched all these, why aren't cows a unit? Or should I take that question to the philosophy review?

[@michaelsommers2356]

Because there's no such thing as a standard cow.

[@F.E.Terman]

Loved the video. However I think earth is more like 6x10^24 kg. Also, do pulsars really blink optically, or rather was it just the radiopulse that gave them their name?

[@CarlosM720]

I love this content.

[@goodlight4113]

Regarding slowing down the earth. Does it matter that the people running, and pushing against the ground? Do they counteract their own force? Wouldn't just erecting lots and lots of tall buildings slow down the earth more, like a whole bunch of Space Elevators?

[@geoffreybartlett9293]

Were you being humorous of femorous? What diff equation has a derivative of rdrrr?

[@xmarkxspot293]

So what is the outcome if the angle of the dangle is equal to the heat of the meat?

[@jindrichnovak3830]

@Physics Girl 10:47 If you assume that there is no friction, would the ball spin? Wouldn't it just slide down the ramp as the box, without spinning?

[@emersonpropst2886]

She specifies the cube has no friction, but mentions the ball has friction so that it does spin (presumably a very small amount of friction so the ball spins, but not so much that the friction alone would change the outcome)

[@jindrichnovak3830]

@@emersonpropst2886 Ok thanks

[@RayKosby]

In the demo the water seems to do a 180 and head back outward away from the center in the slowmo. Why is that?

[@panchalmihir3174]

I still didn't get that, ofcourse angular momentum is function of mass distribution over radius, v=rw. Yet what about the direction?

[@timsinnett3401]

I just watched all this, and i still dont understand why the water overtakes the spin....... but i dont do physics.... why am i watching this u might ask... Cos its addictive, Dianna is very enjoyable to listen to, despite having 1,000,000 questions about everything she says, and i like being confused lol. Subbed!

[@shreejithshaji7418]

Do hardwork every day 💛

[@markmd9]

I was very curious to find out what is the relation between angular momentum and that skeleton

[@crazypyrokitty]

Lol and I was thinking why would you expect it to do anything else?! 😆

[@jonperez8569]

What would you see if you face 2 mirrors perfectly at each other?

[@boopathirajak3488]

Just Awesome..!

[@davidcaldwell8977]

So love watching you.

[@glennkrieger]

Ok. Super amazing video with a super amazing host. I just subscribed. There's a interesting caveat to this angular momentum thing and the earth's spin. Occasionally the earth's spin varies. Like, all the time actually. Very, very small changes though. In the year 2020 the earth beat it's own spin time for a day a total of 28 times. That's the most every recorded in a single year since recording started around 50 years ago or so. One more reason 2020 was just... Fill in the blank.

[@xliquidflames]

I knew it would go ahead of the curved tube because I saw Steve Mould's video about the same thing. Then Destin (Smarter Everyday) and Steve did a colab about it and filmed it on a Phantom.

[@danieletodaro9705]

8:33 that is an amazing sphere!!

[@purplealice]

The classic demonstration of angular momentum is a figure skater doing a spin - they pull their arms close to their body to speed up their rotation (which increases the number of points their routine earns)

[@mythkiller5583]

She keeps all the mathematical stuffs very simple and make the topic as practical as she could. I feel like, I am having a crush on her

[@Eclass96]

I know what you mean...

[@certion0523]

Join the line.... Although, I must admit, when Kari appeared near the end, she again made me focussed like the old mythbuster days...😚😚😚

[@mythkiller5583]

@@certion0523 Everywhere I go, I see competitions😐😐

[@mythkiller5583]

@@Eclass96 what?🤔

[@Eclass96]

@@mythkiller5583 I too have a new crush...

[@FutureTerminal]

Please continue this series for physics above the High school level as well