8.01x - Lect 20 - Angular Momentum, Torques, Conservation of Angular Momentum

[@AlexLiszt]

Dear professor Lewin, I think you've made an enormous contribution to Science by inspiring thousands of young students and showing the beauty of Physics. I'm studying to become a scientist and your amazing lectures are extremely helpful. Thank you!

[@lecturesbywalterlewin.they9259]

+Alex Zilio Thank you Alex for your kind words. I am delighted that my lectures are "extremely" ,helpful for you.

[@Mukta410]

I completely agree.

[@amankumarjha4530]

@@frankdimeglio8216 in your way

[@dixiegriffinjr.267]

I taught many courses during a 30 year career teaching engineering. Dynamics was my favorite course. I learned something new every time I taught it. I find your lectures very informative. I really appreciate how you corrected your mistakes in your lecture and I am sure your students do too.

[@Mukta410]

Yes, you are right.

[@Carol87828]

cheers from Brazil, I've recommended your lessons in 8.01 to all my classical mechanics classmates, and we have emulated almost all your experiments so far.

[@lecturesbywalterlewin.they9259]

Excellent!

[@mudasirbhutto1904]

Hi

[@themanavthakur]

After filling lakhs of fees, real knowledge is found on KZbin for free.🔥 Respect for sir🙏

[@mathurhp]

Prof. Lewin, you are a blessing. Thank you for making it open source.

[@obayev]

The transition from mundane earthly objects to awe-inspiring neutron stars and supernovas was incredible! Thank you for another great lecture, Professor!

[@lecturesbywalterlewin.they9259]

My pleasure!

[@rijulbarot4973]

This part of mechanics is also considered to be the toughest in the entire JEE syllabus, but Professor Lewin made it crystal clear in my head. Thank you sir!

[@ManyaSingh-hm9og]

How your jee exam went? Can i prefer his lectures for my jee prep?

[@ArdaBatinTank]

Thankkk you so much for recording this lectures. I live in Turkey!! At my university, I can't really get inspiring classes to love Physics. But the last 20 minutes of this lecture had a concern to inspire us about the Physics and the nature. I study Physics and you, sir, are making me inspired on your every lecture!

[@lecturesbywalterlewin.they9259]

You're very welcome!

[@semasamur5619]

Which university do you study

[@thewii552]

Sir, I would just like to thank you for your excellent teaching. I was extremely confused after my physics professor taught (a much worse version of) this lecture, but watching your video cleared up all questions I have. Your detail and clarity are second to none; certainly the best teacher of physics I have ever seen!

[@lecturesbywalterlewin.they9259]

Thank you for your kind words.

[@attilamarta2899]

"you passed the course" i love his lectures :D

[@jacksonschanneljohannsen6478]

"If you have problems with this, you are not alone" A very good sign off for physics students. I remember having trouble in my classes thinking I was completely missing the point. It developes over time and I learned far too late that everyone else were experiencing the same.

[@user-wn1jf7pg6x]

I was about to study fluid mechanics before i’ve been mesmerized by the explanations in this video.

[@bharatiyainnovator3185]

I am a 15 yr old from India ( Bharat ) and preparing for jee. I want to thank you. Your lectures are extremely helpful. Professor, My love for physics started just because of you . 🇮🇳

[@lecturesbywalterlewin.they9259]

Best of luck

[@qcislander]

... and after all these years... a few days ago, Jocelyn Bell finally *was* awarded the Nobel prize she's deserved all this time. As I understand it, she intends to donate the entire cash-award part of her prize. I know nothing of her financial circumstances, but even most well-off people would be unlikely to be so magnanimous with "windfall" wealth. You do know her, so please offer her a *whacking* high-five from me, won't you? :-)

[@lecturesbywalterlewin.they9259]

she did not get the Nobel Prize instead she got a $3.5 million prize which is way more than a Nobel Prize. I have congratulated her!

[@qcislander]

Thank you so much for correcting me, Walter: I won't make that mistake again. She *deserves* the recognition and more all the same, but the Nobel snub still rankles. What amazes me is that (as you and others have described her) she's humble, gracious and generous enough to let all that slide *and* offer up her huge cash prize to support others. Yeah... a woman can be a mensch. :-) :-) :-)

[@prabhaker_zeroscore]

You are really dedicated sir. Sacrifice for science .

[@quanta_reletum6643]

For the love of physics

[@alank4467]

Sir, it was so good of you to mention Jocelyn Bell in your lecture. I recently watched a documentary of her by The New York times, 'Almost Famous'. Even though she did not get any recognition, she is still happy.

[@teamindiasupportgroup3907]

Love you sir. Sir you are a gem for indian students preparing for IIT JEE. This topic specially is very irritating, but you made it very easy. Thanks a lot sir

[@inesabreu2524]

Dear professor, Even if I fail my physics course I just want to show my gratittude to you: I've always hated physics since 7th grade and I can finally find it interesting and enjoyable, as it is! You really have a gift for teaching, every matter is very clear now, thank you so much!

[@mushpi]

class 7 e ami physics er namE jantam na 🙄

[@lakshya2441]

Thankyou Sir, for sharing such a beautiful lecture with us. I live in India, and I am a senior high school student. I have watched many lectures of 8.01x and 8.02x several times over since they all are densely packed with knowledge but they all made my concepts crystal clear.

[@lecturesbywalterlewin.they9259]

You are most welcome

[@thewhitecoat7775]

Dear sir I m from india i wanna say that you are phenomenal u make me love PHYSICS which i hated Thanks a lot loads of respect sir hope u live for million years and make futures of students all around tge world

[@tiktokpurple1]

SOMEONE IN THE COMMENTS SAID that these lectures was delivered in 2002 when I was not in that world but after a long time in 2019 I'm watching it ..and it is also helpful for me but a little bit because 1st reason is that I don't know english too much and 2nd reason is i am studying physics but i am in lower level e.g 11 class and in our course ,there is no too much details ..BUT SIR YOU ARE GOOD TEACHER...APPRECIATION FROM MY BEAUTIFUL COUNTRY PAKISTAN..............

[@robhannum]

been a mechanical engineer for over 25 years.. these are great stuff.

[@nicolem.1028]

19:59 epic voice crack

[@kalpanarms9597]

Hello professor, I sometimes feel all of this so mind boggling! We humans were able to understand stuff which are sooo far away, by just using the laws of physics! It was a really great lecture! Thank you professor! 😊

[@lecturesbywalterlewin.they9259]

You are very welcome

[@ilikekids]

👍🏻

[@pratz_Avyu]

I am from India and I literally love your lectures as they make me feel physics . I am gonna rock my test. I am understanding all the concept....to clear jee advanced its highly recommendable sir to watch your videos Lots of love from India

[@lecturesbywalterlewin.they9259]

Best of luck!

[@UtkarshMishra-n8j]

Can you give me suggestion to which video i should watch in this channel😢😢😢

[@lecturesbywalterlewin.they9259]

Lectures by Walter Lewin. They will make you ♥ Physics. kzbin.info/door/iEHVhv0SBMpP75JbzJShqw 300+ videos. Many of them with high resolution. This channel has all my lectures and talks.

[@AdityaSingh-oh8li]

Lectures by Walter Lewin. They will make you ♥ Physics. sir, the link is down. please check.

[@xnonqme3716]

for those of you curious about the calculation preformed at ~34:22. you need to calculate the potential energy of the star (which is ofcourse spherical) in the 2 cases (R = 7*10^5 km and R = 10 km); that is the work required to bring all the particles of the star from infinity to their respective positions on the sphere.

[@anthonylee2158]

Prof. Lewin's lecture is always the best !

[@AKBARCLASSES]

Dear Sir, Thanks is not enough... Now I can understand that what a beauty in Physics! Love from Kishanganj Bihar India...

[@devanshisharma2447]

thankyou sir- i loved the supernova explosion picture!! THANKYOU SOS SO SO MUCH- YOU ARE AN INSPIRATION TO ME.

[@introvertworld8325]

Dear professor Lewin sir, your are a great concept clearer.

[@bismitaguha7946]

sir, when will 8.04 and 8.05 be uploaded...waiting.. I really started loving physics

[@lecturesbywalterlewin.they9259]

If you have finished 8.01, 8.02 and 8.03 then go to MIT OCW and watch 8.04 and after that 8.05. 8.05 is lectured by Barton Zwieback. He is a very good lecturer, I have attended several of his lectures at MIT.

[@amankumarjha4530]

Sir I am in class 11 from India and I am preparing for jee I really appreciate your work in physics and your ability in teaching I am feeling honoured to get the knowledge of physics I am definitely not so rich to pay the expensive fees here in institutes I will grateful and lucky to attend your lectures thank you sir love from India 🙋

[@nekhillkumar7397]

How much are you scoring in Physics in JEE Advanced Mock Tests per paper of around 66 marks , and which coaching institute?

[@TheSpyFardin]

Best physics teacher ever!!!!! Love you from Bangladesh!🇧🇩💝💝😍😍😍😍😍😍

[@MrRkesh]

still watching these lectures! i love it, I am 32 and everyday learning something new.

[@lecturesbywalterlewin.they9259]

Wonderful!

[@ytuniverselife6857]

It's been an honour to study from you sir , thus I'm obliged to srudy from you for my jee exams

[@berireylmaz971]

İlk defa bir fizik dersinde eğlendiğimi düşündüm.. Thanks for sharing with us.

[@4lex355]

This lectures are pretty cool. I was a math student (working now) and i find myself watching this lectures. Cheers from portugal.

[@lecturesbywalterlewin.they9259]

:)

[@alexs6284]

thankeew soo much love from india

[@markrudis305]

thank you very much for your videos! you make a giant difference in the world.

[@lecturesbywalterlewin.they9259]

thank you Mark!

[@BhanuSharma1993]

now thats i call teaching it his best....best teacher i have seen in my life.

[@lecturesbywalterlewin.they9259]

Bhanu Sharma Thank you Bhanu

[@BhanuSharma1993]

really from dumb to pro .... credit goes to sir walter lewin ... n to ... thnxx for uploading whole course ...good work

[@lecturesbywalterlewin.they9259]

Bhanu Sharma click on "playlist" you view wayyyyyyy more than my course lectures alone!

[@BhanuSharma1993]

yup...i saw ur playlist ...but the problem is the all have japanese title...nd i don't know japanese language...all i know is ...."watashi wa bhanu te imashu"...:)

[@lecturesbywalterlewin.they9259]

all have Japanese titles ???????????? For me all have English titles. Why are your Japanese? This is absurd.

[@sagarsingh32112]

At 2:49 maybe {r(perp.)c} represents the perpendicular distance of point Q from center of mass of M. And suppose the body is in pure translation , then , we do not need to consider rhe rotational Angular momentum.

[@lecturesbywalterlewin.they9259]

if an object is in pure translation then there is angular momentum relative to all points except points on the "straight" line of the movement. Depending on the problem you need to solve this angular momentum can be CRUCIAL and can not be ignored.

[@sanskartiwari2996]

These lectures surely do have a classical feel about them

[@mrkakotube]

Walter, your words about Jocelyn Bell were very nice.

[@arunbhardwaj2744]

sir,,,i am a little confused at 24:45 as you are pulling your arms nearer to the axis of rotation you are accelerating as your angular velocity keeps on increasing and finally reaches its maximum value due to conservation of angular momentum but to provide an angular acceleration there must be some external force acting on the system which is absent here, then how you are accelerating,,, like if we take an example of a bomb blast into two fragments each fragment gets some velocity due to internal forces but if we look at each fragment separately then this internal force is actually external for them which makes their velocity from 0 to a certain value but to the whole system the net force is 0.i,e we can actually visualize what forces are giving them velocities here..........but in this case of increasing angular velocity what is going on? or IT IS NOT NECESSARY TO HAVE A NET EXTERNAL TORQUE ON THE SYSTEM TO PROVIDE IT AN ANGULAR ACCELERATION??

[@carultch]

Given a body that remains rigid that cannot redistribute its mass, a torque is required to change its angular velocity, i.e. for it to have an angular acceleration. For a system that CAN redistribute its mass internally, an external torque is not the only way to give it an angular acceleration. Redistributing the mass, and changing the moment of inertia, will cause its angular velocity to change. There is a torque that acts on a body as it moves radially in a rotating reference frame, and this is a consequence of the Coriolis effect. It is this internal torque that occurs between Professor Lewin and the two barbells, that enables the barbells to apply a torque to speed him up. Try walking along the radial handrail on a spinning merry-go-round, and you will experience this effect. You will feel an apparent force pulling you tangentially forward, when you walk radially inward, and you will feel an apparent force pulling you tangentially backward, when you walk radially outward. To stay at the same position as the handrail, you experience a constraint force as a reaction to these apparent forces that are a consequence of the Coriolis effect. The constraint force (tension or compression in your arms) is what enables the speed of the merry-go-round to change, as you redistriute the mass of the system by moving radially on it.

[@Praveensingh-vj3fg]

Thanks sir, this is extremely useful for me, these videos are treasure for me

[@lecturesbywalterlewin.they9259]

Glad to hear that

[@gauthamumasankar]

Amazing lecture sir.... Thanks for motivating us to love physics. Have you taught general relativity and 8.05 quantum mechanics sir??

[@lecturesbywalterlewin.they9259]

+Gautham Umashankar No I have not taught 8.05 at MIT. It's taught in general by theoreticians. At present you can take the 8.05x course on EdX. It's taught by Professor Barton Zwiebach. He is a great teacher.

[@gauthamumasankar]

+Lectures by Walter Lewin. They will make you ♥ Physics. Thanks a lot sir!!

[@surendrakverma555]

Excellent lecture Sir. Thanks and Regards 🙏🙏🙏🙏🙏🙏🙏🙏🙏

[@lecturesbywalterlewin.they9259]

Most welcome

[@FMTrades]

Sacrifice for the sake of science. Doesn't matter being a finance student, still I love Physics. Respect from INDIA 🇮🇳

[@harrybruijs2614]

In 1054 everyone who could write was possibly more concentrated on the great Schisma in the Christian Church between the Latin Church and the Greek Orthodox Church. It could be an explanation, there are however mentions of a similar occurance but 2 month earlier. I cannot explain this, but the mention in the Chinese literature is from 3 centuries later.

[@SatyamSingh-xb1fw]

awesome lecture sir . god bless you 💖💖💖

[@simranjoharle4220]

I really thank the person who has made this channel......feel so privileged to watch Mr.Walter Lewin's lectures at a click of a button.

[@lecturesbywalterlewin.they9259]

*This channel was created in Febr 2015 by my Dutch friend Daniel Dekkers.* It has become way more popular than "For the Allure of Physics" (created in Dec 2014) which also carries my 94 MIT course lectures + my Farewell Lecture at MIT "For the Love of Physics" of May 16, 2011. That lecture alone has been viewed by more than 6 million people.

[@simranjoharle4220]

Sir, you have always been an inspiration to me and your lectures always motivate me to pursue my dream of getting into research and studying astronomy.....only the fact that you replied makes me feel so special. Believe it or not but this is like a blessing to me. -Regards

[@lemont2005]

Please, Sir! Don`t answer if it´s too complicated for you to explain it ,ok? Why don´t earth (in the orbital moviment ) looses energy ( as an electron would do in its hipotetical orbital movement around the nucleus) and go into an spiral moviment towards the final colision with the sun? Sorry about my English! You are our hero!!!!!

[@lecturesbywalterlewin.they9259]

Yes you got it. In classical physics accelerated charges produce EM waves. That's the result of Maxwell's eqs. However, in Newtonian mechanics masses that are accelerated do not radiate any energy. HOWVER, in GR (Einstein) accelerated masses do radiate energy; in the form of gravitational waves (GW). The acceleration of the Earth in orbit (v^2/R) is so low that it cannot even be measured. However, two black holes in a close binary system radiate a large amount of energy (GW). Thus the orbits shrink and ultimately they wil merge, the black holes will spiral into each other producing a HUGE "last Surge of GW. This was first observed on Sept 14, 2015 with LIGO. LIGO was perceived 40 years ago by my friend Rai Weiss (former colleague of mine at MIT). I predict that he will share the Nobel Prize of Physics this year for that fabulous result. GW were predicted 100 years by Einstein.

[@sumeetsharma7256]

Brilliant Lewin Sir

[@cheesywiz9443]

Thank you so much for sharing all these marvelous lectures with us ! I am currently in high school and binge watching through all your videos . I have one question from this lecture and it would be great if you could help me out with it! In 33:39 , you estimated the amount of energy released as the radius shrinks but mass remains unchanged... i was wondering how I could calculate the amount of energy released? Can I get any clue or relations that I could get started with? Thanks!

[@lecturesbywalterlewin.they9259]

total energy (KE + PE) is conserved. When the star implodes the PE decreases (the smaller R the less PE) thus KE goes up.

[@vishaljain4915]

@@lecturesbywalterlewin.they9259 so to work out the loss in PE, work out the increase in rotational KE?

[@ashwina5472]

sir, does the spin angular momentum remains the same for any reference point but orbital angular momentum does not

[@lecturesbywalterlewin.they9259]

Spin angular momentum of the Earth is an intrinsic property it's omega*I. Orbital angular momentum of the Earth is ONLY conserved relative to the Sun, NOT relative to any other point. If you choose a point somewhere on the orbit, when the Earth is at that point the orbital angular momentum relative to that point is ZERO.

[@VickysTuition]

@ 26:18 For others confused like me, Even though the points are at different distances from Q --> r1 & r2 ... the torques are equal in magnitude since r1sin(theta1) = r2sin(theta2) ... the component of distance perpendicular to direction of force

[@boombokhayat5901]

Thank you

[@amankumarjha4530]

And yes congratulations for 1 million subscribers 👌👌

[@JaiPrakash-bk3uv]

sir I have a question. sir whenever there is angular accelration on particle there is always an torque acting on it. is am I right

[@lecturesbywalterlewin.they9259]

yes, that is correct torque=I*alpha. alpha is dw/dt

[@carultch]

If its rotational inertia remains unchanged (individual rigid body), it requires a net torque to act upon it, in order to have an angular acceleration. An object can have angular acceleration without torque, while changing its rotational inertia. Unlike linear inertia (i.e. mass), that requires changing the identity of the body to change its mass (like a rocket loosing its propellant mass, or farm equipment collecting a harvest), rotational inertia can change just by redistributing the mass. As you can see with the turntable demonstration in this lecture.

[@rambabu100293]

You are great sir....physics is real magic.

[@hongyang1995]

FAST in China announced the first pulsar it detected and that bring me back here. Thank you Prof. Lewis

[@lecturesbywalterlewin.they9259]

SUPERRRRR en.wikipedia.org/wiki/Five_hundred_meter_Aperture_Spherical_Telescope

[@aiquelindo]

@35:25 the supernova explosion *did not* occur in the year 1054, that's when it was observed! It happened 5,000 years before that date or whatever the distance to the Crab Nebula is in light years.

[@sahithnc]

Namaste Sir, (Indian greetings) Please help me!! A thin rod AB of mass M and length L is rotating with angular speed w about vertical axis passing through its end B on a horizontal smooth table. If at some instant the hinge at end B of rod is opened then, can you please explain why the angular momentum of the rod remains conserved about the center of mass of the rod during the whole process? And I have another one sir A cylinder of height h , diameter h/2 and mass M and with a homogeneous mass distribution is placed on a horizontal table. One end of a string running over a pulley is fastened to the top of the cylinder, a body of mass m is hung from the other end and the system is released. Friction is negligible everywhere. Strings and pulleys can be assumed to be light. At what minimum ratio of m/M will the cylinder lift?

[@Peddayana]

My notes : 1.Momentum Intrinsic ,angular momentum is not,it depends on point about which it is calculated .2. Angular momentum about C,representing Centre of circle,is constant,.while momentum is changing,in uniform circular motion . 3.V at every point of disk is different but angular velocity is not. 4.Angular momentum is always constant about any point in space for rotation about center of mass,spin angular momentum is thus intrinsic and unique ,e.g earth rotation about axis. You could then talk about THE Angular momentum.

[@sadhgurusfunniestandwittie3620]

We would love to see you in a conversation with Sadhguru. He has been to MIT twice, as a guest lecturer.

[@jacobvandijk6525]

CAN'T STRESS THE IMPORTANCE OF THE FIRST 17 MINS. OF THIS LECTURE ENOUGH! But, of course, the Earth has no INTRINSIC angular momentum (17:02)! The spinning around its own axis is not intrinsic, but a consequence of the way our solar system is created.

[@juniorstudy3959]

Please make a video on the application of physics in various real life situations 🙏🙏🙏🙏

[@mastipathsala912]

Thank you sir for this lecture you are a great physist in this world

[@lecturesbywalterlewin.they9259]

It's my pleasure

[@mrmurthy4587]

world best teacher

[@mauriciobarda]

Sir excellent class. Thanks again, I have to stop my self from writing excellent class every time I see one of your videos but I will make an exception here. Now I can see a supernova picture and really enjoy it. I agree with the people calling you an inspiration.

[@lecturesbywalterlewin.they9259]

+Mauricio Barda thanks for your kind words

[@biggerbrother7]

I like watching these video. I would like to see an explanation about how gas can collapse on itself since gravity is small force compared to electromagnetic force . There must be something more than F=GmM/r2.

[@lecturesbywalterlewin.they9259]

gas collapse onto itself during core collpase of a star (supernova explosiion) *BECOZ of GRAVITY*

[@bigfigwasp8689]

Hello Walter Lewin! When you were on the turntable you had your angular momentum L conserved. Your rotational KE was L^2/2I, I being your moment of inertia. When you drew in your hands I decreased so your rotational KE must have increased. Where did you get that KE?

[@lecturesbywalterlewin.they9259]

yes it increased. As I pulled my arms in, I had to do work.

[@BroadeningHorizonsos]

The angular momentum of a particle performing uniform circular motion is L. If the kinetic energy of particle is doubled and frequency is halved, then angular momentum becomes???????? I think the answer should be (√2)L but the answer given in L/4 .............. who is correct??

[@lecturesbywalterlewin.they9259]

KE=0.5mv^2, L=mvR, v=omega*R thus L is also m*omega*R^2. KE is also 0.5m*(omega*R)^2. To double the KE and at the same time to reduce omega by a factor of 2, R^2 will have to go up by a factor of 8, thus L will go up by a factor of 4.

[@BroadeningHorizonsos]

thnx

[@asishbauri7433]

Professor lewin u r awesome

[@alexrussev6425]

sir u took moment of inertia of dish has mr^2 but the moment of inertia is 1\2mr^2 ????

[@jonathansum9084]

The moment of inertia of a hoop is Mr^2, and the Cylinder is 1\2mr^2.

[@monroestephenson1997]

Hello Dr. Lewin I absolutely love your lectures they are much more fun than most physics lectures (I don't believe they have your passion). I am doing a research paper on Cold Fusion (not for physics) and was just wondering if you could give me your opinion on whether it is possible or not. Thank you for all you do!

[@lecturesbywalterlewin.they9259]

+Jack Jackson DO NOT waste your time. Cold Fusion is a myth it is NOT possible. This is NOT even a maybe it's absolutely not ossible. The Pons a Fleischmann publications are nonsense. Their results could never be confirmed not even by P and F themselves. www.google.com/#safe=active&q=pons+and+fleischmann

[@monroestephenson1997]

+Lectures by Walter Lewin. They will make you ♥ Physics. Thank you it's just a paper on Conspiracy theories and I thought that I might as well have some fun with "physics"

[@rongjianliang3845]

Dear professor Lewin, I have a question: is the linear momentum also defined to a specified point as the position vector does? As there is no subscript about the linear momentum in the video. Then how to take derivatives of the angular momentum in a different reference frame to get torques, especially not an inertial frame? Will some fictitious forces come out? Thanks a lot!

[@lecturesbywalterlewin.they9259]

watch my 8.01 lecture where I cover linear momentum

[@diwakartiwari9155]

Great one

[@geethag696]

Simply amazing!

[@veys7035]

Sir , I have confused about angular momentum is not beeing intrinsic property while momentum is an intrinsic property. As the angular momentum depends on the point we chose, aren`t the momentum also depends on frame of reference. It seems to me both of them are not intrinsic property.

[@vijayagaisamudre3217]

Sir on your channel you've 8.01, 8.02 and 8.03 and in this video you said about 8.05 on general relativity. Can you tell me how can i study further after finishing till 8.03

[@lecturesbywalterlewin.they9259]

MIT's QM1 (8.04) and QM2 (8.05) are on my channel. Great lecturers

[@vijayagaisamudre3217]

Okay I'll see to it

[@MaheshSharma-dy6sq]

well , professor does black hole is spinning very fast as it''s moment of inertia is negligible , i think?

[@lecturesbywalterlewin.they9259]

physics.stackexchange.com/questions/310881/do-black-holes-have-a-moment-of-inertia

[@lecturesbywalterlewin.they9259]

www.quora.com/Does-inertia-exist-inside-black-holes

[@riteshsinghphysics239]

And we find amazing lecture on angular momentum

[@talentlogic1512]

Absolute legend, Walter Lewin.

[@dean464]

Thanks for the lectures professor, you really make me love physics and I'm in such love, that i was thinking in give up my course to start a physics course!

[@lecturesbywalterlewin.they9259]

:)

[@snipez285]

Hello Prof Lewin, if there is a mouse on the edge of a rotating disk and we know the moment of inertia of the disk about the centre of mass, then we can find the total moment of inertia which includes the moment of inertia of the mouse measured with respect to the CM of disk. Lets say this disk is moving with constant w. Then the mouse moves towards the center of mass of the disk. Then we can use conservation of Angular momentum to get the new angular velocity. But which moment of inertia do we use for when the mouse is in the centre of mass of the disk. Is it just the moment of inertia about the CM of the disk without the mouse on it, or has the mass of the mouse affected the moment of inertia of the disk about the centre of mass? Thank you!

[@lecturesbywalterlewin.they9259]

the answer is in my 8.01 lectures. WATCH them or use google

[@alexandartheserb7861]

36:58 Supernova not recorded in Europe is not mystery if European history between 4-12 century was counterfeited. Star of Bethlehem could be supernova, meaning 1053 years could be stolen from official history later by clergy in Renessance in 15-16 c.

[@khanhtoanlevo1965]

Professor Lewin, I wonder why non-conservative internal force of a rigid system do no work?

[@haupham5086]

I could understand the conservation law of angular momentum when I was in 3rd or 4th grade. I knew that rotating bodies like wheels tend to keep the direction of their axes. Therefore, when I was in 5th grade, I felt confident to learn how to ride a bike because I knew that the bike would not fell as long as the wheels rotated. Is it correct? Is the bike based on the conservation law of angular momentum ? How did you calculate delta U when you shrink the sun to 10km?

[@lecturesbywalterlewin.they9259]

yes, the stability of a bike depends on angular momentum of the wheels. However, that not the only thing. Use google

[@haupham5086]

Lectures by Walter Lewin. They will make you ♥ Physics. Does static friction force matter?

[@haupham5086]

Lectures by Walter Lewin. They will make you ♥ Physics. I think delta U should be negative because W gravity = delta K = -delta U => gravitational potential energy converted to kinetic energy. Did I misunderstand something?

[@sanskarbhargava4852]

sir in your turn table experiment treating your central body and arms as different parts. if we see your central body will have increased angular velocity and angular moment um should then increase how so.is conservation of angular momentum not valid then

[@lecturesbywalterlewin.they9259]

In the absence of external torque, angular momentum is conserved. Watch my "ice skater" demo. I*omega is conserved. If I increase I omega goes down and vice versa.

[@lecturesbywalterlewin.they9259]

kzbin.info/www/bejne/eIHMq4R6fsiEbZI

[@turtle8558]

You know this is going to be hard when there are two corrections in the first 9 minutes

[@lemont2005]

Thanks for your time!

[@lecturesbywalterlewin.they9259]

I sent you a revised version of my earlier answer. Plse read it.

[@AnuragYadav-ic8fl]

hello Sir,as u mentioned a body(scale in this case) when hit at some point other than CM will rotate about CM.So when a body is hinged then it'll rotate about hinged point.My confusion is related to center of percussion(COP).Is it really that at different impact points with respect to (COP) body will try to rotate either clockwise(CW) or anti clock(ACW)?The force on hinge will be either right or left according to supposed CW or ACW rotation?

[@lecturesbywalterlewin.they9259]

I cover all this in my 8.01 lectures

[@StanJan]

Excellent. Thank you

[@lecturesbywalterlewin.they9259]

:)

[@SwordSeraph]

Hi Professor Lewin, At around 6 minutes, you chose the example of the Earth's orbit around the sun to demonstrate a constant angular momentum despite a changing velocity. But isn't the orbit of the Earth around the sun elliptical? If so, wouldn't the position vector also change and lead to a changing angular momentum?

[@lecturesbywalterlewin.they9259]

Yes the orbits are elliptical, YET angular momentum of all planets (with the Sun as origin) is conserved. Torque=dL/dt and Torque is rXF r(vector) and F(vector) are in opposite direction. The sin of 180 degrees is zero. Thus the cross product is zero, thus the torque on a planet (with the sun as origin) is ZERO. Thus L (magnitude and direction relative to the sun) of a planet in orbit cannot change. E.g., compare apogee with perigee. If the the ratio of the distances sun=>planet (apogee/perigee) is 3 (highly elliptical) the speed at perigee is 3 times higher than at apogee, BECAUSE angular momentum is conserved.

[@SwordSeraph]

Alright, let me make sure I understood you correctly. F(vector) here refers to the gravitational attraction between the sun and the planet? (Edited: Mentioned in lecture. Yes it does.). Since Torque = dL/dt = 0, L = constant. If all that is true, I do see that the speed of a planet in an elliptical orbit must change to keep L conserved. But I'm a bit confused because, in the lecture, you said the speed does not change. Are you assuming a circular orbit?

[@lecturesbywalterlewin.they9259]

If I said that the speed is constant I must have talked exclusively about circular orbits. I recommend STRONGLY that you watch this lecture. I discuss your issue in detail. kzbin.info/www/bejne/b6incqBjmdGbqpY

[@SwordSeraph]

Will do. Thanks for answering my questions.

[@lecturesbywalterlewin.they9259]

Do not make the mistake to think that if r is twice at large that v is than twice as small and vice versa. It's the magnitude of the cross product that does not change. However, at apogee and perigee the angle between v and r are 90 degrees. Thus ONLY there is it OK to say that of r at apogee is 3 times larger than r at perigee, that then the speed at apogee is 3 .times smaller than at perigee.

[@debabhai1474]

Sir i love physics so much that i have got tatooed two times e=mc² and another one is integral tatoo on my bicep love you from india.sir

[@nikithkora1945]

Will watching these lectures help me in JEE

[@prakharbhalla9461]

At 34:00 you calculated change in gravitational potential energy. But as much I know gravitational potential energy is due to gravitational pull between two objects. so with which object this gravitional pull to the star is being considered?

[@lecturesbywalterlewin.they9259]

I cannot add to the clarity of my lecture in which I define Grav PE (zero at infinity). --MmG/r (r being the distance of m to the center of M). Watch my lecture again or use google

[@sadhgurusfunniestandwittie3620]

Hi good sir. What is the appx age of these students? Which class is it. Is it 11th standard? Or first year in college?

[@lecturesbywalterlewin.they9259]

1st year college

[@sadhgurusfunniestandwittie3620]

17 years appx age I guess?

[@AyanHussain05]

@@sadhgurusfunniestandwittie3620 Around 19 or 20

[@sinakhorsand6206]

Hello Pr. Lewin... I have a question in this lesson that is Fr that you defined as a gravitational force or attract force from sun to earth in showing of conservation of angular momentum, is equal to centeripital force in the circular motion of earth around the sun?? Thanks a lot!!....

[@lecturesbywalterlewin.they9259]

ONLY orbital ang mom is conserved relative to the sun; It's not conserved relative to any other point. The grav force from the sun on Earth is the centripetal force on Earth in its orbit around the sun.

[@sinakhorsand6206]

Thanks a lot!!!!! Pr.Lewin

[@sinakhorsand6206]

Excuse me Pr.Lewin... I have another question about your answer for my last question... and is that as you say we get center...force equal with gravita....force; so It can be true but I suppose that rotating obj stay without any moving around in that position and hasn't got any rotation; so in that case we have gravity...force for this obj from that another object independent of it's rotation case(mood) so I mean that gravit...force can't be equal with centeri....force.. . ( and I don't suppose that rotating obj ,the earth,because it can't be stop and stay in a certain location to the sun for example...) is my thinking true.....??

[@lecturesbywalterlewin.they9259]

@@sinakhorsand6206 In the case of circular orbits the gravitational attraction by the sun is exactly the centripetal force on a planet. The Earth orbit is close to circular. In the case of highly ellipsoidal orbits, watch my 8.01 lecture on that topic. The stress in a planet generates its own centripetal force for the spin rotation of a planet, that has NOTHING to do with the sun.