8.01x - Lect 20 - Angular Momentum, Torques, Conservation of Angular Momentum
Вставка
- Опубліковано 15 чер 2024
- Angular Momentum - Torques - Conservation of Angular Momentum - Spinning Neutron Stars - Stellar Collapse
Assignments Lecture 19 and 20: freepdfhosting.com/4a64566212.pdf
Solutions Lecture 19 and 20: freepdfhosting.com/972f51a6e8.pdf - Наука та технологія
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!
+Alex Zilio Thank you Alex for your kind words. I am delighted that my lectures are "extremely" ,helpful for you.
I completely agree.
@@frankdimeglio8216 in your way
After filling lakhs of fees, real knowledge is found on UA-cam for free.🔥
Respect for sir🙏
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.
Yes, you are right.
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.
Excellent!
Hi
The transition from mundane earthly objects to awe-inspiring neutron stars and supernovas was incredible! Thank you for another great lecture, Professor!
My pleasure!
Prof. Lewin, you are a blessing. Thank you for making it open source.
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!
How your jee exam went?
Can i prefer his lectures for my jee prep?
"you passed the course" i love his lectures :D
Lectures by Walter Lewin. They will make you ♥ Physics.
ua-cam.com/channels/iEHVhv0SBMpP75JbzJShqw.html
300+ videos. Many of them with high resolution. This channel has all my lectures and talks.
Lectures by Walter Lewin. They will make you ♥ Physics. sir, the link is down. please check.
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!
You're very welcome!
Which university do you study
19:59 epic voice crack
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!
Thank you for your kind words.
I was about to study fluid mechanics before i’ve been mesmerized by the explanations in this video.
Prof. Lewin's lecture is always the best !
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.
thankyou sir- i loved the supernova explosion picture!! THANKYOU SOS SO SO MUCH- YOU ARE AN INSPIRATION TO ME.
You are really dedicated sir.
Sacrifice for science .
For the love of physics
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
been a mechanical engineer for over 25 years.. these are great stuff.
Dear Sir, Thanks is not enough... Now I can understand that what a beauty in Physics! Love from Kishanganj Bihar India...
Dear professor Lewin sir, your are a great concept clearer.
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 . 🇮🇳
Best of luck
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!
class 7 e ami physics er namE jantam na 🙄
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..............
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! 😊
You are very welcome
👍🏻
İlk defa bir fizik dersinde eğlendiğimi düşündüm..
Thanks for sharing with us.
This lectures are pretty cool. I was a math student (working now) and i find myself watching this lectures. Cheers from portugal.
:)
Best physics teacher ever!!!!!
Love you from Bangladesh!🇧🇩💝💝😍😍😍😍😍😍
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
Best of luck!
Walter, your words about Jocelyn Bell were very nice.
And yes congratulations for 1 million subscribers 👌👌
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.
You are most welcome
You are great sir....physics is real magic.
still watching these lectures! i love it, I am 32 and everyday learning something new.
Wonderful!
... 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? :-)
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!
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. :-) :-) :-)
thank you very much for your videos!
you make a giant difference in the world.
thank you Mark!
It's been an honour to study from you sir , thus I'm obliged to srudy from you for my jee exams
awesome lecture sir . god bless you 💖💖💖
Simply amazing!
FAST in China announced the first pulsar it detected and that bring me back here. Thank you Prof. Lewis
SUPERRRRR en.wikipedia.org/wiki/Five_hundred_meter_Aperture_Spherical_Telescope
thankeew soo much
love from india
Brilliant Lewin Sir
And we find amazing lecture on angular momentum
These lectures surely do have a classical feel about them
Thanks sir, this is extremely useful for me, these videos are treasure for me
Glad to hear that
Professor lewin u r awesome
world best teacher
Thanks for your time!
I sent you a revised version of my earlier answer. Plse read it.
Excellent. Thank you
:)
Sacrifice for the sake of science. Doesn't matter being a finance student, still I love Physics. Respect from INDIA 🇮🇳
Thnx a Million for uploading this and all other videos !
Manish Singh You are welcome
Manish Singh you are welcome. This website
"Lectures by Walter Lewin. THey will make you love Physics" is the ONLY complete site. It has all my lectures.
enjoy!
\\/\///////////
+Lectures by Walter Lewin. They will make you ♥ Physics. Would you ever consider doing another complete lecture series? These are so fantastic.
now thats i call teaching it his best....best teacher i have seen in my life.
Bhanu Sharma Thank you Bhanu
really from dumb to pro .... credit goes to sir walter lewin ... n to ... thnxx for uploading whole course ...good work
Bhanu Sharma click on "playlist" you view wayyyyyyy more than my course lectures alone!
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"...:)
all have Japanese titles ????????????
For me all have English titles. Why are your Japanese?
This is absurd.
sir, when will 8.04 and 8.05 be uploaded...waiting..
I really started loving physics
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.
@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.
Best teacher!
:)
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.
*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.
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
Excellent lecture Sir. Thanks and Regards 🙏🙏🙏🙏🙏🙏🙏🙏🙏
Most welcome
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!
total energy (KE + PE) is conserved. When the star implodes the PE decreases (the smaller R the less PE) thus KE goes up.
@@lecturesbywalterlewin.they9259 so to work out the loss in PE, work out the increase in rotational KE?
Thank you sir for this lecture you are a great physist in this world
It's my pleasure
Do u also have a video on some most important common rotational questions involving the concepts
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!
:)
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!
watch my 8.01 lecture where I cover linear momentum
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 🙋
How much are you scoring in Physics in JEE Advanced Mock Tests per paper of around 66 marks , and which coaching institute?
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?
yes it increased. As I pulled my arms in, I had to do work.
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.
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.
You know this is going to be hard when there are two corrections in the first 9 minutes
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?
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!
+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
+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"
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!
the answer is in my 8.01 lectures. WATCH them or use google
Please make a video on the application of physics in various real life situations 🙏🙏🙏🙏
Amazing lecture sir.... Thanks for motivating us to love physics. Have you taught general relativity and 8.05 quantum mechanics sir??
+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.
+Lectures by Walter Lewin. They will make you ♥ Physics. Thanks a lot sir!!
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.
gas collapse onto itself during core collpase of a star (supernova explosiion) *BECOZ of GRAVITY*
Professor Lewin, I wonder why non-conservative internal force of a rigid system do no work?
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?
I cover all this in my 8.01 lectures
thank you a lot
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?
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
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.
sir, does the spin angular momentum remains the same for any reference point but orbital angular momentum does not
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.
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!!!!!
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.
Bless Walter Lewin for editing in his corrections
25:13 Such a good demonstration + made my laugh :D
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.
+Mauricio Barda thanks for your kind words
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
yes, that is correct
torque=I*alpha. alpha is dw/dt
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.
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.
thanx for the lecture .. at time 11;11 in this lecture u said , gravitational force in this direction but graviational froce always act downward??? then how force become zero
+Harish Yadav downward on earth, but toward the larger body in space. I would watch earlier lectures for this (gravity is actually attractive force between all objects
how do we find the angular momentum of an object(rotating about it's centre of mass) about a point other than it's com
If an object of mass m is rotating about its com with angular momentum A and if it is also translating with velocity V, then the angular momentum relative to any point P is A + m*dXV, d is the position vector from P to the com. X indicates cross product.
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.
24:54 I am surprised He is perfectly spinning without feeling dizzy
24:44 what happens to the rotational kinetic energy? I understand when you pull your arms in it increases and that comes from reduction of internal chemical energy used for the work to bring arms in. But when you pull your arms out how is energy conserved?? You lose rotational kinetic energy, but you don't gain any internal energy. Where does the lost kinetic energy go? Who gains that energy?
I understand you could extend the arms almost effortlessly, but still in this case what balances the loss of KE in that closed system?
(from the lab frame there is no centrifugal force)
Good question. To understand what happens to the rotational kinetic energy when his arms extend and his body slows down when rotating, consider a linear motion example. Suppose there is a 1 kg bottle of water on top of a 1 meter table above the floor. You pick it up, and carry it to set it gently on the floor. It starts at rest, and ends at rest, so something must've happened to the 9.8 Joules of potential energy in this example, as that energy cannot be in the bottle's kinetic energy. Your muscles generate thermal energy when they act as brakes for objects in motion, absorbing the work done on them, and dissipating the energy as heat.
That's what happens to his rotational kinetic energy as well, when he extends his arms while rotating on that platform. The kinetic energy becomes work done on his muscles, and when work is done on a person's muscles, heat is dissipated. Since our bodies aren't built for regenerative braking of objects in motion, we also get tired from doing negative work on objects in motion, just as we would get tired from doing positive work to put objects in motion.
How can we determine the direction of angular momentum with just the vectors of velocity and direction? 6:43
professor, is there any change in medium from space to inner part of black hole that cause the speed of light to become 0 there as it is opticaly very dense?
physics.stackexchange.com/questions/26515/what-is-exactly-the-density-of-a-black-hole-and-how-can-it-be-calculated
sir u took moment of inertia of dish has mr^2 but the moment of inertia is 1\2mr^2 ????
The moment of inertia of a hoop is Mr^2, and the Cylinder is 1\2mr^2.
2:41 Respect sir.🙏
Can you please explain why rotations about the center of mass gives the same value for angular momentum irrespective of the point of origin... I've done my homework on Google but it keeps redirecting me towards QM.
use google, simple math
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?
yes, the stability of a bike depends on angular momentum of the wheels. However, that not the only thing. Use google
Lectures by Walter Lewin. They will make you ♥ Physics. Does static friction force matter?
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?
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??
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.
Walter Lewin, Master of the dotted chalk line on the blackboard. : )
sir can you please tell me how you are finding out direction of angular momentum. I thought we must go from the first vector towards the second vector and see in which direction would a screw move if we rotated it in the direction in which we go (first vector to second vector). But everytime you seem to give an opposite direction
angular momentum is r cross p r and p are vectors
I have a question about the right-hand rule, if whatever is in the z-axis is "coming out of the blackboard," does that make it negative, and going into the blackboard positive?
x X y = z that's a righthanded coordinate system. PERIOD it does not matter how you draw it as long as x X y = z.
@@lecturesbywalterlewin.they9259 okay, thank you
How do you get the potential energy change at 34:08? I don't get it.