Lec 15 | 8.01 Physics I: Classical Mechanics, Fall 1999

[MrDivyanshugurukul]

@moah2012, what u have heard about energy is very much correct, but energy does not mean only kinetic energy or only potential energy and not even the sum of both. There are various forms of energy. And one form of energy gets converted in some other forms of energy. Hence, kinetic energy or potential energy, for a given system may change, even if no work has been done on it but total energy remains constant.

[moah2012]

he just said kinetic energy is destroyed... all my life I heard "energy can't be created or destroyed" someone explain this to me??!!

[schendu]

5:14 - Yeah Shit just got real!

[nightreaper999]

sucks i dont kno calc yet. we just went over this in ap physics b. cant wait to understand this :D

[ihmlihml]

Tell my why I took AP Physics as a junior in high school...

[ckaiser524]

The camera should stay on the teacher, it's really annoying when i'm trying to read the equations and it pops over to a guy staring at the bored... great teacher tho.

[MrYomantanepali]

the greatest teacher ever... 

[purpleleach1]

I took Honors Physics as a junior and I am taking AP Physics B as a senior. His videos will help me get a 5 on both of the AP Physics C exams and will help me do great on my SAT II Physics exam because he is excellent at explaining and demonstrating physical phenomena.

[Ilsonvp]

I'm from Brazil!!!

Very nice!!!

Gostaria de dizer que Curso Física (bacharel) e gostaria de ter um Professor com a metade (1/2) dessa sabedoria para ensinas.... Muito obrigado!!! estou assistindo todos!!!

Thank you!!!

[KSSC0013]

lost me at momentum

[ultimatestg]

He's dutch i think.

Belgian people have no such accent.

[lesleyhenriquez]

This is 3rd year highschool. I remember this.

His accent, is he South-African or Dutch or Belgian?

[NemmyG]

@lesleyhenriquez Dutch

[MissTrapShooter7]

Preach it brother! XD very helpful

[djmohaamed]

GREAT VIDEO.. LEARNED LOTS FROM THESE SERIES.. THANKS FOR THE UPLOAD.. by the way 0.193 x 2.00 doesnt equal 0.396 but 0.386 xD

keep up the great work :p

[khwezi1986]

I am in South Africa doing PHYSICS 1A at University of Johannesburg and we are doing the same stuff and I understand the work better now Thank you am definately recommending this to my friends

[steven5456]

7:15 Georgie's crab demo ;)

[steven5456]

@steven5456 omg nvm, 47:15 the actual thing!!! lol

[myattassaye]

This is the best physics teacher on the planet

[anakore2]

5:14

OOOHHH MY GOD

XDXDXDXDXd

[partonfilaton]

hey is there a reason that it is too easy? are these the actual lectures for physics students at mit?

[LiquidOX2H2]

@partonfilaton it's not the subject, it;s the professor. And if there's such a professor, it MUST be MIT ;)

[Algerian5462]

@partonfilaton I think you are from Europe where college physics classes at the freshman level are more advanced but here in the US this is the entry level and later thins become more advanced. This is due to that physics is not well taught in high school and in some high schools physics is elective only.

[tomczyce]

@partonfilaton lectures i think are almost always easier than assignments, and sometimes tests.

[TheMethadoneParty]

im glad im not getting tested on this lol im to high to go to school

[Algerian5462]

5) I think the CM in the equilateral triangle example is not shifted to the 2m mass on the height from 2m mass to the left side, but rather in the middle of the height. The calculations are correct but there was a mismatch fixing the CM by geometry and should be exactly in the middle of the height not shifted to the 2m mass, because CM of m and m on the left is 2m positioned in the middle of the left side and CM of 2m and 2m is exactly in the middle of the height

[LaureanoLuna]

If the calculatioms are correct, the height of the CM is 0,22l, which is not half the height of the triangle but a fourth of it.

[Algerian5462]

@LaureanoLuna I submitted my last comment to youtube to finish my calculation explaining that the CM is indeed the middle point on the height( from the left corner to the left side) but my message was not appearing probably I reached the maximum allowed number of comments.

So I am writing it here: The height is h = (sqrt(3)/2)l using pythagore or h = l/cos(30) = (sqrt(3)/2)l

Ycm was found Ycm = (sqrt(3)/8)l

[Algerian5462]

@LaureanoLuna I Continue:

so the length of the segment from the left corner to CM point (which is the hypotenuse in the right triangle having as side Ycm) is: ((sqrt(3)/2)l)/sin(30) = (sqrt(3)/4)l which is exactly half the height of the height h I found above (h = (sqrt(3)/2)l) .

Finally By saying "height" I mean height of the triangle but you talked about the Y component of the CM (Ycm = (sqrt(3)/8)l = 0.22l) which is not the same.

[Algerian5462]

@LaureanoLuna I finish : The Professor said that the CM point is shifted towards the left corner on the "height" which is not true but the CM is exactly in the middle point of the triangle height (Height of a triangle is always defined as the perpendicular from a corner to the opposite side).

The coordinates (Ycm and Xcm) calculation of the CM are correct but geometrically the Professor did not show the CM at the right place on the triangle height.

[Algerian5462]

@LaureanoLuna Sorry I mean the "right corner" not the "left corner" in all what I said above.

[Algerian5462]

3) His example of two colliding objects sticking together after collision should be the last example to give because it is a special example of total inelatsic collision.

4) He should give a visiual example on how elastic and inelastic collisions happen. For example taking two pencils and when colliding them he can roll one pencil on the other to show that friction is a loss of energy(during collision) not transfered to the two pencils at the final state so Ek(before) is larger than Ek(final)

[Algerian5462]

Here are my comments, be a physics teacher myself:

1) The teacher here passed quickly on how the total momentum is conserved if no external forces. He should for example say: Fext=0=dP/dt=(Pafter - Pbefore)/dt so Pafter-Pbefore= 0 so (Pafter=m1v1 + m2v2 =Pbefore=m1u1+m2u2) and this means total P is conserved.

2) Some confusing in vectors, for example he mixed vectors and numbers (components) and he should said we brake the vectorial relation to components or we project it over a chosen axis.

[Mexi1010]

I am in highschool and my teacher certainly knows the material, but he sure can't teach it. These videos are what teach me. Thank you so much!

[TowerCraft]

yay for HQ

[rafonzo]

He's really the best Teacher i've seen so far !

Btw: At 14:50 he said "Kinetegy" instead of "kinetic energy" xD

[johnzammitpace]

Professor Walter Lewin is regarded today as one of the best lecturer, teacher, in Mechanical Physics today. I watch enthusiastically and learn from his lectures with the ears of eagerness, and with a foolscap notebook to take notes down. I recomend his lectures to any enthusiast of this subject, of whatever age and profession. John E. ZAMMITPACE (MALTA) Avant-Garde composer and jazz guitarist.

[92yara]

I don't get why the all the internal forces in a system cancel each other out and so the net force is only the sum of the external forces and thus momentum is conserved. Can someone explain that to me?

[92yara]

nevermind i got it.

[royalsnowman]

Actually, the word "System" is free to choose. Imagine I have mass A and B. If A moves and hit B (rest), A will slow down and B will move. At the "contact time", there are normal forces in between. A "pushes" on B; B "pushes" on A. Those "pushes" are equal in magnitude but opp in directn. If you calculate the initial (total) momentum and the final one, you will get same answer. I hope that answer your qtn :)

[ladysuzaku90]

wow

i wanna go to MIT!!

[GLeNss]

I wanna go their for my masters since Im in college already. Mit accept me or Ill burn your house down :P

[truthasauthority]

hey y do you think KE gets less when the total momentum is the same

[gunshin2]

In real life situations thats often the case, as energy in collisions is lost as heat energy and sound energy, etc

If no KE is lost, then the collision is considered elastic

And offcourse it is impossible for there to be a gain in KE

[Phibins]

gUNSHIN IS RIGHT, the rule of thumb is energy is not conserved in impacts, or collisions. unless its elastic.

[ekid2k]

Just watching to study for the honors physics midterm at USC

[xexejesse]

dang!!! you need a scientific calculator in this thing!!! prrfff ill just buy one...

[tihanu]

this high quality version is very nice. much easier to see the equations!

[cdumeohyeah]

I understood about 60% of it- Im an undergraduate student (3rd year) in Applied Physics at Georgia State University.

[neilbuster]

holy mother of God! It's so fucking cool.

[l0uds0undz]

I couldn't understand shit!

[scientificboysclub]

awesome

!! I wanna join MIT

[jebovasusername]

amazing