Lec 16 | 8.01 Physics I: Classical Mechanics, Fall 1999

[Kricardose]

I am very happy to see the vidoe after you give this Elastic and InelasticCenter of Mass Frame of Reference

[bebeheuy]

I Love The Video Collisions Elastic and Inelastic Center of Mass Frame of Reference It Can Increase My Knowledge

[Mjhond]

Steady I Really Like This Video Collisions Elastic and Inelastic Center of Mass Frame of Reference

[berkamilan]

i need that blackboard and chalks :D and the lecturer knows how to teach! lol need it all

[NeoJamro]

this guy, Walter Lewin, has made me love physics again ^.^ (currently 3rd year physics major tutoring physics and mechanics full time)

[Libservative79]

I love this guy because my professor is so completely incompetent!!

[MrYomantanepali]

where can i download these videos ? I want to download it...

[madiele92]

@MrYomantanepali click on the link in the description, then go on "video lecture" section, now chose a video, below the video there is a download link

[purpleleach1]

You can kind of see that the ping pong ball goes twice as fast as the billiard ball collides into it. The Billiard ball goes some distance after the collision while the pingpong ball goes about twice that distance from the collision.

[MustafaRadiy]

Wow i don't understand anything but well done

[MrJunmatsushima]

wow i literally want to marry all ur videos

[ongteu33]

Nice! I think ima check out all your vids...

[Airickk2021]

too easy, everyone knows that Pie = Delicious . .

[Osman9100]

wall act like is external force. 

[HypnosKilla]

I wonder y i am watching this when I am 13?

[mezoleo4u]

WTF i learn this things at last class at school....in Greece these are a piece of cake for someone who studies some hours a day....

[07ziul]

" You have to massage the algebra a little bit..." Too funny =D

[deluksic]

07:55 thumbs up for WTF?! face!!! hahaha

[xiKUDx]

Im smart. S-m-r-t smart

[schmoogenx]

I believe I will pass AP Physics C now.....

[wwwwhhhhaaaah]

good

[LordIllidan98]

All I remember lol from YouTUbe, is the FBD, right is acceleration, left is friction, north is force or newton, south is gravity which is always 9.8m/s prime2.

Lol

[sickguy123]

you are complicating the problem. The guy already explained it before .

m1 << m2 that is the mass of the ball is much smaller then that of the wall. so using the equations he gives says the velocity of the wall SHOULD be 0 if momentum is conserved.

look at 8.15 onwards

[c155cece]

@sickguy123 wouldn't say what aphididae did is a complication, it is a part of mathematical proof, in both equations for momentum and energy of the wall replace the v2 with 2*m1*v1 / m2, now do a limit for m2-->infinity ... you will find in the first quation m2/m2 and in the equation for energy m2/(m2*m2) ...

[recyclingismylife]

i love how he's always so excited by the results of the experiments. 20:05

[aphididae]

I'm a newbie but... if momentum is conserved and it's like the case that m2 >> m1, then we have v_1 ' = - v_1, so if we use the conservation of momentum:

m1*v1 + 0 = -m1*v1 + m2*v

so 2 * m1*v1 = m2*v

2*m1*v1 / m2 = v

I think why the wall doesn't move is quite clear from this. (or from 23:34, if you prefer)

[MisterAlexUk]

I hate lessons... for some reason I find this guy really watchable.

I don't like the way he said you can destroy energy, though.

[ace666999]

erm this is the sort of stuff im doing in A level mechanics :| is it that advanced ?!

[dakamalu]

No,

This is high school physics.

But he is not bad.

Don't care if it's MIT, it is high school physics. I know. I teach it.

[alwaysdrifting]

"Not bad". He's of the same calibre of Feynman in terms of explaining physics.

In fairness, high school physics only scrapes over Mechanical physics compared to the full course of 801.

[TheOvermaster]

@alwaysdrifting I find Feynman (so far in the lectures) not that great at explaining things. He has a knack for showing great physical insight - and I find the lectures to be absolutely engrossing, but I also find I have to read some things over a lot before I get it - whilst with this lecturer, that simply is not the case. Feynman is good - don't get me wrong, it's just it's quite obvious that only the smartest of the students would have "got" it the first time through.

[MisterAlexUk]

Me too D;

I hate M2.

[ace666999]

LOL I like M2 but its is bare hard!! juat got an E in my mock :D

[facts098]

The momentum of the sys (tennis ball -m1 with v 1velocity and the wall m2 with zero velocity )before collision is m1v1 after collusion m1 v'1(but v1=v'1) so this way momentum conserved. Or shall we think the wall to have velocity of v relative to the ball! It would have been very good explanation is given for the said phenomenaalso By the lecturer Dr Walter Lewin too Our excellent lecturer

[PomyslChwile]

hail to our excelent lecturer!!

[anegligibleperson]

I'm stumped. How can the wall have momentum but no kinetic energy? Could anyone help?

[prabhakar0bharat]

lisn...actually the wall has no momentum..the principle of conservation of momentum states that we can conserve momentum only if the net force on the sys is 0..do u think the net force on the sys is 0?..no it isnt..wat about the forces which r holding the wall intact at its position??..so dere is no conservation of momentum...the normal reaction provides an impulse to the tennis ball and the impulse in return to the wall is beared by it....so no momentum..no kinetic energy..does that make sense?

[summerday44]

well good thiking but the trick is that we are making a wrong mathematical step i am sending youiu a mail to say it

[jellyfish501]

@prabhakar0bharat well, a good idea, u found a hall in this question. but you can always take the system as the whole earth, or even the universe, well than there no ext forces.

But even withe just a big wall.

If the mass of the wall is very very big compared with the ball, the he gets the momentum - that 2mv (m and v of the ball. But the mass of the wall (M) is so big that V goes to zero...

and V goes to zero well V squard goes even stronger. (you can calcute this limit)

and K E goes to zero.

[thwei64]

@prabhakar0bharat wrong conservation of momentum states that momentum is conserved if there is no external force acting on the system. Contact force is an internal force which is similar to colliding balls where there is contact force but since its equal and opposite it is an internal force .

[summerday44]

a wrong mathematical step, that the trick

[susirupines]

momentum is mv. If it has 0 speed, it has 0 momentum. I think he wrote 2mv2 because v2 is equal to zero, it is equivalent ito write 2mv2=0 or 0=0.

[dnp97001]

The elastic problem about the tennis ball and the wall is a limiting problem. Solve it like this. Take the ball to have mass m and the wall to have a finite mass M. Solve for the velocities of the ball and the wall after the collision. Once you do this, take the limit as M goes to infinity. Then the velocity of the ball after the collision becomes -v and the velocity of the wall after the collision goes to zero where +v is the velocity of the ball before the collision.

[dnp97001]

To add to what I wrote before, the momentum of the wall goes to 2mv as the mass of the wall M goes to infinity.