Lec 13 | 8.01 Physics I: Classical Mechanics, Fall 1999

[rolandhensle24]

This is a very "rotten" video. =p

[curtisbadger]

Accidentally came on this video...brain has now melted!!

[wyattorr820]

This video is a favorite on Barbados

[fatherjon908]

at 10:45 damn she looks cute haha

[codyahernek]

hahaha one hour later I still feel stupid.

[thecomanche1]

At  time 26:32 u=mg r (1 - cos theta). where does the 1 in that equation come from? from U = mg r - (r cos theta)???

[wideeyedraven15]

@thecomanche1 because it's: mg( R-Rcostheta), the R is factored out and you get the above equation. It's confusing but listen to him and it gets clearer in this instance. Cheers.



[thecomanche1]

@wideeyedraven15 Yes your right. Got It!, boy I must have been a bit sleepy at the time. Must remember not to post when tired. Thanks for the reply.

[mdgreg]

@jamesmkwan Would the Result Be the Same With a Hollow ball, of the Same D and Mass? Inertia?

[mdgreg]

He is "True to his Charge".

[jadx99]

oooo lecture 13..

[Quiethoof]

I will answer your question here.

The answer is 'diffusion'... and this is the same for human thought process... high consentration to low consentration... the very reason(force) he is teaching, and you are learning(potential)... the very reason for my will in him.

Statement the human man makes at 15:00 is of three dots... the space reserved for you.

[silverlight33]

jamesmkwan@ , ball doesn't rotate because there is no friction.

[misterman567]

woooooow 34:20 !

'scuse me? anything wrong? i dont think so. thank you.'

chill out man

[NilsMcCloud]

@misterman567 He was being VERY, VERY calm. Trust me, I've been around professors with much less patience.

[shemulable]

i was waiting for him to rub the board

[HunterDX77M]

What is the reason he was alluding to at the end of the video with the two arcs?

[abuye2011]

At 23:31, he wrote aformula of x double dot which is written wrongly... it should be

double dot X + K.X/ M=0

Ofcourse he forget it without noticing.....

[marwanxyz123]

@abuye2011 he corrected it at 24:00

[arsenelupin123]

I like how this guy does Hamiltonian and Lagrangian mechanics without ever mentioning the words Hamiltonian and Lagrangian. Refreshing.

[bharathsf]

@arsenelupin123 In which video does he explain this...i want to learn about this

[arsenelupin123]

@bharathsf

As far as I remember, he doesn't do it explicitly in this course. That is probably covered in the second mechanics course.

But I am referring to the part where write the energy of the harmonic oscillator and differentiates it to get the equation of motion, which is one of the main points of Lagrangian and Hamiltonian mechanics.

Maybe you can find another video on this. This all depends on your experience with these things.

[scout6686]

what level pf math is required for this class?

[cstebs]

@scout6686 I'm not sure at MIT however here Physics I requires one year of Calculus just so you can do simple derivatives and integrals.

[LiquidOX2H2]

It's so funny to see how less and less people go to these lectures xD. The professor is amazing !!!

[alf1230castillo]

34:23  haha! "Excuse me? Anything wrong? I don't think so, thank you."

[andreirocks1992]

besides the rotational kinetic energy, the ball is moving much faster so the air drag is more significant

[mridularul1]

hehe many people here are telling that they are 17 and they understand what he is teaching.I'm 14 and i also understand what he is teaching, that lowers the min age limit isn't it =D. BTW Professor Walter Lewin is the best person i've ever seen who teaches with ultimate perfection

[partonfilaton]

@mridularul1 i take that to be except for the calculus...

[sssss3841]

@partonfilaton doesn't have to be the case, i knew calculus when i was 14, btw im only 16 now

[x1x2x3ct]

@sssss3841 I'm 8 and know quantum field theory so gtfo nigger

[colinbarushok]

@x1x2x3ct

I'm 2 and I wrote quantum field theory

[mridularul1]

He looks great in black dress =)

[mhmalik25]

He left a CLIFF HANGER for a physics lecture.. THIS DUDE ROCKS!!!

[untrots]

Yes We Can! :)

[01Rater10]

im 17, i have no idea what this guy is talking about :)

[wittsmith]

I'm 17, I understand physics -.-

[thelastwords]

really?

I'm 17 and this makes complete sense lol

[903mosher]

where does walter teach? i would love to take his physics class

[solidsound901]

@903mosher MIT. Take a look at the video description/channel. Walter Lewin and Gilbert Strang are gods :D!

[radioactive1415]

oh!!! the ball is ROLLING. the car on the air track is TRANSLATING. so there's a huge difference because you can't compare linear kinetic energy to rotational kinetic energy; they are two different classes of mechanical energy.

I love Walter Lewin. He's the best physics teacher ever! =]

[jamesmkwan]

I know, right! The only thing I hate is how he never reveals the answer to the puzzle. I mean, I wish he answered it in like the next lecture or something.

[PatinyaHan]

I love the way he explains. That's cool

[1888junkteam]

excellent work!

[mst7eel]

the osculating ball has an angular velocity which affects the result.

i think that's the reason.

[amrosik]

@mst7eel jep.

[LucasChau]

In the second experiment,r is lower.Hence,theta is very greater.The formula is only work when theta is small.Thus,the prediction show wrong.

[taygr555]

u suck

[jamesmkwan]

The ball rotates. Therefore, it has rotational KE not just the linear KE.

[atreusK]

@jamesmkwan I thought so too but he said there was negligible friction so there's no rotational KE.

[jamesmkwan]

Umm. if there's no friction, it does't mean theres no Rotational KE: KE[rot]=1/2 * I * w^2

[atreusK]

the amount of energy that's rotational is even greater because a sphere's moment of inertia is 4/3 MR^2.

[partonfilaton]

@jamesmkwan thanks

[geoffreyefloyd]

How does he make the dotted lines with his chalk so easily?

[wrprince]

@geoffreyefloyd just angle the chalk into the direction you are drawing. the chalk skips over the board.

[geoffreyefloyd]

@wrprince lol nice...thx

[rumidom]

@geoffreyefloyd

magic , im sure it cant be explained scientifically

[behnamasid]

@geoffreyefloyd If you notice he holds the chalk perpendicular to the board, 

[xsaz321]

big chalk and vibration

[chengyman2009]

@geoffreyefloyd He makes the dotted like by putting the chalk directly on the chalkboard the friction between the board and chalk causes the chalk itself to skid thus creating the dotted line...I tried it ;)

[flyingpatte]

The ball has a moment of inertia that is 2/5*m*r^2. Since the ball is rotating as it oscillates forth and back it gets rotational kinetik energy that is 1/2*I*m*(v/r)^2 = 1/5*(m*v)^2. This is quite a bit of the total energy of the system. That means that the balls v-max is less than it would be without any moment of inertia. Since the mean-speed is less during the oscillations the time T also is larger. (The mechanical energy is though conserved and U-max is the same, if friction ignores)

[vadimgrin]

About the small angel approx : he compares the cosθ and the approx using a calculator. But the calculator does his calculation using the same approx(Taylor series of a cosθ), so there is no wonder his error margin is so small.

[flyingpatte]

If the calculator use the same approx, I guess that it uses many more terms of the Taylor-serie and therefore it gives a value that is extremly close the correct one. What he shows is that it is quite ok to only take the two first terms when you work with small angles. If you work with very small angles it's even ok to only take the first term, which he mentions.

[cgibbard]

It seems hard to tell the exact effect it would have from the data he gives about the ball, but it seems plausible to me that enough of the potential energy gets converted not just into linear kinetic energy to move the ball along the track, but to the angular momentum of the ball itself to skew the result. Making some very rough estimates about the properties of the ball and how fast it is at peak, I get that the energy associated to its angular momentum is ~0.008 joules, about 17% of the ME.

[Gilb604]

what is some of the approaches to mathematical problem solving? like the IMO ones?

[tadple2512]

i believe inertia may be the cause

[facts098]

The case of "The steel ball pendulam "Prof said friction is not the cause !?could it be because the curviture is big so the previous assumption will not hold?Any body have idea please coment!

[physieur]

I think that's because theta in ball bearing = 13 deg is quite larger for the assumption of approximation, but for air track it's about 1.5 deg which gives acceptable results

[1V10ntyPyth0n]

no, thats not the case... (hint: you have to look at the equations, what assumptions are made there...)

[X3QT0R]

Answer to his last "brain teaser" may be that .. some of the energy goes into the "rolling" of that ball.

[WarzSchoolchild]

I wonder what would happen, if one made a pendulum from the front wheel and fork of a bicycle. Remove the tyre, and just wind a lot of steel wire round the rim to make it heavy.

Make sure it's ballanced

First swing the pendulum, allowing the wheel to rotate on it's bearing, then stop the wheel from rotating. In the first case, a horizontal spoke (Perhaps painted) should remain horizontal, but it can't in case 2.

It would be interesting to time both experiments, and see what happens.

[WarzSchoolchild]

It's possible to make 'Trick' ball bearings by electroforming a dual-layer ball, Electrodeposited Silver has a hardness Vickers No. 100 and density 10.50 g/cm^3. Steel is only 7.8 g/cm^3, The trick is to make two balls exactly the same weight and size, but one of them is solid steel, and the other is Silver with a core of carbon. They roll at different speeds. WHY?

[zmansporscheboxster]

Is it density.

[WarzSchoolchild]

Hmmmm? "Let's try a very heavy ball bearing, one made of Platinum or even Gold" It rolls!

[WarzSchoolchild]

Johann Bernoulli in "Acta Eruditorum" June 1696 posed a problem,

"Given two points A and B in a vertical plane, what is the curve traced out by a point acted on only by gravity, which starts at A and reaches B in the shortest time."

Isaac Newton in the hurry of the great recoinage, did not come home till 4PM...very much tired, but did not sleep till he had solved it, which was by 4AM.

"I do not love to be dunned [pestered] and teased by foreigners about mathematical things ... "

[zombierobopirate]

its some sort of cycloid right?

[dsmith1427]

In the inertial reference frame of the World Wide Web, Who is a Foreigner?