# Lec 06: Newton’s First, Second, and Third Laws | 8.01 Classical Mechanics, Fall 1999 (Walter Lewin)

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Last time we discussed

that an acceleration is caused by a push or by a pull. Today we will express this

more qualitatively in three laws which

are called Newton’s Laws. The first law really goes back to the first part

of the 17th century. It was Galileo who expressed what he called

the law of inertia and I will read you his law. “A body at rest remains at rest “and a body in motion

continues to move “at constant velocity

along a straight line unless acted upon

by an external force.” And now I will read to you Newton’s own words

in his famous book,Principia. “Every body perseveres

in its state of rest “or of uniform motion

in a right line “unless it is compelled

to change that state by forces impressed upon it.” Now, Newton’s First Law is clearly against

our daily experiences. Things that move don’t move

along a straight line and don’t continue to move, and

the reason is, there’s gravity. And there is another reason. Even if you remove gravity then there is friction,

there’s air drag. And so things will

always come to a halt. But we believe, though, that

in the absence of any forces indeed an object, if it had

a certain velocity would continue along a straight

line forever and ever and ever. Now, this law,

this very fundamental law does not hold

in all reference frames. For instance, it doesn’t hold

in a reference frame which itself is

being accelerated. Imagine that I accelerate

myself right here. Either I jump on my horse,

or I take my bicycle or my motorcycle or my car and you see me being accelerated

in this direction. And you sit there and you say,

“Aha, his velocity is changing. “Therefore, according

to the First Law, there must be a force on him.” And you say, “Hey, there,

do you feel that force?” And I said, “Yeah, I do! “I really feel that,

I feel someone’s pushing me.” Consistent with the first law. Perfect, the First Law works

for you. Now I’m here. I’m being accelerated

in this direction and you all come towards me being accelerated

in this direction. I say, “Aha, the First Law

should work so these people

should feel a push.” I say, “Hey, there! Do you feel the push?” And you say, “I feel nothing. There is no push,

there is no pull.” Therefore, the First Law doesn’t

work from my frame of reference if I’m being accelerated

towards you. So now comes the question,

when does the First Law work? Well, the First Law works

when the frame of reference is what we call an “inertial”

frame of reference. And an inertial frame

of reference would then be a frame in which there are

no accelerations of any kind. Is that possible? Is 26.100…

is this lecture hall an inertial reference frame? For one, the earth rotates

about its own axis and 26.100 goes with it. That gives you

a centripetal acceleration. Number two, the earth

goes around the sun. That gives it

a centripetal acceleration including the earth, including

you, including 26.100. The sun goes around the Milky

Way, and you can go on and on. So clearly 26.100 is not

an inertial reference frame. We can try to make an estimate on how large

these accelerations are that we experience

here in 26.100 and let’s start with the one that is due

to the earth’s rotation. So here’s the earth… rotating

with angular velocity omega and here is the equator, and

the earth has a certain radius. The radius of the earth…

this is the symbol for earth. Now, I know that 26.100 is here but let’s just take the worst

case that you’re on the equator. You’re… (no audio ) You go around like this

and in order to do that you need a centripetal

acceleration, a c which, as we have seen last

time, equals omega squared R. How large is that one? Well, the period of rotation

for the earth is 24 hours times 3,600 seconds so omega equals two pi

divided by 24 times 3,600 and that would then be

in radians per second. And so you can calculate now

what omega squared R earth is if you know that the radius

of the earth is about 6,400 kilometers. Make sure you convert this

to meters, of course. And you will find, then that the centripetal

acceleration at the equator which is the worst case–

it’s less here– is 0.034 meters

per second squared. And this is way, way less–

this is 300 times smaller than the gravitational

acceleration that you experience

here on Earth. And if we take the motion

of the earth around the sun then it is an additional factor

of five times lower. In other words,

these accelerations even though they’re real

and they can be measured easily with today’s

high-tech instrumentation– they are much, much lower

than what we are used to which is

the gravitational acceleration. And therefore,

in spite of these accelerations we will accept this hall as a reasonably good

inertial frame of reference in which the First Law

then should hold. Can Newton’s Law be proven? The answer is no, because

it’s impossible to be sure that your reference frame

is without any accelerations. Do we believe in this? Yes, we do. We believe in it

since it is consistent within the uncertainty

of the measurements with all experiments

that have been done. Now we come to the Second Law,

Newton’s Second Law. I have a spring… Forget gravity for now– you can do this somewhere

in outer space. This is the relaxed length

of the spring and I extend the spring. I extend it over a certain

amount, a certain distance– unimportant how much. And I know that I when I do that

that there will be a pull– non-negotiable. I put a mass, m1, here,

and I measure the acceleration that this pull causes

on this mass immediately after I release it. I can measure that. So I measure

an acceleration, a1. Now I replace this object

by mass m2 but the extension is the same,

so the pull must be same. The spring doesn’t know what the

mass is at the other end, right? So the pull is the same. I put m2 there, different mass and I measure

the new acceleration, a2. It is now an experimental fact

that m1 a1 equals m2 a2. And this product, ma,

we call the force. That is our definition

of force. So the same pull

on a ten times larger mass would give a ten times

lower acceleration. The Second Law

I will read to you: “A force action on a body

gives it an acceleration which is in the direction

of the force…” That’s also important– the acceleration is in

the direction of the force. “And has a magnitude

given by ma.” ma is the magnitude and the direction is

the direction of the force. And so now we will write

this in all glorious detail. This is the Second Law by Newton perhaps the most important law

in all of physics but certainly in all of 801: F equals ma. The units of this force are kilograms times meters

per second squared. In honor of the great man,

we call that “one newton.” Like the First Law,

the Second Law only holds in inertial reference frames. Can the Second Law be proven? No. Do we believe in it? Yes. Why do we believe in it? Because all experiments

and all measurements within the uncertainty

of the measurements are in agreement

with the Second Law. Now you may object

and you may say “This is strange,

what you’ve been doing. “How can you

ever determine a mass “if there is no force somewhere? “Because if you want

to determine the mass “maybe you put it on a scale, “and when you put it on a scale

to determine the mass “you made use

of gravitational force “so isn’t that some kind

of a circular argument that you’re using?” And your answer is “No.” I can be somewhere

in outer space where there is no gravity. I have two pieces of cheese;

they are identical in size. This is cheese without holes,

by the way. They are identical in size. The sum of the two has

double the mass of one. Mass is determined

by how many molecules– how many atoms I have. I don’t need gravity to have

a relative scale of masses so I can determine the relative

scale of these masses without ever using the force. So this is a very legitimate way of checking up

on the Second Law. Since all objects in

this lecture hall and the earth fall with the constant

acceleration, which is g we can write down

that the gravitational force would be m times

this acceleration, g. Normally I write an “a” for it,

but I make an exception now because gravity, I call it

“gravitational force.” And so you see

that the gravitational force due to the earth

on a particular mass is linearly proportional

with the mass. If the mass becomes

ten times larger then the force due to gravity

goes up by a factor of ten. Suppose I have here

this softball in my hands. In the reference frame… 26.100 we will accept to be

an inertial reference frame. It’s not being accelerated

in our reference frame. That means the force on it

must be zero. So here is that ball. And we know if it has mass, m– which in this case

is about half a kilogram– that there must be

a force here, mg which is about five newtons,

or half a kilogram. But the net force is zero. Therefore it is very clear that I, Walter Lewin,

must push up with a force from my hand onto the ball,

which is about the same… which is exactly the same,

five newtons. Only now is

there no acceleration so I can write down

that force of Walter Lewin plus the force of gravity

equals zero. Because it’s

a one-dimensional problem you could say that the force

of Walter Lewin equals minus mg. F equals ma. Notice that there is

no statement made on velocity or speed. As long as you know f

and as long as you know m a is uniquely specified. No information is needed

on the speed. So that would mean,

if we take gravity and an object was falling down

with five meters per second that the law would hold. If it would fall down

with 5,000 meters per second it would also hold. Will it always hold? No. Once your speed approaches

the speed of light then Newtonian mechanics

no longer works. Then you have to use Einstein’s

theory of special relativity. So this is only valid

as long as we have speeds that are substantially smaller,

say, than the speed of light. Now we come

to Newton’s Third Law: “If one object exerts

a force on another “the other exerts the same force in opposite direction

on the one.” I’ll read it again. “If one object exerts

a force on another “the other exerts the same force in opposite direction

on the one.” And I normally summarize that

as follows, the Third Law as “Action equals

minus reaction.” And the minus sign indicates,

then, that it opposes so you sit on your seats and you are pulled down on

your seats because of gravity and the seats will push back

on you with the same force. Action equals minus reaction. I held the baseball in my hand. The baseball pushes on my hand

with a certain force. I push on the baseball

with the same force. I push against the wall

with a certain force. The wall pushes back

in the opposite direction with exactly the same force. The Third Law always holds. Whether the objects

are moving or accelerated makes no difference. All moments in time, the force– we call it actually the “contact

force” between two objects– one on the other is always

the same as the other on one but in the opposite direction. Let us work out

a very simple example. We have an object

which has a mass, m1. We have object number one

and m1 is five kilograms. And here, attached to it,

is an object two and m2 equals 15 kilograms. There is a force and the force is coming in

from this direction. This is the force– and the magnitude

of the force is 20 newtons. What is the acceleration

of this system? F equals ma. Clearly the mass is

the sum of the two– this force acts on both– so we get m1 plus m2 times a. This is 20, this is 20 so a equals

one meters per second squared in the same direction as f. So the whole system

is being accelerated with one meters

per second squared. Now watch me closely. Now I single out this object– here it is… object number two. Object number one, while

this acceleration takes place must be pushing

on object number two. Otherwise object number two

could never be accelerated. I call that force f12 the force

that one exerts on two. I know that number two

has an acceleration of one. That’s a given already. So here comes f equals ma. f12 equals m2 times a. We know a is one,

we know m2 is 15 so we see that the magnitude

of the force 12 is 15 newtons. This force is 15. Now I’m going to isolate

number one out. Here is number one. Number one experiences

this force, f, which was the 20 and it must experience

a contact force from number two. Somehow, number two must

be pushing on number one if one is pushing on number two. And I call that force “f21.” I know that number one

is being accelerated and I know the magnitude is

one meter per second squared. That’s non-negotiable, and so we have that f,

this one, plus f21 must be m1 times a. This is one, this is five,

this is 20 and so this one, you can

already see, is minus 15. F21 is in this direction and the magnitude is exactly

the same as f12. So you see? One is pushing on two with

15 newtons in this direction. Two is pushing back on one

with 15 newtons and the whole system

is being accelerated with one meter

per second squared. Now, in these two examples– the one whereby I had

the baseball on my hand– you saw that it was consistent

with the Third Law. In this example, you also see that it’s consistent

with the Third Law. The contact force

from one on the other is the same as

from the other on one but in opposite signs. Is this a proof? No. Can the Third Law be proven? No. Do we believe in it? Yes. Why do we believe in it? Because all measurements,

all experiments within the uncertainties are

consistent with the Third Law. Action equals minus reaction. It is something

that you experience every day. I remember I had

a garden hose on the lawn and I would open the faucet and the garden hose would start

to snake backwards. Why? Water squirts out. The garden hose pushes onto

the water in this direction. The water pushes back onto the

garden hose and it snakes back. Action equals minus reaction. You take a balloon. You take a balloon

and you blow up the balloon and you let the air out. The balloon pushes onto the air. The air must push

onto the balloon. And therefore,

when you let it go the balloon will go

in this direction which is the basic idea

behind the rocket. (huffing and puffing ) I love to play with balloons,

don’t you? So, if I do it like this,

and I let it go the air will come out

in this direction and so then it means the balloon is pushing on the air

in this direction. the air must be pushing on

the balloon in this direction. There it goes. (whistles ) It didn’t make it to the moon but you saw the idea

of a rocket. Action equals minus reaction. If you fire a gun, the gun

exerts a force on the bullet the bullet exerts

an equal force on the gun which is called the recoil. You feel that in your hands

and your shoulder. I have here a marvelous device which is a beautiful example of

“action equals minus reaction.” I show you from above

what it looks like. You’ll see more details later. This rotates about this axis

rather freely– the axis is vertical– and we have here a reservoir

of water, which we will heat up. It turns into steam and these are hollow tubes

and the steam will squirt out. And so when the steam squirts

out in this direction the tube exerts a force

on the steam in this direction so the steam exerts an equal

force in the opposite direction and so the thing will start

to rotate like this. And I would like

to demonstrate that. You can see it now there. With a little bit of luck,

there you see it. So we’re going to heat it. (torch hissing ) Walking. When you walk,

you push against the floor. The floor pushes back at you and if the floor

wouldn’t push back at you you couldn’t even walk,

you couldn’t go forwards. If you walk on ice,

very slippery– you can’t go anywhere, because

you can’t push on the ice so the ice won’t push back

on you. That’s another example

where you see action equals minus reaction. This engine is called

“Hero’s engine.” Hero, according

to the Greek legend was a priestess of Aphrodite. Let’s first look at it. She was a priestess of Aphrodite

and her lover, Leander would swim across the Hellespont

every night to be with her. And then one night

the poor guy drowned and Hero threw herself

into the sea. Very romantic thing to do but, of course, also

not a very smart thing to do. On the other hand, it must

have been a smart lady if she invented,

really, this engine. Yesterday, I looked

at the Web, “ask.com.” It’s wonderful–

you can ask any question. You can say, “How old am I?” Now, you may not get

the right answer but you can ask any question. And I typed in, “Hero’s engine.” And out popped a very nice high-

tech version of Hero’s engine. A soda can– you pop four holes

in the soda can at the bottom. So here’s your soda can. You pop four holes in here,

but when you put a nail in there you bend every time

the nail to the same side so the holes are slanted. You put it in water you lift it out of water

and you have a Hero’s engine. And I made it for you–

it took me only five minutes. I went to one of MIT’s machines,

got myself a soda put the holes in it,

and here it is. It’s in the water there. When I lift it out,

you will see the water squirts. There it goes. High-tech version

of Hero’s engine. Also makes a bit of a mess,

but okay. All right. Try to make one– it’s fun

and it’s very quick. It doesn’t take

much time at all. There are some bizarre

consequences of these laws. Imagine that an object

is falling towards the earth. An apple is falling

towards the earth from a height, say, of,

hmm, I’d say 100 meters. And let’s calculate

how long it takes for this apple to hit the earth which should for you

be trivial, of course. So here’s the earth… and the mass of the earth is about 6 times 10

to the 24 kilograms. And here at a distance, h– for which we will take

100 meters– is this apple, m, which, say,

has a mass of half a kilogram. There’s a force

from the earth onto the apple and this is that force. And the magnitude of that force

is mg and that is 5 newton. I make g ten and

just round it off a little. Now, how long does it take

this object to hit the earth? So, we know that

1/2 gt squared equals h. It doesn’t start with any

initial speed, so that is 100. G is 10, this is 5,

so t squared is 20. So t is about 4½ seconds. So after 4½ seconds, it hits

the earth– so far, so good. But now, according

to the Third Law the earth must experience exactly the same force

as the apple does but in opposite direction. So therefore the earth will

experience this same force, f– 5 newton, in this direction. What is the earth going to do? Well, the earth is going to fall

towards the apple– f equals ma. So the force on the earth

is the mass of the earth times the acceleration

of the earth. The force, we know, is 5. We know the mass,

6 times 10 to the 24 so the acceleration will be 5

divided by 6 times 10 to the 24 which is about 8 times 10 to the minus 25 meters

per second squared. How long will the earth fall? Well, the earth will fall

roughly 4½ seconds before they collide. How far does the earth move

in the 4½ seconds? Well, it moves

one-half a earth t squared. That’s the distance

that it moves. We know a and we know t squared,

which is 20. One-half times 20 is 10 so that means this distance

becomes that number times 10. It’s about 8 times 10

to the minus 24 meters. The earth moves 8 times 10

to the minus 24 meters. That, of course, is

impossible to measure. But just imagine what

a wonderful concept this is! When this ball falls back to me the earth and you and I and MIT

are falling towards the ball. Every time that the ball

comes down we’re falling towards the ball. Imagine the power I have

over you and over the earth! But you may want

to think about this– if I throw the ball up,

going to be away from the earth I’ll bet you anything that the earth will also

go away from the ball. So as I do this,

casually playing– believe me, man,

what a glorious feeling it is– earth is going down, earth

is coming towards the ball. The earth is going down

and I’m part of the earth and I’m shaking this earth

up and down by simply playing

with this ball. That is the consequence

of Newton’s Third Law even though the amount

by which the earth moves is, of course,

too small to be measured. I now want to work out with you

a rather detailed example of something in which we combine

what we have learned today– a down-to-earth problem– the kind of a problem

that you might see on an exam

or on an assignment. We hang an object on two strings and one string makes an angle

of 60 degrees with the vertical and the other makes an angle

of 45 degrees with the vertical. So this is the one

that makes an angle… oh, 60 degrees with the horizon,

30 degrees with the vertical and this one, 45 degrees. Let’s assume that the strings

have negligible mass. So they are attached here

to the ceiling and I hang here an object, m. Well, if there’s an object m for sure there will be

a force mg, gravitational force. This object is hanging there,

it’s not being accelerated so the net acceleration

must be zero. And so one string must

be pulling in this direction and the other string must

be pulling in this direction so that the net force

on the system is zero. Let’s call this pull,

for now, “T1.” We’ll call that the tension

in the string and we call the tension

in this string “T2.” And the question now is how

large is T1 and how large is T2? There are various ways

you can do this. One way that always works–

pretty safe– you call this the x direction. You may choose which direction

you call “plus.” I call this plus,

I call this negative. And you could call this

the y direction and you may call this plus

and this negative. I know, from Newton’s

Second Law– F equals ma– that there is no acceleration,

so this must be zero so the sum of all forces

on that mass must be zero. These three forces must eat

each other up, so to speak.

Well, if that’s the case,

then the sum of all forces in the x direction

must also be zero because there’s no acceleration

in the x direction and the sum of all forces

in the y direction must be zero. And so I am going

to decompose them– something we have done before. I am going to decompose

the forces into an x and

into a y direction. So here comes

the x component of T1 and its magnitude is T1

times the cosine of 60 degrees. Now I want to know

what this one is. This one is T1 times

the sine of 60 degrees. This projection, T2,

cosign 45 degrees and the y component,

T2 times the sine of 45 degrees. So we go into the x direction. In the x direction

I have T1 cosign 60 degrees minus T2 cosign 45 degrees

equals zero– that’s one equation. The cosine of 60 degrees

is one-half and the cosine of 45 degrees

is one-half square root two. Now I go to the y direction. This is plus, this is minus,

so we get one component here which is T1 times

the sine of 60 degrees plus T2 times the sine

of 45 degreesminus mg. It’s in the opposite direction–

must be zero. That’s my second equation. The sine of 60 degrees equals

one-half the square root three and the sine of 45 degrees is the same as the cosine

one-half square root two. Notice I have two equations

with two unknowns. If you tell me what m is I should be able to solve

for T1 and for T2. In fact, if we add them up it’s going to be very easy

because we lose this because we have both

one-half square root two. And so you see immediately here

that one-half times T1 plus one-half square root three

times T1 equals mg and so you find that

the tension 1 equals two mg divided by one

plus the square root of three. I can go back now

to this equation– T1 times one-half equals T2 times one-half

square root of two. I lose my half and so T2 equals T1 divided

by the square root of two. So the bottom line is,

you tell me what m is I’ll tell you what T1 is

and I’ll tell you what T2 is. Suppose we take a mass

of four kilograms– m equals four kilograms,

so mg is about 40 if we make g ten for simplicity. Then T1, if you put in

the numbers, is about 29.3 and T2… 29.3 newtons and T2 is about 20.7 newtons,

I believe. It’s very difficult to rig

this up as an experiment but I’ve tried that. I’ll show you in a minute. I want you to know

that there is another method which is perhaps

even more elegant and which you may consider in which there is

no decomposition in the two directions. Here is mg– that’s a given. And we know that the other

directions are also given– this angle of 30 degrees here

and this angle of 45 degrees. If these two forces

must cancel out this one why don’t I flip this one over? Here it comes. I flip it over. There it is. T1 and T2 now, together,

must add up to this one. Then the problem is solved,

then the net force is zero. Well, that’s easy– I do this. And now I have constructed a complete fair construction

of T1 and of T2. No physics anymore now,

it’s all over. You know this angle here, 45

degrees, so this is 45 degrees. This is 30, this is 30. You know all the angles and

you know this magnitude is mg so it’s a high school problem. You have a triangle with

all the angles and one side; you can calculate

the other sides and you should find exactly

the same answer, of course. We made an attempt to rig it up. How do we measure tension? Well, we put in these lines,

scales, tension meters and that is problematic,

believe me. We put in here a tension meter,

we put in here a tension meter and the bottom one, we hang on a

string with a tension meter and then here we put

four kilograms. These scales are not masses. That’s already problematic. The scales are

not very accurate so we may not even come close

to these numbers. For sure,

if I put four kilograms here then I would like this one

to read 40 newtons or somewhere

in that neighborhood depending on

how accurate my meters are. These are springs,

and the springs extend and when the springs extend,

you see a handle… a hand go. You can clearly see

how that works because if there is a force

on that bottom scale in this direction, which is mg,

and it’s not being accelerated then the string

must pull upwards and so… in order to make

the net force zero. And if you have a pull down here

and you have a pull up here and you have in here a spring then you see you have

a way of measuring that force. We often do that– we measure with springs

the tension in strings. For whatever it’s worth, I will

show you what we rigged up. Now a measurement without

knowledge of uncertainties is meaningless–

I told you that. So maybe this is meaningless,

what I am going to do now. Let me do something meaningless

for once. And remember, when I show it,

you can always close your eyes so that you haven’t seen it. So we have here something

that approaches this 60 degrees and this approaches

the 45 degrees and we’re going to hang

four kilograms at the bottom. There it is, and here it is. All right, this one–

it’s not too far from 40. It’s not an embarrassment. This one is not too far

from 20.7. This one is a bit

on the low side. Maybe I can push it up a little. I think that’s close to 30;

it’s not bad. So you see, it’s very difficult

to get these angles right but it’s not too far off. So let’s remove this again because this will block

your view. These scales were calibrated

in newtons, as you could see. Now we come

to something very delicate. Now I need your alertness

and I need your help. I have a block–

you see it there– and that block weighs

two kilograms. A red block. So here it is. It’s red. And I have two strings. It’s hanging from a black string

here and a black string there. Ignore that red string,

that is just a safety. But it’s avery thin thread

here and here. And they are as close

as we can make them the same. They come from the same batch. This one has a mass

of two kilograms and this string has no mass. This is two kilograms. So what will be the tension

in the upper string which is string number one? This is string number two. Well, this string must be able

to carry this two kilograms so the tension has to be

20 newtons. So you will find here

the tension– call it T1– which is about 20 newtons. So it’s pulling

up on this object. It’s also pulling down

from the ceiling, by the way. Think about it, it’s pulling

from the ceiling. The tension is here, 20 newtons. We could put in here

one of these scales and you would see

approximately 20 newtons. What is the tension here? Well, the tension here

is very close to zero. There’s nothing hanging on it

and the string has no weight so there’s no tension there–

you can see that. Now I am going to pull on here and I’m going to increase

the tension on the bottom one until one of the two breaks. So this tension goes up and up and therefore, since this object

is not being accelerated– we’re going to get a force

down now on this object– this tension must increase,

right? You see that? If I have a force on this one… so there’s a force here,

and there is mg then, of course, this string

must now be mg plus this force.

So the tension will go up here

and the tension will go up here. The strings are as identical

as they can be. Which of the strings

will break first? What do you think? LEWIN:

Excuse me? (student answers

unintelligibly ) I can’t hear you. STUDENT:

The one on top. LEWIN:

The one on top. Who is in favor

of the one on top? Who says no, the bottom one? (Student answers

unintelligibly ) LEWIN:

Who says they won’t break

at all? Okay, let’s take a look at it. The one on top–

that’s the most likely, right? Three, two, one, zero. The bottom one broke. My goodness. Newton’s Second Law is at stake. Newton’s Third Law is at stake. The whole world is at stake! Something is not working. I increased tension here,

this one didn’t break. This one’s stronger, perhaps. No, I don’t cheat on you;

I’m not a magician. I want to teach you physics. Did we overlook something? You know,

I’ll give you a second chance. We’ll do it again. Let’s have another vote. So I’ll give you a chance

to change your minds. It’s nothing wrong in life,

changing your mind. It’s one of the greatest things

that you can do. What do you think

will happen now? Who is in favor still

of the top one? Seeing is believing. You still insist on the top one? Who is now in favor

of the bottom one? Ah, many of you

got converted, right? Okay, there we go. Three, two, one, zero. The top one broke. So some of you were right. Now I’m getting so confused. I can’t believe it anymore. First we argued

that the top one should break but it didn’t–

the bottom one broke. Then we had another vote

and then the top one broke. Is someone pulling our leg? I suggest we do it

one more time. I suggest we do it one more time and whatever’s going to happen,

that’s the winner. If the top one breaks,

that’s the winner. If the bottom one breaks, well,

then, we have to accept that. But I want you to vote again. I want you to vote again

on this decisive measurement whether the top one will break

first or the bottom one? Who is in favor of the top one? Many of you are scared, right? You’re notvoting anymore! (class laughs ) LEWIN:

I can tell, you’re not voting. Who is in favor

of the bottom one? Only ten people are voting. (class laughs ) LEWIN:

Let’s do this

in an undemocratic way. You may decide–

what’s your name? Alicia? Georgia, close enough. (laughter ) You may decide

whether the top one or the bottom one will break. Isn’t that great?

Doesn’t it give you

a fantastic amount of power? The bottom one. The bottom one. You ready? Three, two, one, zero. The bottom one broke. You were right. You will pass this course. Thank you,

and see you Wednesday. By the way, think about this,

think about this.

Abdul ABPost authorthe director is the worst, why the fuck he is filming the students!! I cannot concentrate.

IamdynamitePost authorcan someone explain the last experiment? it was hard to see with the bad quality of the video. The 2kg block is hanging by the top string, how does the bottom string get involved?

Louie McConnellPost authorare there any problem sets/exams?

qualquanPost authorProfessor Lewin isn't the 3rd Law of Newton (action = reaction) just conservation of momentum and not a new law at all? I see no difference.

follow the lightPost authorI've never understood this. If a particle A exerts a force on particle B and it's equal and opposite. Say, both these particles had significantly different masses. Then the force exerted by a on b pushes the particle b in the direction of the force right? But then particle A remains stationary and if particle B is exerting the same force as A just in the opposite direction then why does particle A not move. I can understand that there is difference of mass and stuff. But mathematically it doesn't hold bc particle a remains stationary and has an accelatrion of 0 which makes the force acting on it 0. where as the equal and opposite force is not 0?!?!?!?! Someone put me out of my misery, ive not understood this since 6th grade!

Seineele MoagiPost authorlmao,akward ending

Ilic SorrentinoPost authorhe is the best. the physics teacher I've always wanted having at school. a pity for the MIT case. I discovered him a little bit late in my life but he really improved my knowledge.

Nikhil BankaPost authorPhysics.Yes I'm in love with it <3

Ksuy APost authori was this was HD Walter Lewin's <3

Alex De La TorrePost authorWouldn't F1on2 be 5N? because the mass is 5kg and the acceleration is 1m/s^2?

MrDJRObert1234Post authorwait, can someone explain to me the balloon example. When i release the balloon the air inside pushes outward so the reaction is in the opposite way which makes it go forward. But why is it going forward? isnt it supposed to stay still? since action equals minus reaction?

Jayden.theKidPost authorQuestion: I'm not from america but I'm kinda wondering: is this what you'd learn in college? or is this part of high school syllabus as well?

Arthur SantanaPost authorDoes anybody know what is 26100 he keeps mentioning?

mipatcheu charlesPost authori just love this lectures

Alp BartuPost authorI think the physics at the end of the lecture takes us back to ''Some thing breaks because the magnitude of acceleration becomes too high'', even tho he exerts the same force, if the impact time differs, the results are pretty plausible.

Allye BakerPost authorHow the hell did he make those dots so fast, for the diameter of the sun?! (4:12ish)

Rohit SaiPost authorwhy did bottom string broke?

colton ellisPost authorThese are arguably 3 of the most powerful laws of physics ever, especially the 2nd law.

Eric ThiesPost authorLove this guy…

12:50, yup that's what my class looks like.

I'm glad to see that even the best lecturers get this response form students.

Prashik NaikPost authorWoW superb lecture

H.BannaiPost authorcan someone please explain why the strings holding the red cube at the end behaved that way ?

girish kulkarniPost authorThat's not the correct statement of Newton's second law the second law is The rate of change of momentum of a body is directly proportional to the applied force. you only arrive at F=MA if and only if the mass dosent varies with time which in cases like rocket propulsion does vary with time!

Medhawini KapoorPost authorawesome

FUMBANANI SOKOPost authorTHANKS PAPA

Paul EdwardPost authorBrazil is here.

a jPost authorVery strange, first a force is defined as mass times acceleration and then one step later it is stated as a law. So when you say F:= m * a ofcourse holds F = m * a. Unbelievable what a mess those physicist make. And it is written more or less the same in all those "advanced" textbooks. Humanity has still a long way to go.

Kamlesh Kumar KamalPost authorSir when will the tension remain same? What are the conditions for it to be same?

John GormleyPost author"Is someone pulling our leg"

Shallu GoyalPost authorAt 30:28 he said that earth would go away from the ball it means gravitational force would become repulsive

Isn't the earth should go towards ball because force is acting still in the same direction

Miguel MonroyPost authorcan act the first and third law at the same time?

Naresh KumarPost authorSir there is question in third law we say action and reaction acts on two differrent bodies hence dont cancel each other just in example of ball thrown on wall returns back but how then book resting on table has zero net force i.e how action reaction are cancelled in this case

alistair leePost authorI have this physics conundrum which I cant answer, please help! https://medium.com/@alee250485/newtons-third-law-of-motion-is-it-always-applicable-11a4ee0e6b7c

Dalton LucasPost authorGood tech all ways teach in order student understand nyc for your lecture.

Nadja ReschPost authorWhat does 'twenty six one hundered' mean?

Priyam ChauhanPost authorPlease someone tell me the introduction of this man… lots of gratitude to him…. he made my concepts clear… please post such lectures daily… we really need it…

krishnaa sharmaPost authorTanks a lot.. Sr I m a big 💪 fan of yours.. My I have ur 👉👤. Mail 📧I'd sr please