Newton’s Third Law of Motion
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Newton’s Third Law of Motion


Now we’re ready for Newton’s
third law. And Newton’s third law, in some
ways, I think is the most fun because it’s– at least it
was to me, the least intuitive of all the laws. But once you really kind of
understand it a lot of things start to make sense. So Newton’s third law
essentially says– actually, I can tell you kind of what
you might have heard. A lot of people say every
action has an equal and opposite reaction. Another way of thinking about
it is, if there’s an object and it exerts a force
on another object. So let’s say I have a– oh,
let me think of something. Let’s say that I have a fist. So
let me draw the fist. So I have a fist and it is punching
someone’s face. So this is a face, and
they’re not happy. And let’s say this fist is
punching the face with a force of– I don’t know–
10 Newtons. Let’s not make it so violent. Let’s say that this is a hand
massaging the face. It’s pressing upon the face with
a force of 10 Newtons. So what Newton’s third law tells
us is that the face is also, I guess, we could
say punching the hand. Or– well no, we’re not
using the violent. The face is also pressing upon
the hand with a force of 10 Newtons in the opposite
direction. I guess you would say
along the same line. So does that make any sense? Because it seems like the hand
is doing something to the face and not the other way around. But if you think about it, when
you press on someone’s face, their face might kind of
press in a little bit, but you also feel something on your
fist or on your hand. I mean think of it, maybe a
better example would be instead of someone’s face,
imagine if it was a tree that you’ve decided to massage
or, I guess, punch. So here is the tree. That is the tree. And the same thing
would happen. If you were to press upon the
tree with– if you were to punch the tree essentially,
the tree is essentially punching back with the
exact same force. And here it makes sense because
your hand will hurt. And maybe in this case, the
face will hurt because the face kind of gives way while
your fist doesn’t. But here the tree’s not giving
way and your fist will. Another way to think about it
is if– well, and this is probably the least intuitive. If I have the earth and
here am I standing on top of the earth. So we already figured out that
the pull of the earth or the force of gravity upon me,
it’s pulling down upon me at 150 pounds. That’s the force. And you know, we could
say the Newton. But pounds is a unit of
force, it’s weight. But also, Newton’s third law
tells us that I’m actually, at the same moment, pulling
on the earth with a force of 150 pounds. And this might not make a lot
of sense to you, but you can think about it this way. When I’m stepping on– let’s
say I’m stepping on a soft surface, like sand
or something. My feet will compress a little
bit, but so does the sand. Well, depending on which
one’s softer. And another way to think about
it also is, if me and the earth are both in deep space. And I am, you could say, falling
towards the earth because the earth is
pulling on me. How do we know that the earth
isn’t falling towards me? I mean it’s kind of arbitrary. There’s no frame of reference. We’re both in deep space. There’s nothing else
to look at. We’re essentially, falling
towards each other. I’m not necessarily falling to
the earth, the earth’s not necessarily falling to me, we’re
just falling towards each other. And that’s another way of
thinking about this. So you could think about every
example where a force applies to something. And if you really think about
it, the force is going the other way as well. For example, if I were to take
I bat to this tree and swing on it really hard. So I were to swing on this tree
really hard, I have a good chance of breaking
that bat. Even though you would have
thought, hey, that bat is applying the force
to the tree. But why is the bat breaking? Because the tree is applying
an equal force to the bat. And actually, if I did it
perfectly, if I had like– let’s say I had two bats
or two swords. For some reason I think I’m
going a little too violent with these example. But I guess you know we’re
talking about forces. So maybe violence is
justified here. But let’s say I have two swords
that are completely identical hitting each other. And let’s say I keep increasing
the force at which they’re kind of going in
opposite directions. At some point, they’re
going to break. If I just keep increasing the
force on– you know, one guy is swinging in one direction,
one guy is swinging in the exact opposite direction. And the force just keeps
increasing, at some point, they’re going to break. And you could say that well–
this guy says well, I was the only guy swinging because
this guy was stationery. And the other guy will say,
well, I was really guy swinging because this
guy was stationary. Not one of them is going to
break, they’re both going to break because even though this
mauve sword was pushing on this blue sword with some force,
the blue sword was essentially pushing back with
the exact same force. So these are completely
identical swords. At some point, they’re
going to break. Another way we could think about
it, one of the swords could have just been held. You know, stationary. It could’ve been held stationary
by somebody and this other sword that– if you
were to press down on it, kept increasing the force with which
you press, at some point they both would break. Because they’re identical. If one was harder than the
other, than that one would stick around. Hopefully that gives you
a little intuition. I mean we could do a bunch
of more examples. I’m trying to think. Oh, let me think
of another one. A less violent one. Let’s say we’re in deep space
again and I have a– I don’t know– I have a basketball. Let me do it in orange. I have a basketball and
it weighs 1 kilogram. And let’s say that I weigh 50
kilograms. So let’s say I push– so my hand pushes on this
ball with a force of– I don’t know– let’s say I push
on that ball with a force of 10 Newtons. 10 Newtons to the right. What Newton’s third law tells
me is that essentially, that basketball is going to push on
my hand with an equal and opposite force. So it’s going to push on me with
a force of 10 Newtons. So what’s going to happen? So we’re touching. I’m pushing on 10 Newtons on
the basketball and we’re in deep space. There’s no gravity from random
stars, et cetera. And then the basketball’s going
to push on me with the force of 10 Newtons
simultaneously. We know F equals ma. So the basketball, so 10 Newtons
is equal to 1 kilogram times acceleration. So acceleration is going to be
10 meters per second squared to the right. So as long as we’re touching,
the basketball’s going to accelerate at 10 meters per
second squared to the right. And simultaneously, I’m going
to accelerate at a certain acceleration to the left. And what’s that going to be? 50 kilograms. We know that the
force to the left is also going to be 10 Newtons. That equals 50 kilograms
times acceleration. So here, the acceleration is
going to be 1/5 meters per second squared. So we’re both in deep space
floating around and I push on this 1 kilogram basketball with
a force of 10 Newtons. As long as I’m pushing on it,
it’s going to accelerate at 10 meters per second squared. But simultaneously, while I’m
pushing on it, it’s exerting an equal and opposite force
on me of 10 Newtons. So I’m going to actually move
back a little bit at a slower acceleration. That’s just because
I have more mass. At 1/5 meters per
second squared. Another example you could
think of is if someone shoots a gun. There’s that– I forgot
the term because I don’t shoot guns. But your shoulder jerks back
as I’ve seen in the movies when a bullet is shot. That’s because the gun is
exerting a force on that bullet and the bullet is
exerting an equal and opposite force on the gun, which
kind of pushes back on your shoulder. And the reason why the bullet
just goes a lot, lot faster forward than you and the gun go
backwards is because your mass is much, much, much
larger than the bullet. Hopefully that gives you a
little bit of intuition on Newton’s third law. And this is kind of
non-intuitive. So look around you in the world,
look at all the forces that are being applied, and I
want you to think about when one force is being applied in
one direction, why does it make sense that another force,
an equal and opposite force, is being applied in the exact
opposite direction? I’ll see you all in
the next video.

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