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.