Best Film on Newton's Third Law. Ever.

- There are a lot of
misconceptions out there,

and this is a video about
one of the most common ones.

So I went around asking people

"What makes the Moon go around the Earth?"

And they told me...

- The Earth puts a
gravitational force on the Moon.

- But does the Moon put a
gravitational pull on the Earth?

- Pull on the Earth? Yes it does,

hence we have tides, et cetera.

- The Moon pulls on the Earth too.

It affects like the tides and women.

(both laughing)

It does, it's very powerful.

- Does the Moon pull on the Earth?

- Probably that as well, yeah.

- So what I want to know
is how does that force,

that the Moon exerts on the Earth,

how does that compare in terms of size

to the force the Earth exerts on the Moon?

- Well, I'm no scientist,

but I think that one would be
more powerful than that one.

- It's got a greater force
coming from the Earth,

'cause it's greater mass.

- 'Cause the Earth probably has more mass.

- 'Cause it's a big mass.

I thought greater mass is
more like, is more force.

- Does the Moon pull on the Earth?

- Yes.

But a lot less.

- Yeah, Not as much as the
Earth pulls on the Moon.

- Yeah, but a little bit.

- But not very strongly.

- Yes, but much, much, much, much smaller.

- Because of its mass, size

- It's got less Mass

- Because it's smaller, it's
much smaller than the Earth.

- Because it's smaller.

- Because it's smaller.

- Allow me to let you
in on a little secret.

Everyone got it wrong.

The force that attracts
the Moon to the Earth

is exactly the same size

as the force that attracts
the Earth to the Moon.

So what's going on here?

Why did everyone get it wrong?

Well I think it comes
down to cause and effect.

The effect of the force on
the Moon is quite clear.

The Moon goes in circles around the Earth,

but the effect of that force on the Earth

is basically negligible.

The Earth barely wobbles at all.

So people interpret this negligible effect

as indicating there's very
little force affecting the Earth,

but that is forgetting the
third key piece of the puzzle

which is inertia.

Inertia is the tendency of mass

to maintain its state of motion.

Since the Earth has a greater mass,

it has a greater inertia.

And so even with the same
amount of force on it,

it doesn't accelerate that much.

Now the funny thing is many
of the people I interviewed

could state Newton's
third law, which is...

Every force has...

- an equal and opposite reaction.

- Something about Newton's law

doesn't seem to fit into that.

(laughing)

- You're good at this.

(laughing)

Which Newton's law are we talking about?

- The whole equal and
opposite force thing?

Yeah, that one.

- So tell me what you're thinking now.

- Uh well, one would think that

if the Earth putting a force on me,

I would be putting an equal force upon it.

- So why didn't they
apply it to this problem?

Well, I think they may
have memorized the words,

but not really believe Newton's
third law in their core.

Do they really feel it in their spleen?

I don't think they did.

So, allow me to try to
convince you, all of you,

spleen included

that Newton's third law really is true.

Let's consider two objects.

Initially they have the same
mass, one kilogram each.

So obviously the gravitational
force of attraction

must be the same on both of the objects.

Now let's add a second
kilogram to the first object.

The force on the second object
will now be twice as great

because that one kilogram
is attracted equally

to each of the kilograms
in the first object.

But what is often forgotten,
is that new kilogram

is also attracted to the second object.

Meaning that the total
force on each object

is still the same.

They're attracted to each other

with an equal and opposite force.

We could add a third kilogram

and we would find the same thing,

the force on both objects
is still the same,

even though the object on the
left has three times the mass

of the object on the right.

So we can see that no
matter what the mass,

any two objects will have
the same gravitational force

towards each other.

Can you feel Newton's third
law in your spleen now?

It should settle inside you
and become a part of you.