- 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.
- 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.