Gravitational force | Middle school physics | Khan Academy

- [Instructor] You probably
know that Earth's gravity

keeps you and everything
around you stuck to Earth,

and similarly, the Sun's gravity

keeps all the planets closed
together in our Solar System.

This, by the way, picture
is not to scale, okay?

But the big question is,

what exactly is gravity,

and what exactly produces gravity?

Is it just the planets and stars

or other things can produce it too?

And we could have even more questions,

like for example,

the astronauts inside the
space station are floating.

So is there gravity over there?

Well, let's find out.

We experience a lot of force
in our day-to-day life,

like friction, tension.

There are other forces like normal forces,

and of course, one of
them is also gravity.

But guess what?

Not all these forces are fundamental.

In fact, most of these forces

are not fundamental forces at all.

All these forces are actually

fundamentally electromagnetic force.

What about gravity?

Gravity is another
fundamental force of nature.

But what exactly is it?

Well, you can think of gravity
as a force of attraction.

It's a fundamental force of attraction

between any two masses.

For example, there's a force
of attraction between Earth

and everything else that is on Earth,

like you, your friend, the house.

Anything else that you can consider,

every single thing is
attracted towards the Earth.

And the reason they're
attracted towards the Earth

is because the Earth has mass,

and you and your friend and
everything else, they have mass.

So any two masses will attract each other

with the force of gravity.

Another example, the Earth and the Moon,

they both have mass,

and therefore the Earth attracts the Moon

with the force of gravity.

Similarly, the Sun and all the
other planets also have mass,

and therefore they attract each other,

again due to a force of gravity.

But wait a second,

we might have some questions over here.

The force of attraction can
happen at a distance, right?

That means gravity is a non-contact force.

You don't need to be in contact
for gravity to be exerted.

But how does it work?

For example, how does the
Earth exert a force on the Moon

without ever being in contact with it?

Well, here's how we like
to think about it, okay?

We like to imagine that Earth produces

what we call a gravitational
field everywhere.

This is a field.

A field exists in the space around it.

And when any other mass comes in contact

with the gravitational field,

like for example, say the Moon,

it now, because of that
field, experiences a force.

So the force is mediated
via the gravitational field.

And I'm sure you might
have even more questions

about what exactly is a field and stuff,

but we'll not delve too deep into it.

But anyways, because of this force,

the Moon is literally
falling towards the Earth,

but it doesn't crash into the Earth

because the Moon is also moving sidewards,

and as a result, it ends
up orbiting the Earth.

Okay, but what about the Moon?

Does it also produce
a gravitational field?

The answer is yes.

The same thing applies over here.

Moon produces also its
own gravitational field,

and when the Earth, which is another mass,

comes in contact with
that gravitational field,

it also experiences a force.

And that should make sense

because we know that forces come in pairs.

So both the Earth and Moon

are gravitationally attracting each other

with exactly equal but opposite forces.

But the next big question could be,

what does the strength
of this force depend on?

Well, since gravity is
a force due to masses,

well, the strength depends on the mass.

In fact, if both the
masses are pretty large,

like in the case of
the Earth and the Moon,

then the force is pretty large.

Even if one of the objects
has a very small mass

but the other object has a big mass,

like for example, if
you consider the Earth

and say a tennis ball.

In this case, the tennis ball

has a very tiny mass compared to Earth,

but the Earth has a such a huge mass.

Even then you get a
comparable force of gravity,

and that's why you do see
the ball falling down.

But what about the gravitational
force of attraction

between say two tennis balls?

Remember, gravity acts
between any two masses,

so there must be a force of, you know,

attraction between them.

But we don't see it.

We don't see two tennis balls

getting attracted to each other.

Why is that?

Well, that's because both of these masses

are incredibly tiny,

and therefore the force of
attraction between them,

the gravitational force is so, so weak

that there are other forces
that can easily balance them.

For example, air resistance.

And that's why we usually do not see

the effects of gravity on, you know,

everyday objects, like two tennis balls.

Or maybe, you know,

gravitational force between
you and your friend.

It's the same story.

There is gravitational attraction
between you two for sure.

But since the masses are so small,

the forces are so tiny,

again, forces like friction
can easily balance them

and you don't see their effects.

But on the other hand,

if you consider the force of attraction

between you and the Earth,

that's pretty large because
the Earth is very massive.

Okay, so the strength of
gravity depends upon the masses.

At least one of the objects
must have a very high mass

for conceivable amount of gravity.

Got it.

What else does the strength
of the force depend on?

Well, again, if we go back
to the Earth and the Moon,

another thing that the force depends on

is the distance between them,

or to be more precise,

the distance between the centers of masses

of the two objects, okay?

What happens as the distance increases?

Well, as the distance increases,

the force actually becomes weaker.

The force becomes smaller.

Gravity becomes weaker

as the distance between objects increases.

The bigger the distance between them,

the weaker the force of gravity.

So here's now a quick question.

What do you think happens
to the force of gravity

between Earth and say you
when you climb the mountain,

when you go to the top of the mountain?

What do you think will happen

to the force of gravity
between you and Earth?

Why don't you pause and think about this?

All right, we might think
that as we climb the mountain,

the distance between us

and the center of the Earth has increased.

We have gone farther away from the center,

so the gravity must get weaker, right?

Now, technically that's true,

but look at the size of the Earth

and compare that to the
size of the mountain.

The size of the mountain is negligible.

It just looks like a dot.

And therefore, for all practical purposes,

climbing the mountain has
not changed the distance

between you and the center of the Earth,

and therefore it pretty
much stays the same.

And that's why the force
of gravity in, again,

everyday life stays pretty much the same.

It does not change with height or altitude

because it's very tiny compared
to the radius of the Earth.

But what if you go very far away?

For example, what if you go as far

as the International Space Station?

Ooh, now you're so far away
from the surface of the Earth

that the distance between you

and the center of the Earth

has become considerably bigger,

and so the force of
gravity does become weaker.

But guess what?

People over there still
feel about 80 to 90%

of the gravity they would feel
on the surface of the planet.

So it's become weaker, but not that weak.

So the folks inside the
ISS will definitely feel

a significant amount of
gravitational force from the Earth.

But wait a second.

Why are the astronauts inside
the space station floating?

If they feel the force of gravity,

why are they floating?

Well, the short answer
is they're not floating.

They are falling.

The whole space station

is actually falling towards the Earth,

giving them the illusion of floating.

But again, the reason
why the space station

will not crash into the Earth

is because just like the Moon,

it is moving sidewards
with a very high velocity.

So it's continuously
falling towards the Earth,

but keeps missing the Earth
because of the sideward motion,

because of the sideward velocity.

In fact, if there was no gravity,

it wouldn't be orbiting the Earth.

So the fact that it's orbiting the Earth

is actually showing us that
there is gravity over there.

So, long story short,
what exactly is gravity?

It's a fundamental force of attraction

between any two masses.

Remember, it's always attractive

and its feature is that
it's a non-contact force.

You don't need to be in contact for that.

And how does it get mediated?

It gets mediated via gravitational fields.

And what does the strength
of the force depend on?

Well, it depends on the masses,

more the mass, more the force,

and it gets weaker with the distance.

Well, more the distance
between the two objects,

between the centers of the objects,

smaller is the force of gravity.