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Any time you drop something,
it falls down.

But - why? Why do you come back down, 
no matter how hard you jump on that trampoline?

When you throw a ball, why does it fall
to the ground instead of flying off into space?

The answer is gravity, but it does 
a lot more than pulling things down.

Gravity is an invisible force that 
pulls objects towards each other.

In most cases, 
when we say "gravity"

we are talking about the earth's gravity,
but anything that has mass has gravity, too.

Yes, even you have gravity! 
Of course, you don't have very much gravity.

The more mass something has,
the more gravitational force it has.

Compared to the gravity
of the earth,

any gravitational force that 
you have is too weak to notice.

Mass is not the same 
as weight.

Mass is a measure of the matter in an object, 
and does not change no matter where you go.

On the other hand, weight measures the pull 
of gravity on an object, and this can change.

Most places on earth have roughly 
the same amount of gravity,

but if you were to leave earth,
the pull of its gravity

would weaken 
the farther away you went,

until eventually 
you would experience zero-g.

This is because gravity 
gets weaker with distance.

There is still gravity in space, because 
everything that exists exerts gravitational force,

but the gravitational pull 
from something like a person

or even a spaceship is so small 
it feels as though you have no weight at all.

If you were to land on another
planet or celestial body,

the pull of gravity would be 
different depending on its mass,

and you would not weigh 
the same as you did on earth.

On the moon, for example, 
gravity is only about 17% of earth's.

This is because the moon 
is much less massive than the earth.

A person who weighs 100 pounds
or 45 kilograms on earth

would weigh only 17 pounds
or 7 and a half kilograms on the moon.

On Mars, the same person 
would weigh 38 pounds or 17 kilograms.

On Jupiter, they would weigh
253 pounds or 106 kilograms.

Gravity is the weakest of the four 
fundamental forces of the universe,

and only gets weaker with distance,
but it is very important for life on Earth.

It is the force that holds us down
to the surface of the planet.

Gravity makes things fall down,
and gives things weight.

It is what holds the Earth together, and keeps
the air we breathe from spinning off into space.

Gravity also holds 
our solar system together.

The earth's gravity holds the moon
in orbit around the Earth.

The sun's gravity holds the Earth and all
the other planets in orbit around the sun.

On a larger scale, even the Milky Way 
galaxy is held together by gravity.

Scientists believe that a supermassive
black hole sits at the center of the Milky Way,

with its gravity holding all the gas, 
dust, stars and systems in place around it.

Black holes have so much mass
in such a small space

that their gravitational pull
is very strong.

They generate such strong gravity 
that not even light can escape from them,

and that's why 
they look black!

So you see, while gravity 
may be an invisible force,

it has a very visible 
impact on our lives.