Gravity for Astronauts in Orbit

When you look at footage or
photographs of astronauts

in space, it doesn't look like
there's any gravity at work

here.

Everything is not falling
down in one direction.

In fact, it's not even
clear what up or down is.

Everything just floats around.

If I were to push
off of this wall,

I would just float
in that direction.

So it doesn't look like there's
this overarching influence

like gravity that's trying
to pull everything down.

But the question is that
these astronauts are still

not too far away from
a supermassive body.

In fact, the space shuttle
gets up only a couple

of hundred miles above
the surface of the Earth.

So the space shuttle, if I
were to draw it to scale,

would probably be
right about there.

And we know that the force of
gravity between two objects

is equal to big G, the
gravitational constant, times

the mass of the first
object, times the mass

of the second object over
the distance between the two

objects squared.

And if the space
shuttle is right here,

only a few hundred miles above
the surface of the Earth,

this r isn't that different.

It's a little bit
further than if you

were at the surface
of the Earth.

Remember that r is measured from
wherever you are to the center,

from the center of
the Earth, or really

the center of the object
to the center of the Earth.

The center of the Earth
represents most of the distance

here.

So if I'm at the
surface of the Earth

or if I'm just a few hundred
miles above the surface

of the Earth, it's not going
to change r that dramatically,

especially in terms
of percentage.

So when you look at it this
way, it seems pretty clear

that the force of
gravity for someone who

is in space only a few
hundred miles above the Earth

should not be that different
than the force of gravity

for someone who is on
the surface of the Earth.

So my question to
you is, what gives?

If there should be
gravity in space,

how can we see all
of these pictures

of people floating
around like this?

And the answer is that
there is gravity in space,

and that these people
actually are falling.

They're just moving fast
enough relative to the Earth

that they keep missing it.

And let me show you what
I'm talking about there.

Let's say I'm sitting
here in Africa,

and I were to shoot
something, if maybe I

have a really good
sling shot, and I

were to sling something super
fast and maybe at a 45 degree

angle, it might take
off a little bit

and eventually
hit another point.

And this would actually already
be a super duper slingshot.

I just made it travel a couple
of thousand miles or at least

over 1,000 miles.

If I make it go a
little bit faster,

if I put a little
bit more force on,

if I just propelled the
projectile a little bit faster,

it might go a
little bit further,

but it will eventually
fall back to the Earth.

Let's try to propel it a
little bit faster than that.

Then it'll still
fall to the Earth.

Let's propel it even
faster than that.

Well then, it's still eventually
going to fall to the Earth.

I think you might see
where this is going.

Let's go even faster than that.

So if we go even
faster than that,

eventually it'll
fall to the Earth.

Even faster than
that, so if you were

to throw an object
even faster than that,

it would then go really far
and then fall to the Earth.

I think you see
what's happening.

Every time you go
faster and faster,

you throw this projectile
faster and faster,

it gets further and
further, up to some velocity

that you release
this projectile,

and whenever it's trying
to fall to the Earth,

it's going so fast that it
keeps missing the Earth.

So it'll keep going around and
around and around the Earth,

and a projectile like that
would essentially be in orbit.

So what's happening
is if there was

no gravity for that projectile,
if there was no gravity,

the projectile would just
go straight away into space.

But because there's gravity,
it's constantly pulling it

towards the center of
the Earth, or the center

of that projectile and
the center of the Earth

are being pulled
towards each other,

I guess is a better
way to think about it.

The force of gravity
is doing that.

And so it's curving its path.

And if it's going fast enough,
if the projectile or whatever

object we're talking about
is going fast enough,

it'll just keep going
round and round the Earth.

And since there is almost,
pretty much, for most purposes

no air if you go high enough,
especially the altitude

that the space shuttle
is, no noticeable drag,

this thing can
just keep on going

for a substantial
amount of time.

Although there is just
a little bit of drag,

and that's why over time you
do have satellites slow down,

because there is just a
little bit of air resistance.

So the answer to
this conundrum is

that there actually is gravity.

It's not a gravity-free
environment.

It's just that the astronauts
and the space shuttle

and everything else that's
in the space shuttle,

it's all falling,
but it's moving

fast enough that it
never hits the Earth.

It keeps missing the Earth.

It keeps going round
and round and round,

but it is completely under
the influence of gravity.

If they were to just
slow themselves down,

if they were to just brake
relative to the Earth,

and if they were to just
put their brakes on right

over there, they would all
just plummet to the Earth.

So there's nothing special
about going 300 or 400 miles up

into space, that all of a
sudden gravity disappears.

The influence of gravity,
actually on some level,

it just keeps going.

You can't, it might
become unnoticeably

small at some point,
but definitely

for only a couple of
hundred miles up in the air,

there is definitely
gravity there.

It's just they're in orbit,
they're going fast enough.

So if they just keep
falling, they're

never going to hit the Earth.

And if you want to
simulate gravity,

and this is actually how
NASA does simulate gravity,

is that they will put
people in a plane,

and they call it
the vomit rocket

because it's known
to make people sick,

and they'll make them go
in a projectile motion.

So if this is the ground,
in a projectile path

or in a parabolic
path I should say,

so the plane will
take off, and it

will do a path exactly the
same as something in free fall

or in a parabolic path.

And so anyone who's
sitting in that plane

will experience free fall.

So if you've ever been
in, if you've ever right

when you jump off of a
or if you've ever bungee

jumped or skydived
or even the feeling

when a roller coaster is
going right over the top,

and it's pulling you
down, and your stomach

feels a little ill, that
feeling of free fall, that's

the exact same feeling
that these astronauts feel

because they're in a
constant state of free fall.

But that is an
indistinguishable feeling from,

if you were just in deep space
and you weren't anywhere close

any noticeable mass, that is
an identical feeling that you

would feel to having no
gravitational force around you.

So hopefully that clarifies
things a little bit.

To someone who's just
sitting in the space shuttle,

and if they had no
windows, there's

no way of them
knowing whether they

are close to a massive
object and they're just

in free fall around it, they're
in orbit, or whether they're

just completely far away
from any massive object,

and they really are in
a state of or in a place

where there's very
little gravity.