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When you look at footage or
photographs of astronauts

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

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here.

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Everything is not falling
down in one direction.

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

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Everything just floats around.

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If I were to push
off of this wall,

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I would just float
in that direction.

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

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like gravity that's trying
to pull everything down.

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But the question is that
these astronauts are still

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not too far away from
a supermassive body.

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In fact, the space shuttle
gets up only a couple

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of hundred miles above
the surface of the Earth.

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

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would probably be
right about there.

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And we know that the force of
gravity between two objects

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is equal to big G, the
gravitational constant, times

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the mass of the first
object, times the mass

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of the second object over
the distance between the two

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objects squared.

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And if the space
shuttle is right here,

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

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this r isn't that different.

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It's a little bit
further than if you

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were at the surface
of the Earth.

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

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from the center of
the Earth, or really

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

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The center of the Earth
represents most of the distance

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here.

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So if I'm at the
surface of the Earth

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

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

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especially in terms
of percentage.

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

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that the force of
gravity for someone who

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is in space only a few
hundred miles above the Earth

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should not be that different
than the force of gravity

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for someone who is on
the surface of the Earth.

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So my question to
you is, what gives?

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If there should be
gravity in space,

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how can we see all
of these pictures

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of people floating
around like this?

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And the answer is that
there is gravity in space,

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and that these people
actually are falling.

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They're just moving fast
enough relative to the Earth

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that they keep missing it.

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

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

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and I were to shoot
something, if maybe I

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have a really good
sling shot, and I

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

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angle, it might take
off a little bit

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and eventually
hit another point.

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And this would actually already
be a super duper slingshot.

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

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over 1,000 miles.

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If I make it go a
little bit faster,

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if I put a little
bit more force on,

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if I just propelled the
projectile a little bit faster,

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it might go a
little bit further,

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but it will eventually
fall back to the Earth.

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

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Then it'll still
fall to the Earth.

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Let's propel it even
faster than that.

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

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I think you might see
where this is going.

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Let's go even faster than that.

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So if we go even
faster than that,

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eventually it'll
fall to the Earth.

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Even faster than
that, so if you were

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to throw an object
even faster than that,

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

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I think you see
what's happening.

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Every time you go
faster and faster,

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you throw this projectile
faster and faster,

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it gets further and
further, up to some velocity

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that you release
this projectile,

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

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

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

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and a projectile like that
would essentially be in orbit.

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So what's happening
is if there was

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

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the projectile would just
go straight away into space.

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

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towards the center of
the Earth, or the center

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of that projectile and
the center of the Earth

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are being pulled
towards each other,

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I guess is a better
way to think about it.

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The force of gravity
is doing that.

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And so it's curving its path.

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

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object we're talking about
is going fast enough,

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

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

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no air if you go high enough,
especially the altitude

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that the space shuttle
is, no noticeable drag,

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this thing can
just keep on going

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for a substantial
amount of time.

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Although there is just
a little bit of drag,

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

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because there is just a
little bit of air resistance.

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So the answer to
this conundrum is

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that there actually is gravity.

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It's not a gravity-free
environment.

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It's just that the astronauts
and the space shuttle

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and everything else that's
in the space shuttle,

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it's all falling,
but it's moving

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fast enough that it
never hits the Earth.

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It keeps missing the Earth.

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It keeps going round
and round and round,

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but it is completely under
the influence of gravity.

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If they were to just
slow themselves down,

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if they were to just brake
relative to the Earth,

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and if they were to just
put their brakes on right

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

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

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into space, that all of a
sudden gravity disappears.

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The influence of gravity,
actually on some level,

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it just keeps going.

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You can't, it might
become unnoticeably

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small at some point,
but definitely

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

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there is definitely
gravity there.

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

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So if they just keep
falling, they're

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never going to hit the Earth.

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And if you want to
simulate gravity,

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and this is actually how
NASA does simulate gravity,

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is that they will put
people in a plane,

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and they call it
the vomit rocket

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because it's known
to make people sick,

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

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So if this is the ground,
in a projectile path

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or in a parabolic
path I should say,

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so the plane will
take off, and it

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

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or in a parabolic path.

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And so anyone who's
sitting in that plane

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will experience free fall.

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

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

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jumped or skydived
or even the feeling

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when a roller coaster is
going right over the top,

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and it's pulling you
down, and your stomach

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

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the exact same feeling
that these astronauts feel

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

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But that is an
indistinguishable feeling from,

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

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any noticeable mass, that is
an identical feeling that you

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would feel to having no
gravitational force around you.

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So hopefully that clarifies
things a little bit.

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

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and if they had no
windows, there's

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no way of them
knowing whether they

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are close to a massive
object and they're just

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

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just completely far away
from any massive object,

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

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where there's very
little gravity.

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