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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.