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In this video, I want
to talk a little bit

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about Newton's
First Law of Motion.

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And this is a translation from
Newton's Principia from Latin

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into English.

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So the First Law,
"Every body persists

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in a state of being at
rest, or moving uniformly

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straightforward, except
insofar as it is compelled

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to change its state
by force impressed."

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So another way to rephrase
what they're saying

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is, that if there's something--
every body persists--

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so everything will
stay at rest, or

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moving with a constant
velocity, unless it

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is compelled to change
its state by force.

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Unless it's acted on by a force,
especially an unbalanced force.

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and I'll explain
that in a second.

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So if I have something that's
at rest, so completely at rest.

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So I have-- and
this is something

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that we've seen before.

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Let's say that I have a rock.

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Let's say that I
have a rock someplace

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and it's laying on
a field of grass,

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I can keep observing that rock.

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And it is unlikely
to move, assuming

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that nothing happens to it.

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If there's no force
applied to that rock,

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that rock will just stay there.

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So the first part
is pretty obvious.

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So, "Every body persists in
a state of being at rest"--

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I'm not going to do
the second part--

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"except insofar as there's some
force being applied to it."

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So clearly a rock
will be at rest,

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unless there's some force
applied to it, unless someone

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here tries to push it or roll
it or do something to it.

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What's less intuitive about the
first law is the second part.

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"Every body persists in,"
either, "being in a state

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of rest or moving
uniformly straight forward,

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except insofar as
it is compelled

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to change its state
by force impressed."

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So this Newton's
first law-- and I

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think I should do a
little aside here,

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because, this right
here is Newton.

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And if this is
Newton's first law,

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why do I have this huge
picture of this guy over here?

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Well, the reason is is because
Newton's first law is really

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just a restatement of
this guy's law of inertia.

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And this guy, another
titan of civilization

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really, this is Galileo Galilei.

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And he is the first person to
formulate the law of inertia.

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And Newton just rephrased it
a little bit and packaged it

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with his other laws.

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But he did many, many,
many other things.

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So you really have
to give Galileo

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credit for Newton's first law.

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So that's why I made
him bigger than here.

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But I was in the
midst of a thought.

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So we understand if
something is at rest,

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it's going to stay at
rest, unless there's

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some force that acts on it.

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And in some
definitions, you'll see

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unless there's some
unbalanced force.

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And the reason why
they say unbalanced

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is, because you could have two
forces that act on something

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and they might balance out.

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For example, I could push
on this side of the rock

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with a certain amount of force.

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And if you push on
this side of the rock

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with the exact same amount of
force, the rock won't move.

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And the only way that it would
move if there's a lot more

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force on one side than
on the other side,

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so if you have an
unbalanced force.

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So if you have a ton
of-- and maybe the rock

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is a bad analogy.

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Let's take ice, because ice is
easier to move, or ice on ice.

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So there's ice right here.

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And then, I have
another block of ice

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sitting on top of that ice.

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So once again, we're
familiar with the idea,

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if there's no force acting
on it that ice won't move.

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But what happens if
I'm pushing on the ice

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with a certain amount
of force on that side,

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and you're pushing on
the ice on that side

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with the same amount of force?

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The ice will still not move.

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So this right here, this
would be a balanced force.

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So the only way for the ice to
change its condition, to change

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its restful condition is
if the force is unbalanced.

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So if we add a little bit
of force on this side,

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so it more than compensates
the force pushing it this way,

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then you're going to see
the ice block start to move,

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start to really accelerate
in that direction.

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But I think this
part is obvious.

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This, you know,
something that's at rest

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will stay at rest, unless
it's being acted on

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by an unbalanced force.

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What's less obvious is
the idea that something

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moving uniformly
straightforward, which

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is another way of
saying something

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having a constant velocity.

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What he's saying is,
is that something

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that has a constant
velocity will continue

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to have that constant
velocity indefinitely,

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unless it is acted on
by an unbalanced force.

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And that's less intuitive.

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Because everything in
our human experience--

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even if I were to push
this block of ice,

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eventually it'll stop.

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It won't just keep
going forever, even

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assuming that this ice field
is infinitely long, that ice

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will eventually stop.

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Or if I throw a tennis ball.

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That tennis ball
will eventually stop.

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It'll eventually
grind to a halt.

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Or if I roll a bowling
ball, or if I, anything.

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We've never seen, at least
in our human experience,

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it looks like everything
will eventually stop.

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So this is a very
unintuitive thing to say,

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that something in
motion will just

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keep going in
motion indefinitely.

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Everything in human intuition
says if you want something

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to keep going in
motion, you have to keep

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putting more force, keep
putting more energy into it

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for it to keep going.

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Your car won't go
forever, unless you keep,

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unless the engine keeps burning
fuel to drive and consuming

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energy.

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So what are they talking about?

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Well, in all of these
examples-- and I

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think this is actually a pretty
brilliant insight from all

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of these fellows is
that-- all of these things

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would have gone on forever.

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The ball would
keep going forever.

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This ice block would
be going on forever,

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except for the fact that
there are unbalanced

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forces acting on
them to stop them.

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So in the case of ice,
even though ice on ice

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doesn't have a lot
of friction, there

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is some friction
between these two.

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And so you have,
in this situation,

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the force of friction
is going to be

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acting against the direction
of the movement of the ice.

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And friction really comes
from, at an atomic level--

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so if you have the actual
water molecules in a lattice

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structure in the ice
cube, and then here are

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the water molecules in a
lattice structure on the ice,

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on the actual kind of sea of
ice that it's traveling on--

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they do kind of bump and
grind into each other.

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Although they're both smooth,
there are imperfections here.

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They bump and grind.

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They generate a
little bit of heat.

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And they'll, essentially, be
working against the movement.

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So there's a force of friction
that's being applied to here.

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And that's why it's stopping.

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Not only a force of
friction, you also

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have some air resistance.

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The ice block is
going to be bumping

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into all sorts of air particles.

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It might not be
noticeable at first,

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but it's definitely going to
keep it from going on forever.

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Same thing with the ball
being tossed to the air.

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Obviously, at some
point, it hits

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the ground because of gravity.

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So that's one
force acting on it.

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But even once it
hits the ground,

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it doesn't keep rolling
forever, once again,

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because of the friction,
especially if there's grass

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here.

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The grass is going to
stop it from going.

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And even while it's in the
air, it's going to slow down.

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It's not going to have
a constant velocity.

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Because you have all
of these air particles

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that are going to
bump into it and exert

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force to slow it down.

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So what was really
brilliant about these guys

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is that they could
imagine a reality where

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you didn't have gravity,
where you did not

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have air slowing things down.

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And they could imagine
that in that reality,

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something would just keep
persisting in its motion.

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And the reason why Galileo,
frankly, was probably good

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at thinking about
that is that he

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studied the orbits of planets.

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And he could, or at
least he's probably

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theorized that, hey, maybe
there's no air out there.

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And that maybe that's why
these planets can just

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keep going round
and round in orbit.

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And I should say their speed,
because their direction is

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changing, but their
speed never slows down,

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because there's
nothing in the space

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to actually slow
down those planets.

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So anyway, hopefully you found
that as fascinating as I do.

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Because on some level,
it's super-duper obvious.

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But on a whole other level,
it's completely not obvious,

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especially this moving
uniformly straightforward.

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And just to make
the point clear,

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if gravity disappeared,
and you had no air,

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and you threw a ball,
that ball literally

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would keep going
in that direction

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forever, unless some other
unbalanced force acted

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to stop it.

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And another way to think
about it-- and this

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is an example that you might
see in everyday life-- is,

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if I'm in an airplane
that's going at a completely

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constant velocity and
there's absolutely

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no turbulence in the airplane.

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So if I'm sitting in the
airplane right over here.

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And it's going at a constant
velocity, completely smooth,

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no turbulence.

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There's really no way for me to
tell whether that airplane is

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moving without looking
out the window.

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Let's assume that there's
no windows in that airplane.

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It's going at a
constant velocity.

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And there's no turbulence.

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And let's say, I
can't hear anything.

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So I can't even
hear the engines.

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There's no way for me to sense
that the plane is moving.

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Because from my
frame of reference,

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it looks completely
identical to if I

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was in that same plane that
was resting on the ground.

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And that's another
way to think about it.

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That it's actually
very intuitive

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that they're similar states,
moving at a constant velocity

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or being at rest.

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And you really can't
tell whether you

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are one or the other.

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