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We're now ready for Newton's
third law of motion.

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And something, once again,
you've probably heard,

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people talk about.

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But in this video, I want to
make sure we really understand

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what Newton is talking
about when he says--

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this is a translation of
the Latin version of it--

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to every action, there's
always-- and just

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to be clear--
Newton was English,

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but he wrote it in Latin
because at that point in time,

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people wrote things
in Latin because it

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was viewed as a more
serious language.

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But anyway-- to every
action, there is always

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an equal and opposite
reaction, or the forces

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of two bodies on each
other are always equal

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and are directed in
opposite directions.

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So what Newton is saying
is that you can't just

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have a force acting
on some object

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without that object also
having an opposite force acting

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on the thing that's
trying to act on it.

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And just to make it clear,
let's say that we have a--

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and we'll talk about these
examples in the second.

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Let's say that I have some
type of block right over here.

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And that I move, and
I press on the block

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and I try to push it forward.

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So this is my hand.

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This is my hand trying
to press on the block

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and exert a force, a net
force in that direction.

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So that the block
moves to the right.

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Maybe this block is
sitting on some type of ice

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so that it can move.

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So let's say that I have some--
that doesn't look like ice--

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I'll give it a more
ice-like color.

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So the block is sitting on,
maybe, some ice like that.

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So Newton's third law
is saying, look, I

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can press on this
block, and sure,

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I'll exert a net
force on this block

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and that net force will
accelerate the block assuming

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that I can overcome
friction, and if it's on ice

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I can do that.

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But that block is going to exert
an equal and opposite force

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on me.

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And for direct evidence--
this is something,

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even though it might not
be so intuitive, when

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it's said-- this equal
and opposite force.

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But direct evidence that it's
exerting an equal and opposite

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force-- is that my hand
will get compressed.

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I could actually feel the
block exerting pressure on me.

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Take your hand right now and
push it against your desk

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or whatever you have
nearby and you are clearly

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exerting a force on the desk.

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So let me draw-- so let's
say I have a desk right here.

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And if I try to push on the
desk-- so once again that's

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my hand right here,
pushing on the desk.

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If I push on the
desk, and I'm actually

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doing it right now while
I record this video.

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You'll see.

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So I'm clearly exerting
a force on the desk,

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if I do it hard
enough, I might even

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get the desk to shake
or tilt a little bit.

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And I'm actually
doing that right now.

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But at the same time, you'll
see that your hand is getting

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compressed, the palm of your
hand is being pressed down.

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And that's because
the desk is exerting

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an equal and opposite
force on you.

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If it wasn't, you
actually wouldn't even

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feel it, because you wouldn't
even feel the pressure.

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Your hand would be
completely uncompressed.

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Another example of that-- say
you're walking in the beach,

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and you have some
sand right here.

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If you were to step on the sand.

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So let's say that
this is your shoe.

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I'll do my best
attempt to draw a shoe.

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So this is the shoe.

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If you were to step on
the sand, clearly you

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are exerting a
force on the sand.

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The force that you're
exerting on the sand

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is the force of your weight.

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The gravitational attraction
between you and the Earth.

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You are exerting
that on the sand.

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The sand is also-- and
another evidence of that

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is that the sand is
going to be displaced.

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You're going to
create a footprint.

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The sand is going to
move out of the way

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because it's being
pushed down so hard.

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So clearly you are exerting
a force on the sand.

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But the sand is also exerting
an equal and opposite force

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on you.

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And what's the evidence of that?

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Well, if you believe,
Newton's second law--

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if you have this
gravitational force on you,

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you should be
accelerating downwards

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unless there is some other
force that balances it out.

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And the force that
balances it out

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is the force that the beach,
or the sand is exerting on you

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upwards.

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And so when you
net them out there

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is a zero net force on you.

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And that's why you
get to stay there.

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Why you don't start
accelerating down

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towards the center of the Earth.

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Other examples of this- this is
maybe the most famous example

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of Newton's third law--
is just how rockets work.

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When you're in a
rocket, either trying

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to escape the atmosphere,
or maybe you're in space,

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there's nothing to push off
of, nothing to push off,

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that lets you accelerate.

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So what you do is you keep stuff
to push off in your fuel tanks,

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and when you allow the
proper chemical reactions

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or the proper combustion
to take place,

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what it does is it expels gases
at ultra high velocities out

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the back of your rockets.

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And each of those
particles you're

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exerting a force on them.

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Enough force even though they're
super small mass for each

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of them, they're going
at super high velocity.

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So they're being
accelerated tremendously.

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So there's an equal
and opposite force

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on the rocket, the thing that
is actually expelling the gas.

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And so that's what
allows a rocket

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to accelerate even when there's
nothing in this direct vicinity

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to push off of.

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It just expels a bunch
of things or accelerates

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a bunch of things at
a super fast rate.

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It exerts a force on
all these particles,

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and that allows an
equal and opposite force

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to accelerate the rocket ahead.

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And another example of
this is, if you ever

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find yourself drifting in space.

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And this is an actually
useful example,

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so that you don't end up
drifting in space forever.

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Let's say, we don't ever
want this to happen.

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This astronaut,
by some chance he

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loses his connection
to this little tool arm

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right here in the space shuttle,
and he starts drifting away.

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What can that
astronaut do to change

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the direction of his motion
so that he drifts back

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to the space shuttle?

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Well you look around, there's
nothing to push off of.

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He doesn't have any
wall to push off of.

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Let's just assume he
doesn't have any rocket jets

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or anything like that.

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What could he do?

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Well the one thing
you could do--

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and this is the situation if
you're ever drifting in space--

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is you should find
the heaviest or I

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should say the most
massive thing on you--

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and we'll explain the difference
between mass and weight

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in a future video-- but
you should find the most

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massive thing that you can
carry, that you can take off

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of you, that you could throw.

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And you should throw it in a
direction opposite yourself.

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So let me put it this way.

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If I throw-- let's say I'm
in space and I'm floating.

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I'll make it look like the glove
of a-- so let's say that this

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the glove of the astronaut.

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There you go.

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That's his hand, that's
the astronaut's hand

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right over here.

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And let's say he find some
piece of equipment on his,

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or she finds some piece
of equipment on them

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that they can throw.

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They can take off
of their tool set.

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And they could find
the most massive object

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that they could throw.

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So what's going to happen
is-- for some period of time,

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while they push
this object away,

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they will be exerting a force
on that object for some period

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of time while they have
contact with the object.

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And that entire
time, that object,

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while it is accelerating--
while the astronaut is exerting

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a force on it-- will be exerting
an equal and opposite force

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on the hand of the astronaut,
or on the astronaut itself.

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So the object will
accelerate in that direction,

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and while the
astronaut is pushing,

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the astronaut will
accelerate in this direction.

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So what you do is you throw
in the opposite direction

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and that'll allow the astronaut
to accelerate towards the space

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shuttle and hopefully
grab on to something.