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Now that we know a little
bit about Newton's First Law,

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let's give ourselves
a little quiz.

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And what I want you
to do is figure out

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which of these statements
are actually true.

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And our first statement is,
"If the net force on a body

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is zero, its velocity
will not change."

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Interesting.

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Statement number two, "An
unbalanced force on a body

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will always impact
the object's speed."

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Also an interesting statement.

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Statement number
three, "The reason

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why initially
moving objects tend

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to come to rest in
our everyday life

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is because they are being
acted on by unbalanced forces."

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And statement four, "An
unbalanced force on an object

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will always change the
object's direction."

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So I'll let you
think about that.

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So let's think about these
statement by statement.

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So our first statement
right over here,

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"If the net force
on a body is zero,

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its velocity will not change."

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This is absolutely true.

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This is actually
even another way

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of rephrasing
Newton's First Law.

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If I have some type
of object that's

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just traveling through
space with some velocity--

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so it has some speed
going in some direction,

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and maybe it's deep space.

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And we can just,
for purity, assume

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that there's no
gravitational interactions.

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There will always be
some minuscule ones,

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but we'll assume no
gravitational interactions.

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Absolutely no
particles that it's

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bumping into, absolute
vacuum of space.

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This thing will
travel on forever.

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Its velocity will not change.

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Neither its speed nor its
direction will change.

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So this one is absolutely true.

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Statement number two, "An
unbalanced force on a body

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will always impact
the object's speed."

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And the key word right
over here is "speed."

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If I had written "impact
the object's velocity," then

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this would be a true statement.

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An unbalanced force
on a body will always

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impact the object's velocity.

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That would be true.

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But we wrote "speed" here.

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Speed is the
magnitude of velocity.

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It does not take into
account the direction.

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And to see why this
second statement is false,

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you could think about
a couple of things.

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And we'll do more
videos on the intuition

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of centripetal acceleration
and centripetal

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forces, inward forces,
if this does not

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make complete intuitive sense
to you just at this moment.

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But imagine we're looking at
an ice skating rink from above.

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And you have an ice skater.

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This is the ice skater's head.

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And they are traveling
in that direction.

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Now imagine right
at that moment,

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they grab a rope that is
nailed to a stake in the ice

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skating rink right over there.

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We're viewing all of this from
above, and this right over here

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is the rope.

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Now what is going to happen?

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Well, the skater
is going to travel.

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Their direction is
actually going to change.

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And they could hold
on to the rope,

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and as long as they
hold on to the rope,

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they'll keep going in circles.

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And when they let
go of the rope,

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they'll start going
in whatever direction

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they were traveling
in when they let go.

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They'll keep going
on in that direction.

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And if we assume very,
very, very small frictions

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from the ice skating
rink, they'll

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actually have the same speed.

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So the force, the inward
force, the tension

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from the rope pulling on the
skater in this situation,

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would have only changed
the skater's direction.

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So and unbalanced force
doesn't necessarily

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have to impact the
object's speed.

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It often does.

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But in that situation, it
would have only impacted

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the skater's direction.

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Another situation like
this-- and once again,

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this involves centripetal
acceleration, inward forces,

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inward acceleration--
is a satellite in orbit,

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or any type of thing in orbit.

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So if that is some
type of planet,

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and this is one of the
planet's moons right over here,

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the reason why it stays in orbit
is because the pull of gravity

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keeps making the object
change its direction, but not

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its speed.

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Its speed is the
exact right speed.

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So this was its
speed right here.

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If the planet wasn't
there, it would just

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keep going on in that
direction forever and forever.

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But the planet right
over here, there's

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an inward force of gravity.

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And we'll talk more about the
force of gravity in the future.

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But this inward
force of gravity is

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going to accelerate this object
inwards while it travels.

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And so after some
period of time,

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this object's velocity
vector-- if you

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add the previous velocity
with how much it's

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changed its new velocity vector.

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Now this is after its traveled
a little bit-- its new velocity

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vector might look
something like this.

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And it's traveling at
the exact right speed

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so that the force
of gravity is always

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at a right angle to
its actual trajectory.

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It's the exact right speed so it
doesn't go off into deep space

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and so it doesn't
plummet into the earth.

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And we'll cover that
in much more detail.

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But the simple answer is,
unbalanced force on a body

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will always impact its velocity.

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It could be its speed,
its direction, or both,

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but it doesn't have to be both.

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It could be just the speed
or just the direction.

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So this is an
incorrect statement.

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Now the third
statement, "The reason

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why initially
moving objects tend

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to come to rest in
our everyday life

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is because they are being
acted on by unbalanced forces."

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This is absolutely true.

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And this is the example we gave.

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If I take an object,
if I take my book

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and I try to slide
it across the desk,

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the reason why it
eventually comes to stop

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is because we have the
unbalanced force of friction--

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the grinding of the
surface of the book

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with the grinding of the table.

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If I'm inside of a
pool or even if there's

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absolutely no
current in the pool,

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and if I were to try to
push some type of object

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inside the water,
it eventually comes

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to stop because of all of the
resistance of the water itself.

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It's providing an unbalanced
force in a direction

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opposite it's motion.

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That is what's slowing it down.

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So in our everyday
life, the reason

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why we don't see these
things go on and on

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forever is that we have
these frictions, these air

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resistants, or the friction
with actual surfaces.

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And then the last statement, "An
unbalanced force on an object

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will always change the
object's direction."

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Well, this one actually is
maybe the most intuitive.

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We always have this situation.

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Let's say I have a
block right over here,

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and it's traveling with some
velocity in that direction--

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five meters per second.

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If I apply an unbalanced
force in that same direction--

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so that's my force
right over there.

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If I apply it in
that same direction,

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I'm just going to accelerate
it in that same direction.

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So I won't
necessarily change it.

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Even if I were to act against
it, I might decelerate it,

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but I won't necessarily
change its direction.

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I could change its direction
by doing something like this,

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but I don't necessarily.

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I'm not always
necessarily changing

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the object's direction.

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So this is not true.

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An unbalanced force on
an object will not always

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change the object's direction.

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It can, like these
circumstances, but not always.

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So "always" is what makes
this very, very, very wrong.