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I want to make a
quick clarification

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to the last video, and then
think about what's friction

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up to when the block
is actually moving.

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So in the last
video, we started off

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with the block being stationary.

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We knew that the
parallel component

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of the force of
gravity on that block

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was 49 newtons downwards,
down the slope.

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And when the block
was stationary,

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we said there must be
an offsetting force.

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And we said that's
the force of friction,

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and it must be 49
newtons upwards.

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And so they completely
net out in that direction.

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Now, what we said is we're going
to keep applying a little bit

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more force until we
can budge this block

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to start accelerating downwards.

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And I said I kept applying
a little bit more force,

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a little bit more force,
until I get to 1 newton,

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and then the block
started to budge.

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So at that point, when
it started to budge,

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I'm applying this 1 newton
over here, right over here.

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There was already
49 newtons of force,

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or the component of
gravity, in this direction.

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So combined, we're
providing 50 newtons

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to just start budging
it, to just overcome

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the force of friction.

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The one thing I
want to clarify here

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is this whole time
the force of friction

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was not constant at 49 newtons.

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When I wasn't messing
with this block,

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and the parallel component of
the force was 49 newtons, then

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the force of friction
was 49 newtons.

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When I started to press
on it a little bit,

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apply a little bit
of force, maybe I

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applied a tenth of a
newton on top of that,

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then the force of friction
was 49 and 1/10 newton,

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because it was still
providing enough force so

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that this block was not moving.

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Then maybe I applied
half a newton.

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And so the total force
in the downward direction

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would have been 49
and 1/2 newtons.

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But if it still was not moving,
then the force of friction

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was still completely
overcoming it.

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So the force of
friction, at that point,

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must have been 49
and 1/2 newtons,

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all the way up to the combined
force in the downward direction

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being 49.999999 newtons.

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And then the force of friction
was still 49.99999 newtons,

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all the way until I hit 50
newtons and then the block

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started to budge, which tells us
that the force of friction now,

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all of a sudden, or at least
the force of static friction all

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of a sudden now
couldn't keep up and it

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started to accelerate downwards.

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So in that static scenario,
the force of friction

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changed as I applied
more or less force

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in this downward direction.

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Now with that out of
the way, let's take

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a different scenario.

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Let me just redraw that same
block, just since all of this

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is getting messy.

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So we have the same block, and
as we said in the last video,

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we're now assuming that
this is wood on wood.

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So this is the wedge.

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This is the block
right over here.

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We know that the component
of gravity that is parallel

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to the plane right
there is 49 newtons.

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We know that this is 49 newtons.

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We know the component
of gravity that

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is perpendicular to the
plane-- we figured out

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this two videos ago-- is 49
square roots of 3 newtons.

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We know that this block
is not accelerating

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in this normal
direction, so there

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must be some force counteracting
gravity in that direction.

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And that's the normal force
of the wedge on the block.

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So that is going
in that direction

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at 49 square roots of 3 newtons.

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And now instead of assuming
that this block is stationary,

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let's assume that it's moving
with a constant velocity.

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So now we're dealing
with-- let me

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do that in a different color.

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So now we're dealing
with a scenario

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where the block has
a constant velocity.

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And for the sake of
this video, we'll

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assume that that constant
velocity is downward.

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And so the constant velocity,
v, is equal to-- I don't know.

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Let's say it is 5 meters
per second down the wedge,

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or down the ramp.

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Or I guess we could say in
the direction that is parallel

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to the surface of the ramp.

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So it's in this direction
right over here.

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So that's the constant velocity.

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So what are all
the forces at play?

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And be very careful here.

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There might be a temptation
that says, OK, there's

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a net force here.

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We're moving.

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So maybe that's the net force
that's causing the move.

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But remember, this
is super important.

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This is Newton's first law.

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If you have a net force, if
you have an unbalanced force,

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it will cause it to accelerate.

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And we are not
accelerating here.

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We have a constant velocity.

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We are not accelerating here.

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So if you're not accelerating
in that direction,

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then that means that the
force in that direction

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must be balanced.

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So there must be
some force acting

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in the exactly
opposite direction that

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keeps this thing from
accelerating downwards.

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And so it must be
exactly 49 newtons

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in the opposite direction.

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And as you can imagine, this
is the force of friction.

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This right over here is
the force of friction.

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And the difference between
this video and the last video

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is last time
friction was static.

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Even at 49 newtons,
the box was stationary.

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You had to keep nudging
until you get to 50 newtons,

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and then it started moving.

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Here we're just jumping
into this picture

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where we just see a
box that's moving down

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the slope at 5
meters per second.

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So we don't know
how much force it

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took to overcome
static friction.

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But we do know that there is
some force of friction that

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is keeping this box from
accelerating, that's keeping it

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at a constant velocity,
that is completely negating

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the parallel component
of the force of gravity,

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parallel to the
surface of this plane.

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So given this, let's
calculate another coefficient

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of friction.

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But this is going to
be the coefficient

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of kinetic friction, because now
we are moving down the block.

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And I'll do a video
on why sometimes

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a coefficient of
static friction can

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be different than
the coefficient

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of kinetic friction.

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So the coefficient of kinetic
friction-- we'll write it.

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So this is the Greek letter
mu, and we put this k here

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for kinetic, or we can kind
of say moving friction.

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It's going to be equal
to the force of friction,

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or I should say the magnitude
of the force of friction

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over the normal force.

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I should say the magnitude
of the normal force.

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And you can derive
this experimentally.

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One, if you just observe
this whole thing going on

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and you knew the
mass of the block,

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so you knew this
component of gravity

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that's going in this direction.

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If you knew this
angle was 30 degrees

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from the last situation,
you could figure out

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this coefficient of
kinetic friction.

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And what's cool
about this is this

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is in general going to be true
for any two materials that

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are like this.

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So maybe this is a
certain type of wood

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on a certain type of wood, or
a certain type of sandpaper

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on a certain type of sandpaper--
whatever you're talking about.

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And then you can use
that to make predictions

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if the incline was different,
or if the mass was different,

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or even if you were
on a different planet,

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or if someone was pressing
down on this block.

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That would change
the normal force.

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So given this right
here, let's figure out--

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for the sake of doing it-- the
coefficient of kinetic friction

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here.

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The force of friction
here, completely offsetting

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the parallel force of gravity
parallel to the surface,

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is 49 newtons.

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And the normal force
here, the force

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of contact between these
two things, this block

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and this wedge, is 49
square roots of 3 newtons.

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So we get 1 over the
square root of 3.

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And let me get
the calculator out

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to get an actual number here.

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So we have 1 divided by
the square root of 3,

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which gives us 0.5--
I'll just round.

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0.58.

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It is equal to 0.58.

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And there's no units here,
because the units cancel out.

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It's a unit-less measurement.

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Now the interesting thing here
is that the way I've set up

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this problem, the coefficient
of kinetic friction

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is lower, if we assume
the same materials,

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than the coefficient
of static friction was.

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And for some materials, they
might not be that different.

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But for other materials,
the kinetic friction

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can be lower than
static friction.

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You never see a situation
where the coefficient

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00:08:22,894 --> 00:08:24,810
of static friction-- at
least that I know of--

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is lower than kinetic friction.

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00:08:26,101 --> 00:08:28,880
But you do see situations
where the coefficient

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of kinetic friction is
lower than the coefficient

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00:08:33,240 --> 00:08:34,780
of static friction.

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Once something is
moving, for some reason--

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00:08:39,860 --> 00:08:43,650
and we'll theorize why
that might be-- friction

202
00:08:43,650 --> 00:08:47,007
is a little less potent than
when something is stationary.

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00:08:47,007 --> 00:08:49,090
So we can say this generally,
that the coefficient

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00:08:49,090 --> 00:08:54,890
of kinetic friction is less
than or equal to the coefficient

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of static friction.

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It's a little bit easier, or
friction provides a little less

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than or equal to the force
when something's moving

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than when something
is stationary.

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So I'll think about
that a little bit deeper

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in the next video.