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I mentioned in the
last several videos

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that the coefficient of kinetic
friction tends to be less,

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sometimes it'll be
roughly equal to,

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the coefficient of
static friction.

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But this might lead
you to-- at least,

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a question that
I've had in my mind,

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and I still have to
some degree-- is why?

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Why is the coefficient of
kinetic friction lower?

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Or why can it be lower?

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And the current best theory--
one I can visualize in my head,

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and based on the
reading that I've done--

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is the difference between-- so
let's think about it this way.

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So if we look at it at a kind
of a regular human level,

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maybe we have a block.

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So this is the static case.

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So let's think about
the static case.

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Let me draw it like this.

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So I'll draw the
static case over here.

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So I have a block that
is stationary on top

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of-- let me do the surface
in a different color-- on top

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of some type of surface
right over here.

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And over here, I'm going
to have a block moving

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at a constant velocity
relative to some surface,

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relative to the same surface.

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And so let me draw it out.

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So this is moving at
some constant velocity.

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And so the
interesting thing here

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is, assuming that these are
the same masses, that these are

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the same surfaces,
is, why should

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the coefficient of
friction here-- why

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should the coefficient of static
friction-- so here, since this

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is stationary,
what's under play is

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the coefficient of
static friction.

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Why should that be larger
than the coefficient

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of kinetic friction over here?

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Why should that be large
than the coefficient

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of kinetic friction?

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Or another way to
think about it is,

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you would need to apply
more force to overcome

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the static friction here, and
start to get this accelerating,

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than you would need to
apply to get this already

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moving body to accelerate.

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Because there would be kind of
a less of a responsive friction

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force.

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So let's think about
that a little bit.

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So what I'm going to do is
zoom in into the atomic level.

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And so when you zoom
in to the atomic level,

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almost nothing is
completely smooth.

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So the surface over here might
look something like this.

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So I'm going to draw the
molecules that make up

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the surface, the
best to my ability.

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So the molecules, when
you zoom up really close

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for the surface, might
look something like this.

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So we're really zooming
into the atomic level,

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unimaginably small level.

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Much smaller than
that box I just drew.

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But I'm just trying
to look at what's

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happening with the atoms
where they contact,

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or the molecules
where they contact?

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And the box's molecules might
look something like this.

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They aren't completely smooth.

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And hopefully this
video also emphasizes

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that all of these forces and
all of this contact that we're

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talking about in these
videos-- and it's actually

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interesting
philosophically-- nothing

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is ever really in
contact with each other.

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You really just have atoms
that are repulsing each other,

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because their electrons
the electromagnetic force

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of repulsion between
them is not allowing

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them to get any closer together.

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So that's all-- when
you push something,

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it's just the electrons in your
hand pushing on the electron--

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or the electronic clouds in your
hand pushing on the electron

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clouds of, say, the
pen you're holding,

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or the key on your
keyboard, or the mug,

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so that it repulses
it and causes

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it to go in the other direction.

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So there's never
any of this thing

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like, what we imagine in
our heads, real contact.

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And if you really want
to blow your mind--

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and watch the chemistry
videos if you want

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understand this-- is
that most of these atoms

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are actually free
spaced themselves.

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That the electron
cloud-- or I guess

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where most of the probability
of finding the electron--

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is huge compared to the
size of the electron,

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or the size of the nucleus.

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So it's kind of just a
lot of free space pushing

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on a lot of other free space
through the electromagnetic

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force.

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But anyway, we're talking
about friction here.

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So if you were to really zoom
in here, when this thing is

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stationary, the surfaces
aren't actually even.

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And so you could imagine that
these molecules that you have,

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sometimes when it's
sitting stationary,

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they might be kind of
fit into each other.

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They've kind of slid in to
maybe these little ruts here

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and there.

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And so if you're trying
to move this object,

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if you're trying to accelerate
it to the left with some force,

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you have to overcome,
essentially,

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either-- for example,
this part right over here

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either has to somehow break
off, or the whole thing

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has to be shifted up a couple
of atoms or a couple molecules.

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Or maybe, this part over
here has to be broken off,

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or has to be shifted
down one atom-- you

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wouldn't notice these things.

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You wouldn't notice the
shifting of a block,

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or the shifting of the floor.

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You wouldn't notice it by
the width of a molecule,

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or diameter of an
atom or molecule.

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But that's essentially what
you're going to have to do.

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Or you have to rip
them off entirely

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in order to start
this thing moving.

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Once something is
already moving--

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and this is at least
how I think about it--

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it doesn't have a chance to
settle into these little ruts.

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So let me draw something
that's already moving.

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And I'll try to draw
a similar surface.

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So I'm trying to
draw the surface that

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looks, essentially, just
like the one I drew.

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So maybe it looks like that.

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This is supposed to
be the same surface.

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But once it's moving, it's not
sitting in these ruts anymore.

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The whole thing is moving.

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So it's kind of
sliding across the top.

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And so now it might look
something like this.

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I'll try my best to draw it.

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Maybe this has been
shifted up a little bit

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so that it could start sliding.

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You've overcame the
static friction.

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So now it is-- I'm trying to
draw the same surface here,

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give or take-- so
now it's moving.

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It doesn't have a chance
to really settle in.

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It has to kind of
bounce along the top.

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And so that's the
best understanding.

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And so the real force
of the friction here,

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as it's moving along it still
might every now and then

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bounce into a little
ruts here and there.

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But you also have any type
of chemical bonds that

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form between the
atoms temporarily

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that keep breaking and forming.

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And in order to keep this
thing, I guess, moving,

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or especially if you
want to accelerate it,

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you're going to have to
keep breaking these bonds.

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And so that's essentially
the force of friction

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that you're overcoming.

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Here you might have
those same bonds.

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And not only do you
have the same bonds,

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but you also have to
overcome these ruts,

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or these little ragged
parts that have a time

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to settle in to
these little nooks

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that you have to
overcome even more.

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So that's the intuition.

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And you know this is actually
still an area of research.

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So it's not like this
cut and dry thing.

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And it's a fun thing
to think about what's

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happening at the atomic level.

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But this is the
general intuition

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of why the coefficient
of static friction

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is higher than the coefficient
of kinetic friction.