I mentioned in the
last several videos that the coefficient of kinetic
friction tends to be less, sometimes it'll be
roughly equal to, the coefficient of
static friction. But this might lead
you to-- at least, a question that
I've had in my mind, and I still have to
some degree-- is why? Why is the coefficient of
kinetic friction lower? Or why can it be lower? And the current best theory--
one I can visualize in my head, and based on the
reading that I've done-- is the difference between-- so
let's think about it this way. So if we look at it at a kind
of a regular human level, maybe we have a block. So this is the static case. So let's think about
the static case. Let me draw it like this. So I'll draw the
static case over here. So I have a block that
is stationary on top of-- let me do the surface
in a different color-- on top of some type of surface
right over here. And over here, I'm going
to have a block moving at a constant velocity
relative to some surface, relative to the same surface. And so let me draw it out. So this is moving at
some constant velocity. And so the
interesting thing here is, assuming that these are
the same masses, that these are the same surfaces,
is, why should the coefficient of
friction here-- why should the coefficient of static
friction-- so here, since this is stationary,
what's under play is the coefficient of
static friction. Why should that be larger
than the coefficient of kinetic friction over here? Why should that be large
than the coefficient of kinetic friction? Or another way to
think about it is, you would need to apply
more force to overcome the static friction here, and
start to get this accelerating, than you would need to
apply to get this already moving body to accelerate. Because there would be kind of
a less of a responsive friction force. So let's think about
that a little bit. So what I'm going to do is
zoom in into the atomic level. And so when you zoom
in to the atomic level, almost nothing is
completely smooth. So the surface over here might
look something like this. So I'm going to draw the
molecules that make up the surface, the
best to my ability. So the molecules, when
you zoom up really close for the surface, might
look something like this. So we're really zooming
into the atomic level, unimaginably small level. Much smaller than
that box I just drew. But I'm just trying
to look at what's happening with the atoms
where they contact, or the molecules
where they contact? And the box's molecules might
look something like this. They aren't completely smooth. And hopefully this
video also emphasizes that all of these forces and
all of this contact that we're talking about in these
videos-- and it's actually interesting
philosophically-- nothing is ever really in
contact with each other. You really just have atoms
that are repulsing each other, because their electrons
the electromagnetic force of repulsion between
them is not allowing them to get any closer together. So that's all-- when
you push something, it's just the electrons in your
hand pushing on the electron-- or the electronic clouds in your
hand pushing on the electron clouds of, say, the
pen you're holding, or the key on your
keyboard, or the mug, so that it repulses
it and causes it to go in the other direction. So there's never
any of this thing like, what we imagine in
our heads, real contact. And if you really want
to blow your mind-- and watch the chemistry
videos if you want understand this-- is
that most of these atoms are actually free
spaced themselves. That the electron
cloud-- or I guess where most of the probability
of finding the electron-- is huge compared to the
size of the electron, or the size of the nucleus. So it's kind of just a
lot of free space pushing on a lot of other free space
through the electromagnetic force. But anyway, we're talking
about friction here. So if you were to really zoom
in here, when this thing is stationary, the surfaces
aren't actually even. And so you could imagine that
these molecules that you have, sometimes when it's
sitting stationary, they might be kind of
fit into each other. They've kind of slid in to
maybe these little ruts here and there. And so if you're trying
to move this object, if you're trying to accelerate
it to the left with some force, you have to overcome,
essentially, either-- for example,
this part right over here either has to somehow break
off, or the whole thing has to be shifted up a couple
of atoms or a couple molecules. Or maybe, this part over
here has to be broken off, or has to be shifted
down one atom-- you wouldn't notice these things. You wouldn't notice the
shifting of a block, or the shifting of the floor. You wouldn't notice it by
the width of a molecule, or diameter of an
atom or molecule. But that's essentially what
you're going to have to do. Or you have to rip
them off entirely in order to start
this thing moving. Once something is
already moving-- and this is at least
how I think about it-- it doesn't have a chance to
settle into these little ruts. So let me draw something
that's already moving. And I'll try to draw
a similar surface. So I'm trying to
draw the surface that looks, essentially, just
like the one I drew. So maybe it looks like that. This is supposed to
be the same surface. But once it's moving, it's not
sitting in these ruts anymore. The whole thing is moving. So it's kind of
sliding across the top. And so now it might look
something like this. I'll try my best to draw it. Maybe this has been
shifted up a little bit so that it could start sliding. You've overcame the
static friction. So now it is-- I'm trying to
draw the same surface here, give or take-- so
now it's moving. It doesn't have a chance
to really settle in. It has to kind of
bounce along the top. And so that's the
best understanding. And so the real force
of the friction here, as it's moving along it still
might every now and then bounce into a little
ruts here and there. But you also have any type
of chemical bonds that form between the
atoms temporarily that keep breaking and forming. And in order to keep this
thing, I guess, moving, or especially if you
want to accelerate it, you're going to have to
keep breaking these bonds. And so that's essentially
the force of friction that you're overcoming. Here you might have
those same bonds. And not only do you
have the same bonds, but you also have to
overcome these ruts, or these little ragged
parts that have a time to settle in to
these little nooks that you have to
overcome even more. So that's the intuition. And you know this is actually
still an area of research. So it's not like this
cut and dry thing. And it's a fun thing
to think about what's happening at the atomic level. But this is the
general intuition of why the coefficient
of static friction is higher than the coefficient
of kinetic friction.