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We considered a case
where an object is moving

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along the surface, but now
let's consider what happens,

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that if we apply a force here,
and the object stays at rest.

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Well, we still have
the normal force

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distributed over the surface.

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But now because the
object is at rest,

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there is another kind
the contact force now

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is what we call static friction,
the tangential part of that.

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And the contact force is
still the perpendicular part,

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the normal force, plus
the tangential part,

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which we refer to
as static friction.

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Now what happens
with static friction

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is that if we don't apply
a force at all, then

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there is no static friction.

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In fact, you can see
in this simulation,

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that if we slowly increase
the applied force,

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then the static friction
force gets bigger and bigger.

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It depends on what we're
doing to the system.

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But at a certain point, when we
apply a large enough external

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force, the object
suddenly starts to slip.

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That's what we call the
just-slipping condition.

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And the reason for
that is static friction

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has reached its
maximum possible value.

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That has to do with the
physics of the interaction

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between the surface
and the object.

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So also, there's a
very subtle thing,

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is that as you see when we pull
the force and it just slips,

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suddenly the friction
has gone from static

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to kinetic friction, and it's
gotten a little bit smaller.

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How do we express
those properties?

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Well, what we have is the idea
that the static friction can

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vary in magnitude between 0
and some type of maximum value,

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depending on the other
constraints on the system.

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And the maximum value
of static friction,

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we have a force law,
which is that it's, again,

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proportional to
the normal force,

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but the coefficient is
called now the coefficient

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of static friction.

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And this coefficient
of static friction

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is slightly bigger
than the coefficient

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of kinetic friction,
which explains

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why that arrow got a little
bit smaller when it just

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started to slip.

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Now again, like
kinetic friction,

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static friction is opposing
this external force.

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And depending on the direction
that the object moves,

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for instance, if we
applied the external force

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in the opposite direction,
here, then the static friction

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that's distributed over the
surface is opposing that force.

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But there may be many systems
in which we're actually not

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quite sure which way the
static friction points.

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And as the course
develops and we

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look at more
complicated examples,

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we'll see that the
direction of static friction

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can depend on all the other
constraints on the system.