We considered a case
where an object is moving
along the surface, but now
let's consider what happens,
that if we apply a force here,
and the object stays at rest.
Well, we still have
the normal force
distributed over the surface.
But now because the
object is at rest,
there is another kind
the contact force now
is what we call static friction,
the tangential part of that.
And the contact force is
still the perpendicular part,
the normal force, plus
the tangential part,
which we refer to
as static friction.
Now what happens
with static friction
is that if we don't apply
a force at all, then
there is no static friction.
In fact, you can see
in this simulation,
that if we slowly increase
the applied force,
then the static friction
force gets bigger and bigger.
It depends on what we're
doing to the system.
But at a certain point, when we
apply a large enough external
force, the object
suddenly starts to slip.
That's what we call the
just-slipping condition.
And the reason for
that is static friction
has reached its
maximum possible value.
That has to do with the
physics of the interaction
between the surface
and the object.
So also, there's a
very subtle thing,
is that as you see when we pull
the force and it just slips,
suddenly the friction
has gone from static
to kinetic friction, and it's
gotten a little bit smaller.
How do we express
those properties?
Well, what we have is the idea
that the static friction can
vary in magnitude between 0
and some type of maximum value,
depending on the other
constraints on the system.
And the maximum value
of static friction,
we have a force law,
which is that it's, again,
proportional to
the normal force,
but the coefficient is
called now the coefficient
of static friction.
And this coefficient
of static friction
is slightly bigger
than the coefficient
of kinetic friction,
which explains
why that arrow got a little
bit smaller when it just
started to slip.
Now again, like
kinetic friction,
static friction is opposing
this external force.
And depending on the direction
that the object moves,
for instance, if we
applied the external force
in the opposite direction,
here, then the static friction
that's distributed over the
surface is opposing that force.
But there may be many systems
in which we're actually not
quite sure which way the
static friction points.
And as the course
develops and we
look at more
complicated examples,
we'll see that the
direction of static friction
can depend on all the other
constraints on the system.
