Static and Kinetic Friction
Suppose that a horizontal force F is applied to a block resting on a rough surface (see Figure 6.1).
As long as the applied force F is less than a certain maximum force (F
max
), the block will not
move. This means that the net force on the block in the horizontal direction is zero. Therefore,
besides the applied force F, there must be a second force f acting on the block. The force f must
have a strength equal to F, and it must be pointing in the opposite direction. This force f is called
the friction force, and because the block does not move, we are dealing with
static friction
.
Experiments have shown that the force of static friction is largely independent of the area of
contact and proportional to the normal force N acting between the block and the surface. The
static friction force is
f <= u
s
N
where u
s
is the
coefficient of static friction
(which is dimensionless). The coefficient of static
friction is approximately constant (independent of N). The maximum force that can be applied
without moving the block is
F
max
= u
s
N
Once the block has been set in motion, the force F needed to keep it in motion with a constant
velocity is usually less than the critical force needed to get the motion started. In this situation we
are dealing with
kinetic friction
and the friction force f
k
is given by
f
k
= u
k
N
where u
k
is the
coefficient of kinetic friction
. The kinetic friction force is independent of the
applied force, but always points in the opposite direction. The equation for f
k
is
not a vector
equation
since f
k
and N do not point in the same direction.
Note:
The friction between car tires and the road is static friction. This friction is crucial when
you try to stop a car. Since the maximum static friction force is larger than the kinetic friction
force, a car can be stopped fastest if we prevent the wheels from locking up. When the wheels
lock up, the friction force is changed to kinetic friction (the tires and the ground are moving with
respect to each other) thereby reducing the acceleration and increasing the time and length
required to bring the car to a halt.
Sample Problem 6-1

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