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Static and kinetic friction
If you have pushed a refrigerator across a kitchen floor, you have felt a
certain series of sensations. At first, you gradually increased your force on
the refrigerator, but it didn’t move. Finally, you supplied enough force to
unstick the fridge, and there was a sudden jerk as the fridge started moving.
Once the fridge is unstuck, you can reduce your force significantly and still
keep it moving.
While you were gradually increasing your force, the floor’s frictional
force on the fridge increased in response. The two forces on the fridge
canceled, and the fridge didn’t accelerate. How did the floor know how to
respond with just the right amount of force. The figures on the left show
one possible model of friction that explains this behavior. (A scientific model
is a description that we expect to be incomplete, approximate, or unrealistic
in some ways, but that nevertheless succeeds in explaining a variety of
phenomena.) Figure (a) shows a microscopic view of the tiny bumps and
holes in the surfaces of the floor and the refrigerator. The weight of the
fridge presses the two surfaces together, and some of the bumps in one
surface will settle as deeply as possible into some of the holes in the other
surface. In figure (b), your leftward force on the fridge has caused it to ride
up a little higher on the bump in the floor labeled with a small arrow. Still
more force is needed to get the fridge over the bump and allow it to start
moving. Of course, this is occurring simultaneously at millions of places on
the two surfaces.
Once you had gotten the fridge moving at constant speed, you found
that you needed to exert less force on it. Since zero total force is needed to
make an object move with constant velocity, the floor’s rightward frictional
force on the fridge has apparently decreased somewhat, making it easier for
you to cancel it out. Our model also gives a plausible explanation for this
fact: as the surfaces slide past each other, they don’t have time to settle down
and mesh with one another, so there is less friction.
Even though this model is intuitively appealing and fairly successful, it
should not be taken too seriously, and in some situations it is misleading.
For instance, fancy racing bikes these days are made with smooth tires that
have no tread — contrary to what we’d expect from our model, this does
not cause any decrease in friction. Machinists know that two very smooth
and clean metal surfaces may stick to each other firmly and be very difficult
to slide apart. This cannot be explained in our model, but makes more sense
in terms of a model in which friction is described as arising from chemical
bonds between the atoms of the two surfaces at their points of contact: very
flat surfaces allow more atoms to come in contact.
Since friction changes its behavior dramatically once the surfaces come
unstuck, we define two separate types of frictional forces. Static friction is
friction that occurs between surfaces that are not slipping over each other.
Slipping surfaces experience kinetic friction. “Kinetic” means having to do
with motion. The forces of static and kinetic friction, notated F
s
and F
k
, are
always parallel to the surface of contact between the two objects.
A model that correctly explains many
properties of friction. The microscopic
bumps and holes in two surfaces dig
into each other, causing a frictional
force.
(a)
(b)
force
Static friction: the tray doesn’t slip on
the waiter’s fingers.
Kinetic friction: the car skids.
Section 5.2Classification and Behavior of Forces