Summary of Friction
SUMMARY OF FRICTION
The maximum force of static friction that exists between two surfaces
is proportional to the normal force and mostly independent of area of
contact. This situation is shown here: N = the total normal force
(force perpendicular to the horizontal surface) which is essentially
the weight of the object. The coefficient in that equation is called
the coeffecient of static friction and that depends on the material:
When the object is actually moving, the friction is said to be kinetic
friction which is generally less than static friction.
If we add more mass we increase the normal force (N) (because the weight
has increased) and hence we have increased the total frictional force.
This is shown here where it can be seen that twice as much force must
be applied to move two bricks instead of one (the force meter reads
twice as large).
In general frictional forces are independent of the area of contact
although this is an empirical observation not a theory.
Consider a metallic brick and a metallic table. The reason that friction is nearly independent of surface area is
if the "microscopic" area of contact of the brick to the table is
independent of the orientation of the brick. If this is not the case,
then friction will have a small dependence on area. In normal
circumstances, with the largest surface area of the brick in contact
with the table there are a large number of "contact" points that
support the load. With the smallest area in contact (brick standing
on end) there are fewer contacts but as long as the area of each
contact is larger due to the higher pressure (same force, smaller unit
area) then there will be no difference in the amount of static friction.
Over wide limits, most materials follow this and hence friction is
largely independent of surface area. iF you have a situation where
the microscopic contact area does not scale in accordance with the
pressure, then static friction will depend upon orientation.
The following example (which we will do in class) demonstrates the
general rule that friction is not dependent on total area of contact
Try the JAVA Friction Experiment: (not complete yet)
Although their are 5 symbols only three of them are active:
You will either get:
- a square block of mass = 100 grams
- square block of mass = 200 grams
- a rectangular block of mass = 200 grams
You may adjust the angle of the incline. The four vectors that
represent the gravitional force (red), the normal force (blue),
the frictional force (green) which opposes the motoin, and the
component of the gravitational force (white) which is trying
to pull the block down. You can adjust the angle such that the
green vector is bigger than the white vector and therefore
nothing will happen. After you have set the angle, hit the
start button and watch the block slide down the incline and watch
how the vector forces change slightly. The simulation runs for
10 seconds; this may not be long enough to cause the block to
completely slide down.