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:

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.

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