Coefficient of Static and Kinetics Friction

The static friction coefficient  ( μ ) between two solid surfaces is defined as the ratio of the tangential force (F) required to produce sliding divided by the normal force between the surfaces (N)

μ = F /N

For a horizontal surface the horizontal force (F) to move a solid resting on a flat surface

F= μ x mass of solid x g.

If a body rests on an incline plane the body is prevented from sliding down because of the frictional resistance. If the angle of the plane is increased there will be an angle at which the body begins to slide down the plane. This is the angle of repose and the tangent of this angle is the same as the coefficient of friction. Coefficient of kinetic friction

When the tangential force F overcomes the frictional force between two surfaces then the surfaces begins to slide relative to each other. In the case of a body resting on a flat surface the body starts to move. The sliding frictional resistance is normally different to the static frictional resistance. The coefficient of sliding friction is expressed using the same formula as the static coefficient and is generally lower than the static coefficient of friction.

Coefficient of friction, friction coefficient or frictional coefficient, as it is also known, cannot be determined by calculations, but by experiment.

Its values depend on the materials in contact with each other, and it ranges from near zero to above one. If coefficient of friction is zero, it means there is no friction existing between the surfaces. However, this is only a hypothetical value, since no surfaces in contact has been found to be frictionless.

The higher the value of coefficient of friction, the greater the frictional force acting between the surfaces, and this means a higher force will be needed to slid one of them over the other.

In comparison, the coefficient of static friction for two particular surfaces is usually found to be higher than their coefficient of kinetic friction. This goes with the fact that static friction is higher than kinetic friction, however, there are certain surfaces whose coefficient of static and kinetic friction are the same. Examples of these kinds of surfaces include Teflon on Teflon surface.

The coefficient of friction, μ , for some surfaces are given below:

• Wood on wood --- 0.3 - 0.5
• Wood on metal --- 0.6
• Metal on metal --- 0.15 - 0.2
• Metal on greased metal --- 0.1
• Wood on stone --- 0.6 - 0.7