Laws of Sliding Friction Unlubricated Surfaces

Laws of Sliding Friction Unlubricated Surfaces

Dry or unlubricated surfaces. Three laws govern the relationship between the frictional force f and the load or weight L of the sliding object for unlubricated or dry surfaces:

(a) For low pressures (normal force per unit area) the friction force is directly proportional to the normal load between the two surfaces. As the pressure increases, the friction does not rise proportionally; but when the pressure become abnormally high, the friction increases at a rapid rate until seizing takes place.

(b) The value of f/L is defined as the coefficient of friction . The friction both in its total amount and its coefficient is independent of the area of contact, so long as the normal force remains the same. This is true for moderate pressures only. For high pressures, this law is modified in the same way as the first case.

(c) At very low velocities, the friction force is independent of the velocity of rubbing. As the velocities increase, the friction decreases.

The third law (c) implies that the force required to set a body in motion is the same as the force required to keep it in motion, but this is not true. Once a body is in motion, the force required to maintain motion is less than the force required to initiate motion and there is some dependency on velocity. These facts reveal two categories of friction: static and kinetic. Static friction is the force required to initiate motion (F_s). Kinetic or dynamic friction is the force required to maintain motion (F_k).