Laws of Friction

Coulomb in 1781 conducted several experiment on t he friction of plane on dry surface. On the basis of his experiments he stated some laws of friction which as follows –

1. Law of static friction
2. Law of dynamic or kinetic friction

Laws of static friction

1. Friction force always opposes the motion (or tendency of motion) of the body.
2. The magnitude of frictional force is exactly equal but opposite in direction to the applied force component in the direction of motion.
3. For two given surfaces maximum friction depends on R_n and is independent of the extent of area of surfaces in contacts.
4. The frictional force depends upon the roughness of the surfaces.
5. The dynamic frictional force is less than static frictional force.
6. Sliding friction is more than rolling friction.
7. Tangent of angle of friction is equal to coefficient of friction

Laws of dynamic friction

1. The frictional force always acts in direction, opposite to that in which the body is moving.
2. For moderate speeds, the force of friction remains fairly constant. But it decreases slightly, with increase of speed.
3. he magnitude of kinetic frictional force bears a constant ratio to the normal reaction R_n between the two surfaces. But this ratio is slightly less than that in case of limiting friction.

Friction force diagram