The coefficient of friction depends on the material and roughness of the two surfaces in contact.

In general μs (coefficient of static friction) is slightly greater than μk (coefficient of kinetic friction).

As long as the normal force is constant, the maximum possible friction does not depend on the area of the surfaces in contact.

Law of limiting friction for dry surfaces:

• When two bodies are in contact, the direction of the force of friction on one of them is opposite to the direction in which this body has the tendency to move.
• When one body is just on the point of sliding over the other body, the maximum force of friction is being exerted; This is called limiting friction.
• The force of friction is dependent upon the types of materials of the two bodies in contact. The coefficient of friction depends on the material and roughness of the two surfaces in contact.
• The limiting frictional force is proportional to the normal force that acts between the bodies in contact.
• The ratio of frictional force to the normal reaction is constant and is known as the coefficient of friction i.e. μ = F/N.
• Limiting static friction is somewhat greater than kinetic friction.
• The kinetic friction is independent of the relative velocity of the bodies in contact.