6.2    Properties of Friction

Experiments show that friction has 3 properties:

1.      If the body does not move, then the static frictional force equals the applied force.

2.      The magnitude of  has its maximum value f_s,max that is related to the magnitude of the normal force  of the body on the surface:  , m_s is the coefficient of static friction for the two surfaces that are in contact.

Script by Dr. Mohamed S. Kariapper using Physlets from

3.      When the applied force  exceeds f_s,max, the body begins to slide. We have accelerated motion toward the left as seen in the above figure. When sliding the magnitude of the frictional force rapidly decreases to a value f_k given by .  is the force of kinetic friction and m_k is the coefficient of kinetic friction. m_s and m_k are property of the two surfaces that are in contact.