• 1) When the surfaces of two objects are in rough contact, and have a tendency to move relative to each other, equal and opposite frictional forces act, one on each of the objects, so as to oppose the potential movement.

• 2) Until it reaches its limiting value, the magnitude of the frictional force F is just sufficient to prevent motion.

• 3) When the limiting value is reached, F= uR, where R is the normal reaction between the surfaces and u is the coefficient of friction for those two surfaces.

• 4) For all rough contacts 0 < F < IiR.

• 5) If a contact is smooth a = 0.