Friction is the force that two surfaces exert on each other when they rub against each other. Some Causes of Friction are: the strength of the force of friction depends on two factors: how hard the surfaces push together and the types of surfaces involved. Smooth surfaces have less friction; bumpy surfaces have more. Friction acts in a direction opposite to the direction of the object’s motion. Because of static friction, you must use more force to start the motion of stationary objects.