can friction cause motion?.
Direction of friction is opposite to the component of net external force parallel to the contact surface. It is the criteria to decide the direction of friction. There are, however, situations in which external force may not be obvious. In the illustration of the above section, there is no external force on the upper block “B”. What would be the direction of friction on block “B”? As a matter of fact, it is the only force (we do not consider vertical normal force as it perpendicular to motion) on block acting parallel to contact surface. Clearly, we can not apply the criteria for determining direction in this case. We actually analyze the forces on the underneath block “A”. The net external force parallel to interface is acting towards right. It, then, follows that the friction on “A” is towards left. Taking cue from this revelation and using Newton’s third law of motion, we decide that the direction of friction on block “B” should be towards right as shown in the figure below. Figure
