10.2 Features of friction force

Friction force differs to other forces in many respects. Importantly, friction adjusts to the way external force is applied on the body. This characteristic provides opportunity to manage this force that minimizes our effort to produce acceleration in bodies. At the same time, friction prohibits some geometric restriction on the way we manage external force to produce acceleration. For example, we can not push an object unless external force is applied outside a region called "cone of friction".

Besides, there are certain misplaced notion about friction that it is a negative force, which can not produce acceleration or which can not do positive work. It is generally believed that friction only moderates the acceleration and work of other external forces negatively. As a matter of fact, we shall find that it is actually the friction, which is ultimately driving our car on the road. We shall, however, save for the details on this aspect till we take up the study of rolling motion. We shall, however, make the beginning in this module to answer this question at a basic level.

Can friction cause motion?

To investigate this question, let us consider the combined motion of two blocks system, which is being pulled with an external force such that two blocks move together as shown in the figure. Let us also consider that the block system lies on a smooth horizontal surface, enabling us to neglect friction between block “A” and the underlying horizontal surface.

The external forces on the two blocks are shown in the force diagrams in the figure below as seen from the ground reference. The upper block “B” is moving along with lower block “A” due to the friction applied by it.

The friction forces at the interface, which act on two bodies, are action and reaction pair and are equal in magnitude and opposite in direction, in accordance with Newton’s third law of motion . We note that friction on block “B” due to “A” is in the same direction as that of external force “F”. The only external force on block “B”, as seen from the ground, is the friction force, $f_s$ .

The accelerations of the composite block system and individual blocks are same as considered in the beginning. From three corresponding force diagrams, including the one corresponding to composite block system, we have three expressions for the common acceleration :

$$\begin{array}{l}a=\frac{F}{m_A+m_B}\\ a=\frac{F-f_s}{m_A}\\ a=\frac{f_s}{m_B}\end{array}$$

From the last equation, it is clear that static friction, as a matter of fact, caused the motion of block “B” with respect to ground by producing acceleration in the body. Here, measurement of motion (acceleration) is in a frame different to one in which the surface in contact lies. The block is accelerated with respect to ground – not with respect to the reference of block “A”, which applies friction force.

Thus, we see that friction is capable to produce acceleration in a body. It is expected also. This is the very nature of a force. Since friction is a force, it is capable to produce acceleration and do positive work. There is, however, a catch. Friction exists only due to other external forces. It does not exist, if there is no external force on the body. But as far as the question "can friction cause motion?. Answer is yes.