Question

The contact force exerted by a body A on another body B is equal to the normal force between the bodies. We conclude that:
A. The surfaces must be frictionless
B. The force of friction between the bodies is zero
C. The magnitude of normal force equals that of friction
D. The bodies may be rough but they don’t slip on each other

Answer 1

Hint:Contact Forces occur due to electromagnetic interaction between bodies at molecular level.

Complete step by step solution:

According to the question, contact force is equal to the normal force. For the body to remain in equilibrium, the net force acting on the body must be zero. Hence, we can conclude that the force of friction acting between the bodies is zero.

Hence Option (B) is correct.

But friction force is equal to zero does not imply that the surface is frictionless. Because the primary function of friction is to decrease relative motion between bodies. So, if the relative motion between the bodies is already zero, then friction will not act on the body.
Hence Option (A) is not necessarily true.

Earlier, we concluded that friction is zero. But from the diagram, we can see that Normal Force is not equal to zero. Hence, we can conclude that Normal Force is not equal to Friction.

Hence Option (C) is incorrect.

Since, we saw that frictional force between the surfaces is zero but the surfaces are not necessarily frictionless. This clearly implies that there is no relative motion between the surfaces. Hence they do not slip on each other.

Hence Option (D) is correct.

Finally, Options (B and D) are correct.

Note: Looks are deceptive! This question may seem pretty easy and straightforward but one need in dept conceptual knowledge of classical mechanics in order to solve such questions.