Question

The force of friction between two bodies is
A. Parallel to the contact surface
B. Perpendicular to the contact surface
C. Inclined at $30^\circ$ to the contact surface
D. none of these

Answer 1

The force generated by two surfaces that contact and slide against each other is called frictional force. It resists the relative motion between the bodies. Friction is the reason why we can walk, run and why bodies don’t slide over each other all the time.

Complete step by step answer:
The property by virtue of which a resisting force is created between two rough bodies which resist the sliding of one body over another is known as friction. The force of friction or frictional force always opposes the motion of one body over the other body in contact with it. Frictional force operates between:
a) two solids in contact
b) a solid and a fluid
c) two fluids.
Frictional force always acts parallel to two planes in contact with each other and in a direction opposite to that of relative motion of the two bodies.

Thus, the correct answer is option A.

Note: The surface of any given body is never perfectly smooth. Even a polished surface which appears smooth to the naked eye shows irregularities when observed under a microscope. When one body is placed in contact with another body, a large number of irregularities interlock among themselves. Therefore, they get cold-welded due to high pressure at sharp points. This phenomenon is called surface adhesion. It is the primary cause of friction.