Question

What causes friction?
(A). The force applied
(B). Irregularities on the two surfaces in contact
(C). The difference in the weight
(D). All of the above

Answer 1

Hint: Friction is a resistive force which resists relative motion between surfaces in contact with each other. Friction is caused due to the irregularities of the surfaces in contact. The coefficient of friction depends on both the surfaces in contact. It is not a characteristic property of any substance.

Complete Step By Step Solution:
Friction is the resistive force which originates from relative sliding between two surfaces in contact. There are two main reasons of friction roughness of the surface and molecular adhesion
Molecular adhesion is the attraction between the molecules of two different surfaces. These molecules form weak bonds with each other. When the two surfaces are moved against each other these bonds resist the motion creating friction.
Roughness of surfaces is also a reason for friction. No matter how smooth a surface looks, it has some irregularities. When two surfaces slide against each other these irregularities create collision with each other obstructing the movement, thus causing friction.
So surfaces with more roughness or more irregular surfaces have high friction coefficient as compared to that of less rough surfaces.
Hence the correct option is B.

Note: The force applied on a body does not affect the friction coefficient of the two surfaces in contact. It can change the magnitude of friction force, but cannot alter the friction coefficient. Roughness of the surface does not affect the friction coefficient significantly until the surface is highly rough to cause abrasion.