Question

Friction arises due to the interlocking of hills and ________ present in a body.
A. Mountains
B. Holes
C. Valleys
D. None

Answer 1

Hint: Friction is a contact force. It tries to oppose relative motion between two surfaces. It arises when the uneven surfaces of two materials get interlocked with each other due to the interlocking of the tiny bulging out and caving in parts of the uneven surfaces.

Complete step-by-step answer:
The frictional force is a contact surface force which comes into effect whenever there is a tendency, or occurrence of relative motion between two surfaces. Friction is a resistive force and dissipates energy of the bodies in motion through the generation of heat.
The below figure will give a better insight to the mechanism of frictional force.

Even though surfaces of objects may seem smooth at a macroscopic level, at a microscopic level, all surfaces are even and have bulging out parts (hills) and caving in parts (valleys).
When there is relative motion between two surfaces, the hills and valleys get interlocked and try to get joined together. This produces an opposition to the relative motion and is the root cause of the frictional force.
To overcome these forces and continue motion, the interlocking between the hills and the valleys has to be broken for which energy is required. This energy is converted into heat and that is why it is said that friction dissipates energy in the form of heat.
Hence, the correct option is C) Valleys.

Note: The frictional force depends on the coefficient of friction$\left( \mu \right)$. This is a measure of the unevenness and interlocking ability of the surfaces. A surface with a higher coefficient of friction is generally rougher and interlocks with other surfaces to a greater extent, thus producing a stronger frictional force.
Usually the coefficient of friction is between 0 and 1, but can be greater than 1.