Question

The force of friction is ____ the normal reaction between the surfaces.
(A) Inversely proportional to
(B) directly proportional to
(C) proportional to the square of
(D) independent of

Answer 1

Hint
When two bodies are in contact with each other, there is a contact force working between them. This force can be resolved into two components, the normal reaction force and the frictional force, where the frictional force is given by, $f=\mu N$ where $\mu$ is coefficient of friction, and $N$ is normal reaction force.

Complete step by step answer
When we have two bodies that are in contact with each other, then there acts a force in between them. Since this force is acting due to the contact between the two bodies, it is termed as the contact force. This contact force can be resolved in two components. The vertical component of the contact force is known as the normal reaction force and the horizontal component is termed as the frictional force. The normal reaction force is that which acts perpendicular to the surfaces in contact. And the frictional force is that which resists the motion between two surfaces.
The frictional force can be obtained from the formula $f=\mu N$ where $\mu$ is known as the coefficient of friction and depends on the two objects that are causing friction.
So from this formula, since $\mu$ is a constant, we can remove it for the proportionality as
$f\propto N$.
Therefore, we can say that the frictional force is directly proportional to the normal reaction force.
Hence, the correct answer is option (B).

Note
When two bodies are placed in contact, attractive forces act between their particles at the surface of contact. As a result, each body exerts a contact force on the other body. These mutual forces are equal and opposite by obeying Newton’s third law. A component of the contact force is called the frictional force.