Question

Frictional force exists in which of the following cases?
A. Exists only for moving bodies.
B. Exists only for bodies which are at rest.
C. Exists for both moving and non-moving bodies
D. None of these

Answer 1

Hint: When two bodies are in contact with each other, they exert a force on one another hence affecting the motion of each other. In order to work for the solution of the above question, we first need to know what actual friction is and how it acts so as to affect the motion of a body.

Complete answer:
We must know that when two bodies are in close contact to each other, their relative motion is resisted by a force coming into action because of the contact between the two bodies. This force is known as friction. The force of friction is always directed in a direction opposite to the direction of motion of the object.

Classification of frictional force:
Depending upon the relative motion of the bodies in contact with each other, friction is divided into two categories, namely,
1. Static Friction - When the two bodies which are in contact with each other are not in relative motion with each other, the friction which comes into action is known as static friction. For example, static friction can resist a body from sloping down on an inclined plane.
2. Kinetic Friction - When the two bodies which are in contact with each other are in relative motion with each other, the friction which comes into action is known as kinetic friction. For example, kinetic friction comes into play when a block slides down an inclined plane.
3. Rolling Friction - The force resisting the motion of a body rolling on a surface is known as rolling friction or rolling resistance. For example, the friction which comes into play in the rolling of a ball or wheel on the ground.
4. Sliding Friction - The friction which comes into action when two bodies are sliding against each other is known as sliding friction. For example, a rock sliding down from a mountain.
There are a few factors affecting the frictional force, which are as follows,
a) The texture of the surface.
b) The angle and position of the object.

In order to have a clear idea of friction, we also need to know about a quantity known as the ‘coefficient of friction’. Coefficient of friction, generally denoted by µ, is a measure of the amount of friction existing between two bodies.

Mathematically, we can calculate friction as,
   ${{F}_{fric}}=\mu \times {{F}_{normal}}$
Where, ${{F}_{_{fric}}}$ is the frictional force to be calculated between the two bodies.
              $\mu $is the coefficient of friction.
${{F}_{normal}}$ is the normal force acting on the surface. The normal force is simply the support force exerted upon an object which remains in contact with another stable object.

So, in the above discussions, we have learnt what friction actually is and how we can determine friction. Now, a very common question which arises in our minds is whether frictional forces act on static bodies! A simple answer to this question is, ‘yes’ as we have seen the definition of static friction stating that there is a force of friction even when the two bodies in contact are not in relative motion with each other. And for bodies in motion, the frictional force always exists which resists smooth relative motion of the two bodies. Therefore, frictional forces act on both moving and non-moving bodies.

So, the correct answer is “Option C”.

Additional Information:
In this solution, we have discussed frictional forces in case of solids, but frictional forces also exist for fluids. It is the force which obstructs the flow of the fluid and it provides resistance between the two surfaces. If the resistance is high, then we classify the fluid to be highly viscous.

Note:
A body remains at rest because of the action of some other force in the horizontal direction which opposes any applied force, resulting in no net force on the body. This force is parallel to the surface of the body in contact with the table and is known as friction.