When a body is moving on a surface, the force of friction is called

The correct answer is Dynamic friction.

Friction is a contact force that opposes the relative motion (or tendency of motion) between two surfaces in contact. Depending on the state of motion of the body, friction is classified into different types. Let us understand each type clearly:

Types of Friction:

1. Static Friction:
This is the friction that acts on a body when it is at rest (stationary) and an external force is trying to move it. Static friction acts as long as the body has not yet started moving. It adjusts itself to exactly balance the applied force, up to a maximum limit.

2. Limiting Friction:
This is the maximum value of static friction. It is the friction force at the instant the body is just about to start moving — the critical threshold between rest and motion. Once the applied force exceeds limiting friction, the body starts to move.

3. Dynamic Friction (Kinetic Friction):
This is the friction that acts on a body when it is already in motion — i.e., sliding or moving on a surface. It is also called kinetic friction or sliding friction. Dynamic friction is generally less than limiting (maximum static) friction, which is why it is easier to keep an object moving than to start moving it from rest.

4. Rolling Friction:
This acts when a body rolls (like a wheel or ball) over a surface, rather than sliding. Rolling friction is much smaller in magnitude compared to dynamic/sliding friction.

Why "Dynamic Friction" is the Correct Answer:

The question states that the body is already moving on a surface. The word "moving" is the key. Since the body is in motion (not at rest, not on the verge of moving, and not rolling), the friction acting on it is specifically called Dynamic friction (also known as kinetic or sliding friction).

It opposes the ongoing motion of the body and acts in the direction opposite to the velocity of the sliding body. Its magnitude is given by:

f_k={\mu }_k\times N

where μk is the coefficient of kinetic (dynamic) friction and N is the normal reaction force from the surface.

Summary Table:

Therefore, when a body is moving on a surface, the force of friction acting on it is called Dynamic friction.