Force is that external cause which when acts on a body changes or tries to change the initial state of rest or motion with uniform velocity of the body.

When a body travels along a circular path, its velocity changes continuously. Naturally an external force always acts on the body towards the Centre of the path.

The external force requited to maintain the circular motion of the body is called centripetal force.

If a body of mass *m* is moving on a circular path radius *R* with uniform speed *v*, then the required centripetal force, **F = mv ^{2}/R**.

In applying the Newton’s laws of motion, we have to consider some forces which can not be assigned to any object in the surrounding. These force are called pseudo force or inertia force.

Centrifugal force is such a pseudo force. It is equal and opposite to centripetal force.

The rotational effect of a force on a body about an axis of rotation is describe in terms of momentum of force.

Momentum of a force about an axis of rotation is measured as the product of magnitude of force and the perpendicular distance of direction of force from the axis of rotation i.e. **Momentum of Force = Force x Moment arm**.

» It is a vector quantity. Its SI unit is newton metre (Nm)

The center of gravity of a body is that point through which the entire weight of body acts. The center of gravity of a body does not change with the change in orientation of body in space.

The weight of a body acts through centre of gravity in the downward direction. Hence a body cab be brought to equilibrium by applying a force equal to its weight in the vertically upward direction through center of gravity.

When a large force acts on a body for very small time, then force is called impulsive force. Impulse is defined as the product of force and time.

**Impulse = force x time = change in momentum.**

» It is a vector quantity and its direction is the direction of force. Its SI unit is Newton Seconds (Ns).