### Q.6:- **Underline the correct alternative:**

(a) When a conservative force does positive work on a body, the potential energy of the body increases/decreases/remains unaltered.

(b) Work done by a body against friction always results in a loss of its kinetic/potential energy.

(c) The rate of change of total momentum of a many-particle system is proportional to the external force/sum of the internal forces on the system.

(d) In an inelastic collision of two bodies, the quantities which do not change after the collision are the total kinetic energy/total linear momentum/total energy of the system of two bodies.

**Answer:-**

**
**(a)

__Decreases__

A conservative force does a positive work on a body when it displaces the body in the direction of force. As a result, the body advances toward the centre of force. It decreases the separation between the two, thereby decreasing the potential energy of the body.

(b) __Kinetic energy__

The work done against the direction of friction reduces the velocity of a body. Hence, there is a loss of kinetic energy of the body.

(c)** **__External force__

Internal forces, irrespective of their direction, cannot produce any change in the total momentum of a body. Hence, the total momentum of a many- particle system is proportional to the external forces acting on the system.

(d) __Total linear momentum__

The total linear momentum always remains conserved whether it is an elastic collision or an inelastic collision.