Underline the correct alterntaive:
a) when a conservative force does positivie work on a body, the potential energy of the body increase/decreases/remains unaltered.
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 enregy of the system of two bodies.

Explanation

(a) Potential energy of the body decrease. The conservative force does positive work on a body, when it displaces the body in the direction of force. The obdy, therefore, approaches the centre of force, decreasing x. Hence, P.E. decreases.
(b) Work is done by a body against friction at the expense of its kinetic energy. Hence K.E. of the body decreases.
(c) Internal forces cannot change the total or net momentum of a system. Hence the rate of change of total momentum of many particle system system is proportional to the external force on the system.
(d) In an inelastic collision of two bodies, the quantities which do not change after the collision are total linear momentum and total energy of the systme of two bodies. The total K.E. changes as some energy appears is other forms.