Question

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 1

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

For conservative forces, the work done is equal to the negative change in potential energy: \( W = -\Delta U \). If \( W \) is positive, then \( \Delta U \) is negative, so the potential energy decreases.

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

Friction is a non-conservative force that opposes motion, so the work done against friction comes at the expense of the body’s kinetic energy, which decreases (often converted into heat).

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

Internal forces occur in action–reaction pairs and cancel in the total, so only external forces can change the net (total) momentum of 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.

In an inelastic collision, total linear momentum is always conserved and total energy is conserved if all forms (kinetic + heat + deformation, etc.) are included, but total kinetic energy decreases.