Question

A horizontal rod rotates about a vertical axis through one end. A ring, which can slide along the rod without friction, is initially close to the axis and then slides to the other end of the rod. In this process, which of the following quantities will be conserved? [L = angular momentum, E_T = total kinetic energy, E_R = rotational kinetic energy.] 

(a) L only 

(b) L and E_T only 

(c) L and E_R only 

(d) E_T only

Answer 1

Correct Answer is: (b) L and E_T only 

As no external torque acts on the system, its angular momentum is conserved. Since there is no loss of energy due to friction, the total energy is conserved. However, the ring acquires some translational kinetic energy as it slides outwards, and hence rotational kinetic energy is not conserved.