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## QuestionPhysicsClass 11

A collision in which there is absolutely no loss of $${K}.{E}.$$ is called an elastic collision. In such a collision, the linear momentum , total energy and kinetic energy, all are conserved.The coefficient of restitution $$/$$ resilience of perfectly elastic collisions is unity.
Read the above passage and answer the following questions $$:$$
(i) When two bodies of equal masses undergo perfectly elastic collision in one dimension, what happens to their velocities ?
(ii) How is this fact applied in a nuclear reactor ?

(i) The velocities of two bodies of equal masses are just interchanged after they undergo perfectly elastic collision in one dimesion.
(ii) Fast moving neutrons in a nuclear reactor are slowed down by makin them collide against the nuclei of a moderator. As nuclei of a moderator, i.e., hydrogen atoms (vix. protons ) have roughtly the same mass as that of neutron, therefore, on collision, their velocities are interchanged, i.e., neutrons slow doen and protons acquire the speed of neutrons. Often hydrogenic materials (e.g., heavy water , paraffin, ets,) are used as moderators in nuclear reactors.