In a nuclear reactor, neutrons lose energy by making collisions with nuclei of atomf of the materials that maay be present in the core of the reactor. If a neutron of mass mn has initial kinetic energy of 5.0 MeV,how much kinetic energy will it lose if it makes head on elastic collision with nucleas of lead mpb = 206mn, carbon mc = 12 mn or hydrogen mh = mn? The book solves this question by getting the final speed in terms of elastic collision. I understand all the example but what I don't understand is we here used elastic collision but isn't elastic collision defined when there is no loss of kinetic energy ? so why then we would use elastic collision in this case ? I know its stated in the problem,but what I don't understand is that kinetic energy is defined for elastic collision to be when there is no loss in kinetic energy thats what I don't understand about this problem.