A neutron having kinetic energy 12.5 eV collides with a hydrogen atom at rest. Neglect the difference in mass between the neutron and the hydrogen atom and assume that the neutron does not leave its line of motion. Find the possible kinetic energies of the neutron after the even.
According to the question as masses of hydrogen atom and neutron are equal and line of motion does not change, hence this is a type of elastic collision in one line/dimension.
Hence the velocity of two particles interchanged as per elastic collision principle.
So neutron will come to rest. Hence its kinetic energy is zero.