An electron of mass m is involved in head-on collision with an atom of mass M , initially at rest . As a result of the collision a certain amount of energy E is stored internally in the atom . What is the minimum initial velocity that the electron must have ?
[Hint : Since energy is stored in the atom , the collision is essentially inelastic . mv = mv' + Mv'' and `(1)/(2) mv^(2) = E + (1)/(2) mv^(2) + (1)/(2) Mv''^(2)` . Solve for v' and apply the condition for real values of v'.)

Answer

`sqrt((2(m + M) E)/(mM))`