posted by Brian .
A proton of mass m moving with a speed of 2.5×106 m/s undergoes a head-on elastic collision with an alpha particle of mass 4m, which is initially at rest. What is the speed of the proton after the collision?
I would use conservation of momentum here. Then, conservation of energy. You will have two equations, two unknowns. Use substitution. THere is a little algebra involved.