Aproton (mass 1 u) moving at 5.40 x 106 m/s collides elastically and head‐on with a second particle moving in the opposite direction at 1.00 x 106 m/s. after the collision, the proton is moving opposite to its initial direction at 5.60 x 106 m/s. find the final velocity (in m/s) of the second particle. [take the proton’s initial velocity to be in the positive direction.]