Question

A proton that has a mass m and is moving at 280 m/s in the + direction undergoes a head-on elastic collision with a stationary oxygen nucleus of mass 16m. Find the velocities of the proton and the oxygen nucleus after the collision.

Answer 1

initial momentum = final momentum
m1v1(initial) + m2v2(initial) = m1v1 (final) + m2v2 (final)

Initial Final
m1 = m m1 = m
v1 = 280 m/s v1 = ?
m2 = 16m m2 = 16m
v2 = 0 v2 = ?

Plug them in and cancel out your ms and solve for v1 or v2. Then go back and plug that solved value into the original equation.