Two balls P and Q, with equal mass, are travelling in a straight line towards each other, as illustrated. Ball P has a velocity of $1.30\,\text{m s}^{-1}$ to the right. Ball Q has a velocity of $0.50\,\text{m s}^{-1}$ to the left. P and Q collide. The collision is perfectly elastic, so the total momentum is conserved. Which diagram shows the correct motion of P and Q after the collision?
Physics 9702 · AS & A Level · Linear momentum and its conservation