In a one-dimensional collision, a 2 kg puck moving at 3 m/s strikes a 3 kg stationary puck and the first puck stops after the collision. What is the final velocity of the second puck?

• A. 2 m/s
• B. 1 m/s
• C. 3 m/s
• D. 4 m/s

Answer: (A)

In a one-dimensional collision, momentum of the system is conserved when there are no external forces. The total momentum before the impact equals the total after.

Initially, the 2 kg puck moves at 3 m/s, giving a momentum of 2 × 3 = 6 kg·m/s. The 3 kg puck is at rest, so its momentum is zero. If the first puck stops after the collision, all that momentum must be carried by the second puck. So the second puck must have momentum 6 kg·m/s. Its velocity is this momentum divided by its mass: 6 / 3 = 2 m/s.

Therefore, the final velocity of the second puck is 2 m/s in the original direction. Other speeds would give a momentum that doesn’t match the initial total, violating momentum conservation. (Energy may not be conserved in this collision due to deformation or heat, but momentum is.)