An inelastic collision

Two small carts move on an ideal horizontal track with negligible external horizontal force during a short collision. Cart A has mass2.0kg and initially moves to the right at3.0m/s. Cart B has mass1.0kg and is initially stationary. The carts latch together after contact. The setup does not claim that mechanical energy is conserved during the impact, and no spring or rebound is involved. Students must first choose the conserved quantity, then calculate the common velocity and compare the kinetic energies before and after. A disagreement arises because one student treats the energy difference as proof that momentum was lost. Use the stated system boundary and the distinct definitions of momentum and kinetic energy to resolve that disagreement.