What stays the same when two carts collide?

A collision can change both carts’ motions. Two carts can bounce apart or stick together. If they stick, they move at the same speed afterward. Total momentum is shared across the pair, not kept separately by each cart. For carts that stick, multiply the moving cart’s mass by its starting speed, then divide by the combined mass to find the shared speed. The system boundary matters: momentum conservation requires negligible net external impulse during the event. Kinetic-energy loss does not imply energy disappears, because energy can move into other forms. The model isolates these accounting principles and should not be interpreted as a prediction of damage or a perfectly elastic real material.