Consider two cases in which cart A moves to the left with momentum \vec{p}_ip → i and then collides with a second cart which is initially at rest on a horizontal, frictionless air track. In case 1, cart A moves to the left after the collision at a speed of 1 m/s. In case 2, cart A moves to the right after the collision with the same speed as in case 1 (1 m/s). After the collision, the magnitude of the second cart's momentum in case 1 is the magnitude of the second cart's momentum in case 2.