Two magnets are held on a track inside a sealed box, with like poles facing each other. The magnets, track, air and box form a closed system. One magnet is released and moves along the track. At a later instant, the magnetic energy store of the magnets is 6.0 J. Since the magnet was released, the kinetic energy store of the moving magnet has increased by 12 J and the thermal energy stores of the track and air have increased by 4.0 J. No other energy stores have changed.