A moving object has kinetic energy, the energy of motion. Kinetic energy grows with mass: at the same speed, a heavier ball carries more. It grows even faster with speed: doubling the speed gives four times the energy. Kinetic energy is measured in joules. When a rolling ball hits a cup, energy is transferred to the cup and the cup slides. A faster or heavier ball transfers more energy and the cup slides farther. Friction then turns the cup's motion energy into warmth and the cup stops.