Illustration of a ball rolling on rails, changing colour to show the relationship between gravitational potential energy and kinetic energy. At left, the ball has high gravitational potential energy (blue). As it rolls down the rails, its potential energy is converted into kinetic energy and its speed increases (purple). As the ball reaches the bottom of the rails, it is travelling at its maximum speed and its kinetic energy is at a maximum (red). As it rolls up the other side of the rails gravity slows it down, converting its kinetic energy to potential energy (purple) again.