Newton's cradle in motion. This device consists of a set of balls resting against one another, which can swing freely from the two bars they hang from. The ball on the right had been pulled away and released. The kinetic energy with which it struck the middle ball was transferred through to the left ball, which forced the ball to swing outwards. Gravity will pull the ball back down, causing it to strike the middle ball. The kinetic energy will be transferred through to the right ball, which will swing outwards again. This process will continue until energy is lost to friction, bringing the balls to a stop.