Newton's cradle, artwork. This device consists of a number of steel balls suspended from a frame (not seen) in a horizontal line. Here, one ball on the end (left) has been pulled back and allowed to swing forward and downward under gravity. When it impacts the stationary line of balls, its momentum is transferred along the line, causing the ball on the opposite end to move upwards to a similar height. This sequence of impacts continues until all the initial energy has been lost to friction and sound. This apparatus, often used as a desk toy, demonstrates the conservation of momentum and energy.