Newton's cradle. This device comprises five metal spheres suspended from a frame. It is used to demonstrate the principles of conservation of momentum and conservation of energy. When two balls are lifted at either side and released simultaneously so that they strike the centre ball, the momentum of the system is conserved and two balls either side are ejected away from the centre ball. This sequence of impacts continues until all the initial energy has been dissipated - through sound and heat in the collisions and in torsional friction in the suspending threads.