Newton's cradle Newton's cradle Newton’s cradle is a device that demonstrates conservation of momentum and energy using a series of swinging spheres. When one sphere at the end is lifted and released, it strikes the stationary spheres, transmitting a force through the stationary spheres that pushes the last sphere upward. The last sphere swings back and strikes the still nearly stationary spheres, repeating the effect in the opposite direction. The device is named after 17th-century English scientist Sir Isaac Newton. It is also known as Newton's balls or Executive Ball Clicker. A typical Newton's cradle consists of a series of identically sized
Energy–momentum relation their rest masses in (): The energies can be eliminated by: similarly the momenta can be eliminated by: where is the angle between the momentum vectors and . Rearranging: Since the invariant mass of the system and the rest masses of each particle are frame-independent, the right hand side is also an invariant (even though the energies and momenta are all measured in a particular frame). Using the de Broglie relations for energy and momentum for matter waves, where is the angular frequency and is the wavevector with magnitude , equal to the wave number, the energy–momentum relation can be