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 expressed in terms of wave quantities: and tidying up by dividing by throughout: This can also be derived from the magnitude of the four-wavevector in a similar way to the four-momentum above. Since the reduced Planck constant and the speed of light both appear and clutter this equation, this is where natural units are especially helpful. Normalizing them so that , we have: The velocity of a bradyon with the relativistic energy–momentum relation can never exceed . On the contrary, it is always greater than for a tachyon whose energy–momentum equation is By contrast, the hypothetical exotic matter has a