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 object in the lab frame with magnitude . The relativistic energy and momentum include the Lorentz factor defined by: Some authors use relativistic mass defined by: although rest mass has a more fundamental significance, and will be used primarily over relativistic mass in this article. Squaring the 3-momentum gives: then solving for and substituting into the Lorentz factor obtains its alternative form in terms of 3-momentum and mass, rather than 3-velocity: Inserting this form of the Lorentz factor into the energy equation: followed by more rearrangement yields (). The elimination of the Lorentz factor also eliminates implicit velocity dependence of