The term is commonly used to refer to machines which display this phenomenon. In the macroscopic world, perpetual motion is not generally considered to be possible. Perpetual motion machines (the Latin term perpetuum mobile is not uncommon) are a class of hypothetical machines which would produce useful energy in a way which would violate the established laws of physics. No genuine perpetual motion machine currently exists, and according to certain fundamental laws in physics they cannot exist. Specifically, perpetual motion machines would violate either the first or second laws of thermodynamics. Perpetual motion machines are divided into two subcategories (some physicists, including the noted professor of thermodynamics Mark W. Zemansky, include a third), defined by which law of thermodynamics would have to be broken in order for the device to be a true perpetual motion machine.