Mechanical energy includes, on the one hand, potential energy or energy of position; for example, the potential energy Epot of a mass m is equal to Epot = m g h, where g is the acceleration due to gravity and h is the height. Potential energy can be found in elastic deformation as well.
There is also kinetic energy or energy of motion, Ekin = ½ m v2, which arises from the motion of a mass m with a velocity v. Similar to kinetic energy of translation, the rotational energy of a body with angular velocity ω and a moment of inertia I with respect to the rotational axis is described by Erot = ½ I ω2.
When mechanical energy is not converted to another energy form (other than potential energy), the principle of conservation of mechanical energy applies:
This describes, for example, the undamped vibration of a pendulum. An experimental proof of the conservation of mechanical energy is the fact that one cannot build a perpetual motion device.
The picture at the left is a reconstruction of a clock presented in 1815 by David Geiser as a perpetual motion device. After his early death in 1817, it became known that the device did not, in fact, run continuously.
Pictured at the right is a reconstruction of a thousand year old design for the construction of a continuously rotating wheel. It is located in the museum of the Institute for the History of Arabic-Islamic Science at the University of Frankfurt a.M.