Mechanical energy nonconservative forces, like frictional forces, will be present, but often they are of negligible values and the mechanical energy's being constant can therefore be a useful approximation. In elastic collisions, the mechanical energy is conserved but in inelastic collisions, some mechanical energy is converted into heat. The equivalence between lost mechanical energy (dissipation) and an increase in temperature was discovered by James Prescott Joule. Many devices are used to convert mechanical energy to or from other forms of energy, e.g. an electric motor converts electrical energy to mechanical energy, an electric generator converts mechanical energy into electrical energy and a
Biproduct the category, and the empty coproduct is always an initial object in the category. Thus an empty, or nullary, biproduct is always a zero object. In the category of abelian groups, biproducts always exist and are given by the direct sum. Note that the zero object is the trivial group. Similarly, biproducts exist in the category of vector spaces over a field. The biproduct is again the direct sum, and the zero object is the trivial vector space. More generally, biproducts exist in the category of modules over a ring. On the other hand, biproducts do not exist in the