The idea that thermodynamic work depends on the speed of the process is not new. The concept of reversible process carried out in a series of infinitesimal steps suggests an extremely slow process. It is generally recognized that the effective voltage of a battery decreases with increasing current, with the reversible voltage represented by the value measured at zero current. The same principle can be applied to mechanical processes. A definition of mechanical work and its application to a system confined by a piston are given.