Momentum is commonly defined as 'mass times velocity'. However, this cannot be a general definition, since it does not include other types of momentum, including macroscopic radiation momentum or the momentum of a photon. Rankine coined a general definition for energy. Is it possible to describe a corresponding definition of momentum?'There is a part of everything which is unexplored...Even in the smallest thing there is something in it which is unknown. We must find it'. Gustav Flaubert (1821-1880) quoted by Guy de Maupassant (1850-1893) .