In this paper it is intended to elaborate a framework in which we can incorporate external forces in the systems prescription with emphasis on Hamiltonian systems with external forces and on the consequences of external forces. An appealing tool for this case is the language of symplectic geometry. Definitions of Hamiltonian systems with external forces are given and it is shown how they fit very naturally into the framework. It is also shown that forces are basic variables and that they have to be included in the definitions of mechanical systems.
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