Statics is the action of one body on another. A force is either a push or a pull. A force tends to move a body in the direction of its action. The action of a force is characterized by its magnitude, by the direction of its action, and by its point of application. Thus, force is a vector quantity, because its effect depends on the direction as well as on the magnitude of the action. Forces are classified as either contact or body forces. A contact force is produced by direct physical contact; an example is the force exerted on a
Conserved quantity order system of differential equations where bold indicates vector quantities, a scalar-valued function "H"(r) is a conserved quantity of the system if, for all time and initial conditions in some specific domain, Note that by using the multivariate chain rule, so that the definition may be written as which contains information specific to the system and can be helpful in finding conserved quantities, or establishing whether or not a conserved quantity exists. For a system defined by the Hamiltonian "H", a function "f" of the generalized coordinates "q" and generalized momenta "p" has time evolution and hence is conserved if