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'''Force''' is defined as rate of change of [[momentum]] of a body <math>\vec F = {d \vec p \over dt} </math>. The [[International System of Units|SI]] unit of force is the [[newton]] and the [[US customary system]] unit is the pound.

In [[classical physics]], '''force''' is defined as the time rate of change of [[momentum]] of a body <math>\vec F = {d \vec p \over dt} </math>. The [[International System of Units|SI]] unit of force is the [[newton]] and the [[US customary system]] unit is the pound.

The momentum of an object is given by <math> \vec p = m \vec v</math>. For non-[[Theory of Relativity|relativistic]] speeds, <math> \vec F = m \vec a </math> when mass can be assumed to be constant.<ref>Marcelo Alonso and Edward J. Finn, ''Fundamental University Physics'', Addison-Wesley.</ref> In these expressions, ''F'' stands for the total vector sum of all forces, ''m'' for the mass of the object, ''a'' for its [[acceleration]] expressed as a vector, ''p'' stands for momentum vector  and ''v'' for velocity vector.  

Classically, the momentum of an object is given by <math> \vec p = m \vec v</math> and [[acceleration]] relates to force via [[Classical Physics|Newton's Second Law]] as <math> \vec F = m \vec a </math> when mass can be assumed to be constant.<ref>Marcelo Alonso and Edward J. Finn, ''Fundamental University Physics'', Addison-Wesley.</ref> In these expressions, ''F'' stands for the total vector sum of all forces, ''m'' for the mass of the object, ''a'' for its [[acceleration]] expressed as a vector, ''p'' stands for momentum vector  and ''v'' for velocity vector. In [[Theory of Relativity|special relativity]], these terms must be modified so as to be [[Lorentz invariant]], which among other things means that all [[inertial reference frame]]s stand on equal footing and have the same prescription for all physical and dynamical quantities, though observers in different [[inertial reference frame]]s they will measure different values for many of them, each observer being correct for his own frame.

There are four known fundamental types of forces occurring in nature<ref>Lewis H. Ryder, ''Quantum Field Theory'', 2nd ed., Cambridge University Press, Cambridge (UK), 1996</ref>:

There are four known fundamental types of forces occurring in nature<ref>Lewis H. Ryder, ''Quantum Field Theory'', 2nd ed., Cambridge University Press, Cambridge (UK), 1996</ref>: