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'''Force''' is defined as rate of change of [[momentum]] of a body ('''F''' = dp/dt). The [[International System of Units|SI]] unit of force is the [[newton]] and the [[US customary system]] unit is the pound.

'''Force''' is defined as rate of change of [[momentum]] of a body ('''F''' = dp/dt). 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 ''p = m v''. For non-[[Theory of Relativity|relativistic]] speeds, ''F = m a''.<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.  

The momentum of an object is given by ''p = m v''. For non-[[Theory of Relativity|relativistic]] speeds, ''F = m a'' 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.  

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>: