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Force is defined as rate of change of [[momentum]] of a body (F = dp/dt). A unit of force is the[[newton]].

'''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 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''.<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>:

;[[Electromagnetism|Electromagnetic]] force

;[[Electromagnetism|Electromagnetic]] force

;[[Gravitation]]al force

;[[Gravitation]]al force

;Strong force

;Strong force

:this is the force that keeps [[atom]]ic [[nucleus|nuclei]] together.

:The force that keeps [[atom]]ic [[nucleus|nuclei]] together.

;Weak force

;Weak force

:this force is (amongst other things) involved in ''beta decay'', in which a [[neutron]] in an atomic nucleus is changed to a [[proton]], emitting an [[electron]] and a [[neutrino]].

:This force is involved in ''beta decay'', in which a [[neutron]] in an atomic nucleus is changed to a [[proton]], emitting an [[electron]] and a [[neutrino]].

It should be noted that the latter two forces have an extremely short range (on the order of femtometers), and that a classical (Newtonian or relativistic) description of these forces is not possible. They can only be desribed using [[quantum field theory]], a relativistic version of [[quantum mechanics]].

It should be noted that the latter two forces have an extremely short range (on the order of femtometers), and that a classical (Newtonian or relativistic) description of these forces is not possible. They can only be described using [[quantum field theory]], a relativistic version of [[quantum mechanics]].

==References==

==References==