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Line 1: − 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.+ − − 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. More generally it is prescribed as <math> \vec F = \frac{d \vec p}{dt} </math>.<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 presciptions 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 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>: − − ;[[Electromagnetism|Electromagnetic]] force − :The force that acts on objects with [[electric charge]]. − ;[[Gravitation]]al force − :The force that attracts any two objects with [[mass]]. − ;Strong force − :The force that keeps [[atom]]ic [[nucleus|nuclei]] together. − ;Weak force − :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 described using [[quantum field theory]], a relativistic version of [[quantum mechanics]]. − − ==References== − <references/> − − [[Category:Physics]] − [[Category:Mechanics]]
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