Difference between revisions of "Force"
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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>: | ||
Revision as of 03:05, October 31, 2007
Force is defined as rate of change of momentum of a body (F = dp/dt). The 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-relativistic speeds, F = m a when mass can be assumed to be constant.[1] 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[2]:
- Electromagnetic force
- The force that acts on objects with electric charge.
- Gravitational force
- The force that attracts any two objects with mass.
- Strong force
- The force that keeps atomic 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.