Difference between revisions of "Force"
DavidB4-bot (talk | contribs) (→top: Spelling/Grammar Check, typos fixed: presciptions → prescriptions) |
(username removed) (See Also section, fixed broken links, added bit about relativity) |
||
| Line 1: | Line 1: | ||
| − | In [[classical physics]], '''force''' is defined as the | + | In [[classical physics]], '''force''' is defined as the rate of change of momentum with respect to time (i.e. the [[derivative]] of momentum with respect to time.) In many everyday scenarios where the speed of a body is much less than that of [[speed of light|light]] (non-[[relativistic]]), and the [[mass]] of the body is constant, it simplifies to the product of [[mass]] and [[acceleration]]. In other words, how hard you push on something determines how rapidly you change its speed. Of course, the heavier it is, the more it resists this change. |
| − | Using | + | Using mathematics, force may be 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 (unit)|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 [[ | + | Classically, the momentum of an object is given by <math> \vec p = m \vec v</math> and [[acceleration]] relates to force via [[Newton's Laws of Motion|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 prescriptions 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> | 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]] | + | ;[[Electromagnetism|Electromagnetic force]] |
:The force that acts on objects with [[electric charge]]. | :The force that acts on objects with [[electric charge]]. | ||
| − | ;[[Gravitation]] | + | ;[[Gravitation|Gravitational force]] |
:The force that attracts any two objects with [[mass]]. | :The force that attracts any two objects with [[mass]]. | ||
| − | ;Strong force | + | ;[[Strong force]] |
| − | :The force that keeps [[atom]]ic [[nucleus|nuclei]] together. | + | :The force that keeps [[atom]]ic [[nucleus|nuclei]] together, as well as binding quarks together to form [[proton]]s and [[neutron]]s. |
| − | ;Weak force | + | ;[[Weak force]] |
| − | :This force is involved in | + | :This force is involved in [[beta decay]], in which a [[neutron]] in an atomic nucleus is changed to a [[proton]], emitting an [[electron]] and an electron anti-[[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]]. | 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]]. | ||
| + | |||
| + | == See Also == | ||
| + | |||
| + | *[[Fundamental interactions|Fundamental forces]] | ||
| + | *[[Momentum]] | ||
==References== | ==References== | ||
<references/> | <references/> | ||
Revision as of 13:51, September 8, 2016
In classical physics, force is defined as the rate of change of momentum with respect to time (i.e. the derivative of momentum with respect to time.) In many everyday scenarios where the speed of a body is much less than that of light (non-relativistic), and the mass of the body is constant, it simplifies to the product of mass and acceleration. In other words, how hard you push on something determines how rapidly you change its speed. Of course, the heavier it is, the more it resists this change.
Using mathematics, force may be defined as the time rate of change of momentum of a body <math>\vec F = {d \vec p \over dt} </math>. The 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 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>.[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. In special relativity, these prescriptions must be modified so as to be Lorentz invariant, which among other things, means that all inertial reference frames stand on equal footing and have the same prescription for all physical and dynamical quantities, though observers in different inertial reference frames 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:[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, as well as binding quarks together to form protons and neutrons.
- 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 an electron anti-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.