View source for Conservative force

{{Short description|Force in which the work done in moving an object depends only on its displacement}}
{{Classical mechanics}}

In [[physics]], a '''conservative force''' is a [[force]] with the property that the total [[Work (physics)|work]] done in moving a particle between two points is independent of the path taken.<ref>[http://hyperphysics.phy-astr.gsu.edu/hbase/pegrav.html#cfor HyperPhysics - Conservative force]</ref>  Equivalently, if a particle travels in a closed loop, the total work done (the sum of the force acting along the path multiplied by the [[Displacement (geometry)|displacement]]) by a conservative force is zero.<ref name=Hand>{{cite book |title=Analytical Mechanics |author =Louis N. Hand, Janet D. Finch |page=41 |isbn=0-521-57572-9 |publisher=Cambridge University Press |year=1998 }}</ref>

A conservative force depends only on the position of the object.  If a force is conservative, it is possible to assign a numerical value for the [[Scalar potential|potential]] at any point and conversely, when an object moves from one location to another, the force changes the [[potential energy]] of the object by an amount that does not depend on the path taken, contributing to the [[mechanical energy]] and the overall [[conservation of energy]]. If the force is not conservative, then defining a scalar potential is not possible, because taking different paths would lead to conflicting potential differences between the start and end points.

[[Gravitation|Gravitational force]] is an example of a conservative force, while [[frictional force]] is an example of a non-conservative force.

Other examples of conservative forces are: [[Hooke's law|force in elastic spring]], [[electrostatic force]] between two electric charges, and [[magnetic force]] between two magnetic poles. The last two forces are called central forces as they act along the line joining the centres of two charged/magnetized bodies. A central force is conservative if and only if it is spherically symmetric.<ref>{{cite book|last1=Taylor|first1=John R.|title=Classical Mechanics|date=2005|publisher=Univ. Science Books|location=Sausalito, Calif.|isbn=1-891389-22-X|pages=133–138}}</ref>

For conservative forces,

<math>\mathbf{F_c} = - \frac{\textit{dU}}{d\mathbf{s}}

</math>

where <math>F_c

</math> is the conservative force, <math>U

</math> is the potential energy, and <math>s

</math> is the position.<ref>{{Cite web |title=Conservative Forces Definition, Formula, Examples |url=https://physicscatalyst.com/mech/conservative-force.php |access-date=2024-01-02 |website=physicscatalyst.com}}</ref>

==Informal definition==
Informally, a conservative force can be thought of as a force that ''conserves'' [[mechanical energy]]. Suppose a particle starts at point A, and there is a force ''F'' acting on it. Then the particle is moved around by other forces, and eventually ends up at A again. Though the particle may still be moving, at that instant when it passes point A again, it has traveled a closed path. If the net work done by ''F'' at this point is 0, then ''F'' passes the closed path test. Any force that passes the closed path test for all possible closed paths is classified as a conservative force.

The [[gravitational force]], [[Hooke's law|spring force]], [[magnetic force]] (according to some definitions, see below) and [[electric force]] (at least in a time-independent magnetic field, see [[Faraday's law of induction]] for details) are examples of conservative forces, while [[friction]] and [[air drag]] are classical examples of non-conservative forces.

For non-conservative forces, the mechanical energy that is lost (not conserved) has to go somewhere else, by [[conservation of energy]]. Usually the energy is turned into [[heat]], for example the heat generated by friction. In addition to heat, friction also often produces some [[sound]] energy. The water drag on a moving boat converts the boat's mechanical energy into not only heat and sound energy, but also wave energy at the edges of its [[Wake (physics)|wake]]. These and other energy losses are irreversible because of the [[second law of thermodynamics]].

==Path independence==
[[File:Conservative Force Gravity Example.svg|thumb]]
A direct consequence of the closed path test is that the work done by a conservative force on a particle moving between any two points does not depend on the path taken by the particle.

This is illustrated in the figure to the right: The work done by the gravitational force on an object depends only on its change in height because the [[gravitational force]] is conservative. The work done by a conservative force is equal to the negative of change in potential energy during that process. For a proof, imagine two paths 1 and 2, both going from point A to point B. The variation of energy for the particle, taking path 1 from A to B and then path 2 backwards from B to A, is 0; thus, the work is the same in path 1 and 2, i.e., the work is independent of the path followed, as long as it goes from A to B.

For example, if a child slides down a frictionless slide, the work done by the gravitational force on the child from the start of the slide to the end is independent of the shape of the slide; it only depends on the vertical displacement of the child.

==Mathematical description==
A [[force field (physics)|force field]] ''F'', defined everywhere in space (or within a [[simply-connected]] volume of space), is called a ''conservative force'' or ''[[conservative vector field]]'' if it meets any of these three ''equivalent'' conditions:

# The [[Curl (mathematics)|curl]] of ''F'' is the zero vector: <math display="block">\vec{\nabla} \times \vec{F} = \vec{0}. </math> where in two dimensions this reduces to: <math display="block"> \frac{\partial F_y}{\partial x} - \frac{\partial F_x}{\partial y} = 0 </math>
# There is zero net [[work (physics)|work]] (''W'') done by the force when moving a particle through a trajectory that starts and ends in the same place: <math display="block">W \equiv \oint_C \vec{F} \cdot \mathrm{d}\vec r = 0.</math>
# The force can be written as the negative [[gradient]] of a [[potential energy|potential]], <math>\Phi</math>: <math display="block">\vec{F} = -\vec{\nabla} \Phi. </math>

{{math proof| title = Proof that these three conditions are equivalent when ''F'' is a [[force field (physics)|force field]]
|proof= {{Main|Conservative vector field}}
{{glossary}}
{{term|1 implies 2}}{{defn|
Let ''C'' be any simple closed path (i.e., a path that starts and ends at the same point and has no self-intersections), and consider a surface ''S'' of which ''C'' is the boundary. Then [[Stokes' theorem]] says that
<math display="block"> \int_S \left(\vec{\nabla} \times \vec{F}\right) \cdot \mathrm{d}\vec{a} = \oint_C \vec{F} \cdot \mathrm{d}\vec{r} </math>
If the curl of '''F''' is zero the left hand side is zero – therefore statement 2 is true.
}}
{{term|2 implies 3}}{{defn|
Assume that statement 2 holds. Let ''c'' be a simple curve from the origin to a point <math>x</math> and define a function
<math display="block">\Phi(x) = -\int_c \vec{F} \cdot \mathrm{d}\vec{r}.</math>
The fact that this function is well-defined (independent of the choice of ''c'') follows from statement 2. Anyway, from the [[fundamental theorem of calculus]], it follows that <math display="block">\vec{F} = -\vec{\nabla} \Phi.</math>
So statement 2 implies statement 3 ([[Gradient theorem#Converse of the gradient theorem|see full proof]]).
}}
{{term|3 implies 1}}{{defn|
Finally, assume that the third statement is true. A well-known vector calculus identity states that the curl of the gradient of any function is 0. (See [[Vector calculus identities#Curl of gradient is zero|proof]].) Therefore, if the third statement is true, then the first statement must be true as well.

This shows that statement 1 implies 2, 2 implies 3, and 3 implies 1. Therefore, all three are equivalent, [[Q.E.D.]]

(The equivalence of 1 and 3 is also known as (one aspect of) [[Helmholtz decomposition|Helmholtz's theorem]].)
}}
{{glossary end}}
}}

The term ''conservative force'' comes from the fact that when a conservative force exists, it conserves mechanical energy. The most familiar conservative forces are [[gravity]], the [[electric force]] (in a time-independent magnetic field, see [[Faraday's law of induction|Faraday's law]]), and [[Hooke's law|spring force]].

Many forces (particularly those that depend on velocity) are not [[force field (physics)|force ''fields'']]. In these cases, the above three conditions are not mathematically equivalent. For example, the [[magnetic force]] satisfies condition 2 (since the work done by a magnetic field on a charged particle is always zero), but does not satisfy condition 3, and condition 1 is not even defined (the force is not a vector field, so one cannot evaluate its curl). Accordingly, some authors classify the magnetic force as conservative,<ref name="srivastava1997mechanics">For example, {{Cite book|title=Mechanics|author=P. K. Srivastava | url=https://books.google.com/books?id=yCw_Hq53ipsC|year=2004|publisher=New Age International Pub. (P) Limited|access-date=2018-11-20 |isbn=9788122411126|page=94}}: "In general, a force which depends explicitly upon the velocity of the particle is not conservative. However, the magnetic force (q'''v'''×'''B''') can be included among conservative forces in the sense that it acts perpendicular to velocity and hence work done is always zero". [https://books.google.com/books?id=yCw_Hq53ipsC Web link]</ref> while others do not.<ref>For example, ''The Magnetic Universe: Geophysical and Astrophysical Dynamo Theory'', Rüdiger and Hollerbach, page 178, [https://books.google.com/books?id=GO1QwZtIYdAC Web link]</ref> The magnetic force is an unusual case; most velocity-dependent forces, such as [[friction]], do not satisfy any of the three conditions, and therefore are unambiguously nonconservative.

==Non-conservative force==

Despite conservation of total energy, non-conservative forces can arise in classical physics due to neglected [[degrees of freedom (physics and chemistry)|degrees of freedom]] or from time-dependent potentials.<ref>Friedhelm Kuypers. Klassische Mechanik. WILEY-VCH 2005. Page 9.</ref>  Many non-conservative  forces may be perceived as macroscopic effects of small-scale conservative forces.<ref name="Tom">Tom W. B. Kibble, Frank H. Berkshire. Classical mechanics. (5th ed).  Imperial College Press 2004 {{ISBN|1860944248}}</ref> For instance, friction may be treated without violating conservation of energy by considering the motion of individual molecules; however, that means every molecule's motion must be considered rather than handling it through statistical methods. For macroscopic systems the non-conservative approximation is far easier to deal with than millions of degrees of freedom.

Examples of non-conservative forces are [[friction]] and non-elastic material [[stress (mechanics)|stress]]. Friction has the effect of transferring some of the energy from the large-scale motion of the bodies to small-scale movements in their interior, and therefore appear non-conservative on a large scale.<ref name="Tom" /> [[General relativity]] is non-conservative, as seen in the [[Tests of general relativity#Perihelion precession of Mercury|anomalous precession]] of Mercury's orbit.{{citation needed|date=May 2021}} However, general relativity does conserve a [[stress–energy–momentum pseudotensor]].

==See also==
* [[Conservative vector field]]
* [[Conservative system]]

==References==
{{Reflist}}

{{DEFAULTSORT:Conservative Force}}
[[Category:Force]]