Editing Conservative force (section)

==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]].