Editing Force (section)

==== Static <span class="anchor" id="Static equilibrium"></span> ====
{{main|Statics|Static equilibrium}}
Static equilibrium was understood well before the invention of classical mechanics. Objects that are not accelerating have zero net force acting on them.<ref>{{cite web
|title=Static Equilibrium
|work=Physics Static Equilibrium (forces and torques)
|publisher=[[University of the Virgin Islands]]
|url=http://www.uvi.edu/Physics/SCI3xxWeb/Structure/StaticEq.html
|access-date=2008-01-02 |archive-url=https://web.archive.org/web/20071019054156/http://www.uvi.edu/Physics/SCI3xxWeb/Structure/StaticEq.html |archive-date=October 19, 2007}}</ref>

The simplest case of static equilibrium occurs when two forces are equal in magnitude but opposite in direction. For example, an object on a level surface is pulled (attracted) downward toward the center of the Earth by the force of gravity. At the same time, a force is applied by the surface that resists the downward force with equal upward force (called a [[normal force]]). The situation produces zero net force and hence no acceleration.<ref name=uniphysics_ch2/>

Pushing against an object that rests on a frictional surface can result in a situation where the object does not move because the applied force is opposed by [[static friction]], generated between the object and the table surface. For a situation with no movement, the static friction force ''exactly'' balances the applied force resulting in no acceleration. The static friction increases or decreases in response to the applied force up to an upper limit determined by the characteristics of the contact between the surface and the object.<ref name=uniphysics_ch2/>

A static equilibrium between two forces is the most usual way of measuring forces, using simple devices such as [[weighing scale]]s and [[spring balance]]s. For example, an object suspended on a vertical [[spring scale]] experiences the force of gravity acting on the object balanced by a force applied by the "spring reaction force", which equals the object's weight. Using such tools, some quantitative force laws were discovered: that the force of gravity is proportional to volume for objects of constant [[density]] (widely exploited for millennia to define standard weights); [[Archimedes' principle]] for buoyancy; Archimedes' analysis of the [[lever]]; [[Boyle's law]] for gas pressure; and [[Hooke's law]] for springs. These were all formulated and experimentally verified before Isaac Newton expounded his [[Newton's laws of motion|three laws of motion]].<ref name=uniphysics_ch2/><ref name=FeynmanVol1 />{{rp|at=ch.12}}<ref name=Kleppner />