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 Hollow Bottles. At the same time, surface forces resist the downward force with equal
upward force (called the normal force). The situation is one of zero net force
and no acceleration.
Pushing against an object 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.
A static equilibrium between two forces is the most usual way of measuring
forces, using simple devices such as weighing scales and spring balances. 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 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 three laws of motion.