Restoring force
Restoring force, in a physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

 context, is a variable force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

 that gives rise to an equilibrium
Mechanical equilibrium
A standard definition of static equilibrium is:This is a strict definition, and often the term "static equilibrium" is used in a more relaxed manner interchangeably with "mechanical equilibrium", as defined next....

 in a physical system. If the system is perturbed away from the equilibrium, the restoring force will tend to bring the system back toward equilibrium.

An example is the action of a spring
Spring (device)
A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...

: an idealized spring exerts a force that is proportional to the amount of deformation of the spring. Pulling the spring to a longer configuration causes it to exert a force that brings the spring back toward its equilibrium length. The amount of force can be determined by multiplying the spring constant of the spring by the amount of stretch.

Restoring forces do not necessarily cancel out a perturbation entirely. For example, taut piano
The piano is a musical instrument played by means of a keyboard. It is one of the most popular instruments in the world. Widely used in classical and jazz music for solo performances, ensemble use, chamber music and accompaniment, the piano is also very popular as an aid to composing and rehearsal...

 strings have a strong restoring force that gives rise to an equilibrium when the string is straight. A struck piano string does not return directly to the straight equilibrium; rather, it oscillates, causing the piano to emit a particular musical note.
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