Action (physics)

Physical quantity of dimension energy × time / From Wikipedia, the free encyclopedia

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In physics, action is a scalar quantity describing how a physical system has changed over time (its dynamics).[clarification needed] Action is significant because the equations of motion of the system can be derived through the principle of stationary action.

Quick facts: Action, Common symbols, SI unit, Other u...
Action
Common symbols
S
SI unitJoule-second
Other units
JHz−1
In SI base unitskgm2s−1
Dimension
Close

In the simple case of a single particle moving with a constant velocity (uniform linear motion), the action is the momentum of the particle times the distance it moves, added up along its path; equivalently, action is twice the particle's kinetic energy times the duration for which it has that amount of energy. For more complicated systems, all such quantities are combined.

More formally, action is a mathematical functional which takes the trajectory (also called path or history) of the system as its argument and has a real number as its result. Generally, the action takes different values for different paths.[1] Action has dimensions of energy × time or momentum × length, and its SI unit is joule-second (like the Planck constant h).[2]