Action | |
---|---|
Common symbols | S |
SI unit | joule-second |
Other units | J⋅Hz−1 |
In SI base units | kg⋅m2⋅s−1 |
Dimension |
In physics, action is a scalar quantity that describes how the balance of kinetic versus potential energy of a physical system changes with trajectory. Action is significant because it is an input to the principle of stationary action, an approach to classical mechanics that is simpler for multiple objects.[1] Action and the variational principle are used in Feynman's formulation of quantum mechanics[2] and in general relativity.[3] For systems with small values of action similar to the Planck constant, quantum effects are significant.[4]
In the simple case of a single particle moving with a constant velocity (thereby undergoing uniform linear motion), the action is the momentum of the particle times the distance it moves, added up along its path; equivalently, action is the difference between the particle's kinetic energy and its potential energy, times the duration for which it has that amount of energy.
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.[5] Action has dimensions of energy × time or momentum × length, and its SI unit is joule-second (like the Planck constant h).[6]
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