Definition:Principle of Stationary Action

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The principle of stationary actions states that the equations of motion of a physical system can be acquired by finding a stationary point of the action.

In other words, the first variation of the action has to vanish.

Also known as

The principle of stationary action is also known as the principle of least action.


The principle of least action attained its name due to classical problems of minimization.

However, if broken trajectories are allowed, the action can sometimes acquire lower values than for any allowed smooth trajectory.

Since smooth trajectories are more realistic, leastness has been weakened to stationarity.