The process or state of acting or of being active.
According to → Newton’s third law of motion, an
external force that is applied to a body and that is counteracted by an equal force
in the opposite direction ( → reaction).
A quantity whose → dimension (ML2T-1)
coincides with that of → angular momentum,
the → impulse of a force, or → energy x
→ time.
The action plays an important part in → analytical mechanics,
→ quantum mechanics, and in a number
of other fields of physics. Initially introduced in analytical mechanics, the
concept of action has become a basic ingredient of modern physics, due
to the role it has played in the generalization of
→ variational principle.
A scalar quantity computed as a function of the path followed by a system during
its evolution between an initial instant ti and a final instant
tf. It is defined by the → integral
of the → Lagrangian between the two instants:
S = ∫L dt
In the framework of the → field theory, the action
is expressed by the integral of the
→ Lagrangian density
over the corresponding space-time volume:
S = ∫Ld d4x.
In classical physics, the path actually followed by the system is the one for which S
is stationary (→ least action problem).
→ quantum of action.
Math.: The action is a → functional,
a mathematical relationship which takes an entire path and produces a single number.
Etymology (EN): Action, from O.Fr. action, from L. actionem,
from agere “to do,” → act.
Etymology (PE): Žireš, verbal noun from žir stem of
žiridan “to act;” → act.
Koneš, noun from kardan “to do, to make,” Mid.Pers.
kardan, O.Pers./Av. kar- “to do, make, build,”
Av. kərənaoiti “makes,” cf. Skt. kr- “to do, to make,”
krnoti “makes,”
karma “act, deed;” PIE base kwer- “to do, to make.”
