That I know of, in most parts of physics 0 movement means the body is resting, all the forces are in equilibrium. Why in orbital mechanic 0 rotation speed doesn't mean tidal lock as it would be the resting rotation? Why isn't an absolute Cartesian reference usually used instead?

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    $\begingroup$ Tidal lock doesn't imply zero rotation, it implies a relationship between rotational period and orbital period. $\endgroup$ Sep 25, 2014 at 2:24

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I would agree that a circular orbit with full tidal lock is more "in equilibrium" (everything is time independent in the rotating frame) than one in which the orbiting object had zero rotation in an absolute Cartesian reference. There is however an important notion for determining whether something is rotating, and that is its angular momentum. An object can have both orbital and spin angular momentum. $$ L_{spin} = I\omega$$ Where $I$ is the momentum of inertia about its spinning axis, and $\omega$ is its rate of rotation.

Since a tidal locked object spins at the same rate as its orbit motion, its spin angular momentum is non-zero. In this very important sense, it's rotation rate is non-zero.


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