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Assume that two bodies are rotating about their center of mass because of their mutual gravitational attraction between them.

Can they be said to be in equilibrium?

I believe that the answer is no, as there exists a centripetal acceleration on the bodies.

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  • $\begingroup$ they will be in dynamic equilibrium $\endgroup$ – Boris Jan 8 '17 at 8:31
  • $\begingroup$ Boris, you should make this an answer, pointing out that in everyday language "equilibrium" = "static equilibrium" and else that it is impossible to not be in "dynamic equilibrium". Therefore, the distinction is unnecessary and "static" may be dropped $\endgroup$ – Rainer Glüge Jan 8 '17 at 11:03
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No, they can not be said to be in equilibrium. In mechanics language, "equilibrium" = "static equilibrium", meaning equilibrium at rest or on a constant velocity translation, excluding rotation or movement along a curved path (all w.r.t. an intertial frame).

The complement "dynamic equilibrium", meaning that inertia forces oppose accelerations, is unnecessary as it is impossible to not be in dynamic equilibrium. Therefore, the distinction is unnecessary and "static" is in general omitted.

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