Summary
An object in equilibrium is at rest. It is not moving (or "translating") and it is not rotating.
There are two conditions of equilibrium.
That a body is not moving (or "translating") means the sum of the forces is zero,
F = 0That a body is not rotating means the sum of the torques is zero,
A torque is a "rotational force" or the rotational equivalent of a force,
The center of mass has been called "a uniquely interesting point". The center of mass is the "average position" of the mass. You may think of any extended object as a rigid, massless shell with all of its mass concentrated at the center of mass.
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