Final answer:
In static equilibrium, the axis of rotation for an object cannot be determined without additional information. The choice of the axis of rotation depends on the specific situation and the pivot point that simplifies the problem the most.
Step-by-step explanation:
For an object in static equilibrium, the axis of rotation that should be used depends on the problem being analyzed. In theoretical problems, when dealing with planar static equilibrium where rotation is about a fixed axis, the standard procedure is to select a frame of reference where the z-axis is the axis of rotation. The choice of the z-axis allows for a simplification because in this scenario, the net torque has only a z-component, making it easier to apply the conditions of equilibrium, specifically for torques.
This selection also means that all forces that contribute non-zero torques lie in the xy-plane, which is perpendicular to the z-axis. Therefore, for planar problems, the z-axis is most typically used as the axis of rotation, especially if it helps simplify the calculations. This however does not exclude the possibility of using the x-axis or y-axis if the situation dictates.
In conclusion, while any axis can be chosen in theory, the z-axis is frequently the most convenient for planar static equilibrium problems as it often simplifies the calculations involved.