Final answer:
In 3D equilibrium, a force does not cause a moment about an axis if its line of action passes through that axis, as this results in a zero moment arm, and therefore no torque is generated.
Step-by-step explanation:
In 3D equilibrium, a force may not cause a moment about an axis when its line of action passes through the axis. A moment or torque is a measure of the tendency of a force to rotate an object about an axis. If the line of action of the force intersects the axis, the moment arm is zero, and thus no moment is produced.
A point is acted on by two forces in equilibrium, the forces have equal magnitudes but opposite directions. This satisfies the first condition for equilibrium, where the net force on the object is zero.
For planar problems, typically three equilibrium conditions are needed. Forces causing torques are considered in the xy-plane where the z-axis is the axis of rotation. The net torque has only a z-component and is calculated from the x- and y-components of external forces.
It is crucial to remember that only the force component perpendicular to the line connecting the axis of rotation and the point of application of the force results in a torque about that axis, by the principle of moments.