Final answer:
The perpendicular force to a force projected onto an axis is the normal force, which in the context of circular motion, equates to the centripetal force. Forces are in equilibrium when they are of equal magnitude but opposite direction, not necessarily when they are perpendicular to each other.
Step-by-step explanation:
The magnitude of the force perpendicular to a force projected onto an axis is defined as the normal force. For instance, when an external force that is horizontal toward the center of curvature is exerted, such as in circular motion, only the normal force has a horizontal component, which must equal the centripetal force, or mv²/r. Furthermore, two forces are in equilibrium when they have equal magnitudes but opposite directions. Shearing forces are applied perpendicular to a length and parallel to the cross-sectional area, resulting in deformation but are not related to the context of the normal force being horizontal and related to centripetal motion.