For purposes of analysis, forces and moments acting on a structure or structural element can be grouped into two categories: loads and reactions. The loads acting on a structure include gravitational forces, inertial forces, friction, wind, lift, drag, hydrostatic pressure, soil pressure, and impacts. Supports are used to prevent a body from moving when subjected to these loads. Reactions are those moments and forces that act on the body as a consequence of the restraint provided by the supports. The magnitudes of reactions are controlled by the magnitudes of the applied loads. It should be understood that a reaction does not necessarily occur as a consequence of attaching a structure to the ground. Each structural element can have reactions due to its being connected to or supported by another structure or structural element. The type of reaction depends on the physical characteristics of the support. In order for a structure or structural element to be stable, there must be a sufficient number of supports to prevent it from undergoing unrestrained displacements.