Statics is the branch of mechanics that deals with forces acting on bodies that are motionless. We can combine forces much as we do velocities. If forces P1 and P2 with resultant R, act at a point, there must be a third force, P3, equal and opposite to R, for equilibrium. We can combine forces similarly over more complex structures. For a bridge whose weight (w) acts as a downward force W at the center, the upward reactions R1 and R2 at the piers must be such that R1 + R2 = - W for it to be in equilibrium. Similarly, we examine the members of the bridge to determine the stresses acting on each.