Newton’s first law: When a body is in equilibriumin an inertial frame of reference—that is, either at rest ormoving with constant velocity—the vector sum offorces acting on it must be zero (Newton’s first law).Free-body diagrams are essential in identifying theforces that act on the body being considered.Newton’s third law (action and reaction) is also frequentlyneeded in equilibrium problems. The two forcesin an action–reaction pair never act on the same body.(See Examples 5.1–5.5.)The normal force exerted on a body by a surface is not