Moment is merely another term meaning torque, which is something producing or tending to produce rotation or torsion. Common examples of moment or torque are numerous. Pushing on a revolving door while walking through it, tightening a nut with a wrench, and turning the steering wheel of a car all involve moments. The moment is present whether there is actual rotation or only a tendency to rotate. If a force is acting some distance away from a point, such as the fulcrum of a lever, it causes a twisting action about the point. This twisting action, or torque, is called a moment. The magnitude of the moment depends upon both the size of the force and the perpendicular distance from the force to the point.

In Figure 3-1, the moment M = (F)(d) is in a clockwise direction. Recall from Section 2-1 the principle of transmissibility states that a force can act anywhere along its line of action. Force F can therefore act at any of the three locations shown in Figure 3-1 and still produce the same moment.

FIGURE 3-1 moment = force the perpendicular distance between the axis and the line of action of the force The units of moment are pound-feet (lb-ft), pound-inches (lb-in.), or kip-feet (kip-ft). When expressed this way, moment is easily distinguished from work, which is stated in units of ft-lb and in.-lb. In the SI metric system, force is expressed in newtons, and distance in meters. The units of moment or torque are newton-meter (Nm). In two-dimensional drawingswhich are frequently usedthe axis appears as a point. Moments taken about a point are indicated as being clockwise or counterclockwise (Figure 3-2). For the sake of uniformity in calculations, we will assume clockwise to be negative and counterclockwise to be positive. Moments are vector quantities, and their direction must be indicated in one of three ways. For example, the same moment can be expressed as 10 lb-ft , 10 lb-ft, or 10 lb-ft clockwise.

FIGURE 3-2

30 N A 70 70 .1 m .3 m 30 NF d F F5 kN B 75 2 KN 3 m A .5 m .2m- 2 kN