Improper lifting and the back A careful study of human anatomy allows medical resear...

Improper lifting and the back A careful study of human anatomy allows medical researchers to use the conditions of equilibrium to estimate the internal forces that body parts exert on each other while a person lifts in a bent position ( Figure 7.27a). Suppose an 800-N (180-lb) person lifts a 220-N (50-lb) barbell in a bent position, as shown in Figure 7.27b. The cable (the back muscle) exerts a tension force uT M on B on the backbone and the support at the bottom of the beam (the disk in the lower back) exerts a compression force u FD on B on the backbone. The backbone in turn exerts the same magnitude force on the 2.5-cm-diameter fluid-filled disks in the lower backbone. Such disk compression can cause serious back problems. A force diagram of this situation is shown in Figure 7.27c. The magnitude of the gravitational force u FE on Bthat Earth exerts on the center of mass of the upper stomachchest region is 300 N. Earth exerts a 380-N force on the head, arms, and 220-N barbell held in the hands. Using the conditions of equilibrium, we estimate that the back muscle exerts a 3400-N (760-lb) force uT M on B on the backbone and that the disk in the lower back exerts a 3700-N (830-lb) force u FD on B on the backbone. This is like supporting a grand piano on the 2.5-cm-diameter disk.

Rank in order the magnitudes of the torques caused by the four forces exerted on the backbone (see Figure 7.27c), with the largest torque listed first.

(a) 1 7 2 7 3 7 4 (b) 2 = 3 7 1 7 4 (c) 3 7 2 7 1 7 4 (d) 2 7 1 7 3 7 4 (e) 1 = 2 = 3 = 4