Consider the person lifting a heavy box with his back, shown in Figure 9.28.

(a) Calculate the magnitude of the force F_B-in the back muscles that is needed to support the upper body plus the box and compare this with his weight. The mass of the upper body is 55.0 kg and the mass of the box is 30.0 kg.

(b) Calculate the magnitude and direction of the force F_V-exerted by the vertebrae on the spine at the indicated pivot point. Again, data in the figure may be taken to be accurate to three significant figures.

Strategy

By now, we sense that the second condition for equilibrium is a good place to start, and inspection of the known values confirms that it can be used to solve for F_B-if the pivot is chosen to be at the hips. The torques created by W_ub and W_box-are clockwise, while that created by F_B-is counterclockwise.

Transcribed Image Text:

8 cm- Fo Pivot point 10% FB 29% 39° 35 cm 50 cm 61° CGub Wub CGbox Wbox