Verify that the force in the elbow joint in Example 9.4 is 407 N, as stated in the text.

Data given in Example 9.4

Calculate the force the biceps muscle must exert to hold the forearm and its load as shown in Figure 9.25, and compare this force with the weight of the forearm plus its load. You may take the data in the figure to be accurate to three significant figures.

Strategy

There are four forces acting on the forearm and its load (the system of interest). The magnitude of the force of the biceps is F_B; that of the elbow joint is F_E; that of the weights of the forearm is w_a, and its load is w_b. Two of these are unknown (F_B and F_E), so that the first condition for equilibrium cannot by itself yield F_B. But if we use the second condition and choose the pivot to be at the elbow, then the torque due to F_E is zero, and the only unknown becomes F_B.

Triceps muscle FE 4.0 cm 7 16 cm FB Biceps muscle m = 2.5 kg CG Wa 38 cm CG 13 (a) (b) PHYSICS m = 4.0 kg CG W CG Wb Free-body diagram FB W