To practice Problem-Solving Strategy 8.1 Static equilibrium problems. When you l

Question

To practice Problem-Solving Strategy 8.1 Static equilibrium problems. When you lift an object by moving only your forearm, the main lifting muscle in your arm is the biceps Suppose the mass of a forearm is 1 50 kg. if the biceps is connected to the forearm a distance = 3.50 cm from the elbow, how much force F_biceps must the biceps exert to hold a 550 g ball at the end of the forearm at distance d_ball = 37 0 cm from the elbow, with the forearm parallel to the floor? How much force F_elbow must the elbow exert? (Figure 1) Determine the sign (+ or -) of the torque about the elbow caused by the biceps, T_forearm, the sign of the weight of the forearm, T_forearm, and the sign of the weight of the F_ball. Express your answers as using + and - separated by commas.

Explanation / Answer

Sum the moments about the elbow:

0.55 * 9.8 * 0.37 + 1.5 * 9.8 * 0.185 = Fb * 0.035

Fb = 134.68 N

in vertical direction

(0.55 + 1.5) * 9.8 + Fe = Fb

Fe = 114.59 N

part F )

tb = 0.035 * 134.68 = +4.7138

tf = 0.185 * 1.5 * 9.8 = -2.7195

tball = 0.37 * 0.55 * 9.8 = 1.9943

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