A 72.0 kg weight lifter is doing arm raises using a 7.50 kg weight in her hand.

Question

A 72.0 kg weight lifter is doing arm raises using a 7.50 kg weight in her hand. Her arm pivots around the below joint, starting 40.0degree below the horizontal. (See the figure Figure 1).) Biometric measurements have shown that both forearms and the hands together account for 6.40 % of a person's weight. Since the upper arm is held vertically, the biceps muscle always acts vertically and is attached to the bones of the forearm 5.50 cm from the below joint. The center of mass of this person's forearm-hand combination is 17.5 cm from the below joint, along the bones of the forearm, and the weight is held 38.0 cm from the below joint. What force does the biceps muscle exert on the forearm? Find the magnitude of the force that the below joint exerts on the forearm.

Explanation / Answer

Part A

Balancing torques of elbow joint

F ( 5.5) = 72x0.064 x9.8 x5.5 cos40 + 7.5x9.8x38 xcos 40

F = 423.6 N

part B)

423.6 = 7.5+(72*0.064) 9.8 + F1

F1 = 304.9N

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