Chapter 8, Problem 85AP

The Iron Cross When a gymnast weighing 750 N executes the iron cross as in Figure l*N.91a, the primary muscles involved in supporting this position are the latissimus dorsi (“lats") and the pectoralis major (“pecs"). The rings exert an upward force on the aims and support the weight of the gymnast. The force exerted by the shoulder joint on the arm is labeled ${\stackrel{\to }{\text{F}}}_{\text{s}}$, while the two muscles exert a total force ${\stackrel{\to }{\text{F}}}_{\text{w}}$ on the arm. Estimate the magnitude of the force ${\stackrel{\to }{\text{F}}}_{\text{w}}$. Note that one ring supports half the weight of the gymnast, which is 375 N as indicated in Figure P8.91b. Assume that the force ${\stackrel{\to }{\text{F}}}_{\text{w}}$ acts at an angle of 45° below the horizontal at a distance of 4.0 cm from the shoulder joint In your estimate, take the distance from the shoulder joint to the hand to be L = 70 cm and ignore the weight of the arm.