College Physics

11th Edition
Raymond A. Serway + 1 other
ISBN: 9781305952300

Chapter
Section

College Physics

11th Edition
Raymond A. Serway + 1 other
ISBN: 9781305952300
Textbook Problem

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 F → s , while the two muscles exert a total force F → w on the arm. Estimate the magnitude of the force F → 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 F → 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.

To determine
The magnitude of the force Fm .

Explanation

Explanation

The length of the arm is 70â€‰cm , the distance from the shoulder joint to the force Fm is 4.0â€‰cm , the weight on the ring is 375â€‰N , and the distance from the shoulder joint to the hand is 70 cm.

The figure shows the forces acting on the shoulder joint and the muscle.

The body of the gymnast in at rest, so the equilibrium condition is,

âˆ‘Ï„=0

The equilibrium condition can be written in terms of Fm as

(Fw)(L)âˆ’(Fmsin45Â°)(l)=0

• Where Fw is the weight on the ring
• L is the length of the arm
• l is the distance from the shoulder join to the force Fm

Rewrite the above expression in terms of Fm

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