The deltoid muscle connects the shoulder blade to the upper arm (humerus), and s

Question

The deltoid muscle connects the shoulder blade to the upper arm (humerus), and so contraction of the deltoid raises the arm (Fig. 10–35). The deltoid tendon connects the humerus 15.0 cm from the shoulder joint at an angle of 20.0 with the horizontal. Suppose the arm weighs 35.0 N and its center of gravity is 38.0 cm from the shoulder joint. A weight W is held in the outstretched hand 90.0 cm from the shoulder joint. Find the tension in the deltoid tendon in terms of W. Find the force the shoulder joint exerts on the humerus if W 40.0 N. Why is this force so large even for such a small weight?

Explanation / Answer

balancing moment about the shuolder point,

M = (15 F sin20) - (38 x 35) - (90 W) = 0

F = 259.24 + 17.54W

putting W = 40 N

F = 961 N ....Ans

because point of application of this force is much closer to shoulder point.

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