By considering the torques about the centersof the ball joints in your shoulders

Question

By considering the torques about the centersof the ball joints in your shoulders, estimate the force yourdeltoid muscles (those muscles on top of the shoulder) must exerton your upper arm to keep your arm held out and extended atshoulder level. Then, estimate the force they must exert when youhold a 10-lb weight out to the side at arm's length. (Assume thatthe arm length is about 60 cm, arm weight is 10 lbs, and thedeltoid muscle attached to the arm is about 20 cm from the shoulderat an angle of about 10°.)

F=_____lb
F(10)=_____lb

Explanation / Answer

a) F*l*sin-1/2*m*g*L=0 F=(m*g*L)/(2*l*sin) F=(10*60)/(2*20*sin10) ˜ 86 lb b) F(10)*l*sin-1/2*m*g*L=0 F(10)=(m*g*L)/(2*l*sin) F(10)=(10*60+2*10*60)/(2*20*sin10) ˜ 260 lb

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