A person holds a 5.0-N baseball in his hand, a distance of 43.0cm from the elbow
ID: 2251765 • Letter: A
Question
A person holds a 5.0-N baseball in his hand, a distance of 43.0cm from the elbow joint, as shown in figure 11-23 (refer to page 366 for the diagram, this is same problem with different numbers). The biceps, attached as a distance of 2.75 cm from the elbow, exerts an upward force of 20.4 N on the forearm. Consider the forearm and hand to be a uniform rod with a mass of 1.2 kg.
a.) Calculate the net torque acting on the forearm and hand. Use the elbow joint as the axis of rotation.
b.) if the net torque obtained in part (a) is nonzero, in which direction will the forearm and hand rotate
c.) Would the torque exerted on the forearm by the biceps increase or decrease if the biceps were attached farther from the elbow joint?
Explanation / Answer
a)
weight of fore arm = mg =1.2*9.8 = 11.76 N
?down-ball=d*F*sin(theta)=0.43*(-5)*sin(90)=-2.15 N
?down-arm = d*F*sin(theta)=0.215*(-11.76)*sin(90)=-2.494 N
?up =d*F*sin(theta)=0.0275*20.4*sin(90)=0.561 N
net torque=0.561-2.15-2.494
=-4.083 N*m ....is answer
b)
if the net torque is nonzero,then forearm and hand rotated downward
c)
the torque exerted on the forearm and hand increase if the biceps were attached farther from the elbow joint.
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