A person holds a 1.42 N baseball in his hand, a distance of 34.0 cm from the elb

Question

A person holds a 1.42 N baseball in his hand, a distance of 34.0 cm from the elbow joint, as shown in the figure . The biceps, attached at a distance of 2.75 cm from the elbow, exerts an upward force of 12.6 N on the forearm. Consider the forearm and hand to be a uniform rod with a mass of 1.30 kg.
Calculate the net torque acting on the forearm and hand. Use the elbow joint as the axis of rotation.

I understand that youre supposed to : 12.6*.0275-(1.2)(9.81)(.17)-(1.42)(.34)...... the part where im not understaning is when you do (1.2)(9.81)(.17) why you use (0.17) i know thats half of 0.34 but how do you know that's the distance youre supposed to use? you just assume the distance of the forearm is half the total distance but why?

Explanation / Answer

this is because we have to use the center of mass of the arm that is at the center of the arm (middle point) if the arm is considered to be of uniform mass distribution.

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