posted by Sara on .
An athlete at the gym holds a 4.33 kg steel ball in his hand. His arm is 68.7 cm long and has a mass of 4.69 kg. What is the magnitude of the torque about his shoulder if he holds his arm straight out to his side, parallel to the floor?
Torque = massball x g + massarm x g/2
= 4.33 kg x 9.8 + 4.69 kg x 9.8/2
= 42.434 + 22.981
= 65.415 N
Why is my answer incorrect?
Your answer is incorrect because to failed to include the length of his arm when calculating the torque. Your answer has the dimension of force, not torque.
The factor of 1/2 is correct for the torque due to the weight of his arm, IF the arm has uniform mass per unit length. Most arms do not, but they probably want you to assume that.