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Forces in the arm
A lower arm and hand together have a mass of 3.2 kg and are approximated by a uniform rod 32 cm long. The arm is held in a horizontal position below a vertical upper arm by the flexor muscle or biceps that is attached 2 cm forward of the elbow joint. The elbow joint is 2.0 cm in from the elbow end of the lower arm bone. What is the force of the biceps on the arm if the upper arm is completely vertical and the lower arm is completely horizontal? What is the magnitude and direction of the force of the upper arm on the lower arm?
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This problem is quite simple if you have a good diagram. Draw this scenario but instead actually drawing an arm, just draw a rod with force arrows on it. From here, it should be obvious that this is a simple equilibrium question where the torques (F * d) sum to zero.