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Rotational equilibrium and tension question

  1. Nov 10, 2006 #1
    I'm having a hard time with this rotational equilibrium problem: There is an attached image that shows the free-body diagram.

    The arm in the figure weighs 36.0 N. The force of gravity acting on the arm acts through point A. Determine the magnitudes of the tension force Ft in the deltoid muscle and the force Fs of the shoulder on the humerus (upper-arm bond) to hold the arm in the position shown.

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  3. Nov 10, 2006 #2


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    Staff: Mentor

    What torques have to be balanced? How are those torques related to the forces and distances shown?
  4. Nov 10, 2006 #3
    Well the entire system is in equilibrium since there is no movement. Therefore the sum of all the Torques = 0 and the sum of all the Forces =0. The forces are 1) the force of gravity going down point A (the center of gravity) 2)the Force exerted by the should on the humerus and 3) the force of tension in the deltoid muscle.

    I know that Torque = rFsina
    so the torque exerted by the tension in the deltoid = 0.21Ftsin168 and this is positive(?)
    and the torque exerted by the force of the shoulder = 0.29Fs(sina) and this is negative (?)
    Is there a torque involved in the center of gravity? I didn't think so. But that's about as far as I could get.
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