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Equilibrium when applied to muscles and joints! Help, please!

  1. Nov 6, 2005 #1
    This seems like it should be easy but Im not sure what to do. Heres the problem:
    The picture to the problem is here: http://www.webassign.net/giancoli/9-65alt.gif
    "(a) Calculate the magnitude of the force FM required of the "deltoid" muscle to hold up the outstretched arm shown in Fig. 9-65. The total mass of the arm is 2.8 kg and = 12°.
    (b) Calculate the magnitude of the force FJ exerted by the shoulder joint on the upper arm.
    I found a) by doing .12 m (sin 12) F - .25 m (2.8 kg) (9.8 m/s^2) = 0. F = 264 N.
    But how do I find the force FJ? Confused (we haven't had the lesson on this yet, but trying to get my homework done early because I have a busy week). THANKS for any help!!!
  2. jcsd
  3. Nov 6, 2005 #2


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    Homework Helper

    your torque looks okay.

    Now that you know all the Forces, you could treat it like a chapter 3 problem
    (Sum of the force components (x,y) = 0 . you DID enjoy that, right?)

    OR, you can sum torques around the tendon attachment point, to = 0
    (pretending that it might rotate around this attachment point)
    This gives the vertical component of the joint Force (Fjx applies no torque).
  4. Nov 6, 2005 #3
    thanks lightgrav, adding up the components worked. answer turned out to be 260 N. Thank you for the help!
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