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Static Equilibrium of the Arm

  1. Oct 31, 2006 #1
    You are able to hold out your arm in an outstretched horizontal position because of the action of the deltoid muscle. Assume the humerus bone has a mass m=3.6, length L=0.66 and its center of mass is a distance L1=0.33 from the scapula. (For this problem ignore the rest of the arm.) The deltoid muscle attaches to the humerus a distance L2=0.15from the scapula. The deltoid muscle makes an angle of 17 degrees with the horizontal, as shown.
    http://session.masteringphysics.com/problemAsset/1011051/15/MTS_st_20_a.jpg
    http://session.masteringphysics.com/problemAsset/1011051/15/MTS_st_20_b.jpg
    Use throughout the problem.

    A. Find the tension in the deltoid muscle.

    I solved this T= 256, because the some of the torques is equal to 0 which is equal to mL1g-Tsin(theta)L2

    B.Using the conditions for static equilibrium, find the magnitude of the vertical component of the force exerted by the scapula on the humerus (where the humerus attaches to the rest of the body).

    This is whre I am lost. I know that there are three vertical forces in this problem: the force of gravity acting on the bone, the force from the vertical component of the muscle tension, and the force exerted by the scapula on the humerus (where it attaches to the rest of the body), but i don't know what to do with this.

    C.Now find the magnitude of the horizontal component of the force exerted by the scapula on the humerus.
    Same thing don't know what to do

    Please Help I have been sitting here for hours
    Thanks
     
  2. jcsd
  3. Oct 31, 2006 #2

    OlderDan

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    Apply the weight of the humerous at its center of mass. Resolve the deltoid tension into horizontal and vertical components. Include the horizontal and vertical components of the force exerted by the scapula. Write two equations of equilibrium; one for vertical and one for horizontal.
     
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