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Homework Help: Just set me in the right path

  1. Aug 27, 2007 #1
    1. The problem statement, all variables and given/known data
    A patient in therapy has a forearm tht weights 24.5 N and that lifts a 112.0 N weight. These two forces have direction vertically downward. The only other significant forces on his forearm come from the biceps muscle (which acts perpendicularly to the forearm) and the force at the elbow. If the biceps produces a pull of 232 N when the forearm is raised 43 degrees above the horizontal, find the magnitude and direction of the force that the elbow exerts on the forearm. (THE SUM OF THE ELBOW FORCE AND THE BICEPS FORCE MUST BALANCE THE WEIGHT OF THE ARM AND THE WEIGHT IT IS CARRYING, SO THE VECTOR SUM MUST BE 136.5 N upward.)

    2. Relevant equations
    Ok, there are three vectors.
    VECTOR A: magnitude of 136.5 N, angle is 270, x comp: 0, y comp: -136.5
    VECTOR B: magnitude of ? , angle is 43, x comp: ?, y comp: ?
    VECTOR C: magnitude of 232 N, angle is 43 degrees west of north, x comp: 232sin43: 158.2; y comp: 232cos43 = 169.7.

    3. The attempt at a solution

    so, since I don't know know what the magnitude of B is, I did this:
    136.5 - 169.7= -33.3 which would be the y component of vector B I am guessing. Then I did this:
    Bsin43 = -33.3; but my magnitude is a negative. So.... What should I do now?

    Also, I really do not want to abuse this priviledge of getting help from geniuses in physics, but I try my best and if I don't get it, then I ask. If you can just guide me in a path that will get me the answer, I will appreciate that. Thanks again.
  2. jcsd
  3. Aug 27, 2007 #2
    you have a diagram?
    I would try to get it though without a diag.

    Edit: Couldn't get it, so a FBD would be good!
    Last edited: Aug 27, 2007
  4. Aug 27, 2007 #3
    Thanks for replying, I was close to the answer and the software accepted the answer, so thank you so much!!
  5. Aug 27, 2007 #4
    oohhh..k, np
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