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Lever arm on muscle

  1. Jun 18, 2013 #1
    1. The problem statement, all variables and given/known data
    The diagram is attached. It is asking which position has the greatest lever arm.

    2. Relevant equations

    3. The attempt at a solution
    So, I know that the elbow is the fulcrum but what I'm struggling with is how to draw the force arrow. Where is the force coming from and where is it pointing? What is a good way to know where to draw the lever arm?

    Attached Files:

  2. jcsd
  3. Jun 18, 2013 #2
    The force comes from the contracting muscle. So look at the direction in which the connecting tissue from the muscle connects to the bone. The force will pull along this direction on the forearm. The lever arm is then the perpendicular distance to this direction.
  4. Jun 18, 2013 #3
    So is the lever arm the forearm?
  5. Jun 19, 2013 #4
    No. It is the perpendicular distance dropped from the fulcrum onto the line of action of the force.
  6. Jun 20, 2013 #5
    So are the black lines that I drew correct for the line of force?
    And the red for the lever arms?

    Attached Files:

  7. Jun 21, 2013 #6
    The question should actually state explicitly for which force the lever arm is required. There are several forces in this diagram, but I am getting the impression its wants you to get the lever arm of the force of the weight held in the hand. The weight F acts downwards along the pink line in the first diagram and its lever arm is the orange L. If the lever arm of the force that the muscle is exerting on the forearm were required it would be the much shorter L in the second diagram.

    Attached Files:

    Last edited: Jun 21, 2013
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