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Homework Help: Moments WRT Forearm

  1. Mar 10, 2014 #1
    1. The problem statement, all variables and given/known data

    Figure 5 below shows an approximation of the system of forces acting on the human forearm. The weight of the forearm is 25N and acts 15cm from the elbow of a girl lifting a dumbbell of weight 50N

    Find the
    1)Force, F, exerted on the human forearm by the muscle in the upper arm
    2)Forces acting at the point, E of the elbow
    3)Tension if theta is 45 degrees

    2. Relevant equations
    sum of ACW = sum of CW
    sum of upward forces = sum of downward forces

    3. The attempt at a solution

    My Attempt is in the pictures below

    Attached Files:

  2. jcsd
  3. Mar 11, 2014 #2


    User Avatar

    Staff: Mentor

    Hi Ronaldo95163. You could use ∑ forces iff you knew the force acting at the elbow, but we don't know anything about force components at E (apart from the fact that they exist). So your first part is wrong. You will have to approach it differently.

    In any case, because of the big mechanical disadvantage seen in this type of lever, you know that the upwards force needed close to the pivot point is going to be >> the two downward forces you have considered.
  4. Mar 11, 2014 #3
    Thanks alot man...wasn't sure if to consider thet force at E first...what I figured tho was that
    Fsin70 = 75 + E

    Solve for E using moments and then substitute it in to find for F
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