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Torque problem?

  1. Oct 16, 2008 #1
    1. The problem statement, all variables and given/known data
    An athlete at the gym holds a 2.2 kg steel ball in his hand. His arm is 74 cm long and has a mass of 4.0 kg. What is the magnitude of the torque about his shoulder if he holds his arm in each of the following ways?
    1) Parallel out to his side
    2) Straight but 35 M below the horizontal


    2. Relevant equations
    t=rf


    3. The attempt at a solution
    I dont even know how to start this one :(
     
  2. jcsd
  3. Oct 16, 2008 #2

    LowlyPion

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    How does the distance of the ball and center of mass of the arm from his shoulder change horizontally? Isn't torque about a point the distance away through which the force acts? Hence the vertical force of gravity acts through the ball and center of mass straight down, so how far away along the horizontal in each of the 2 situations?
     
  4. Oct 16, 2008 #3
    I got the first part by using
    (4*.37) + (2.2*.74) = 3.108*9.8=30.46

    Now im having trouble with the second part, wouldnt I just use that answer times sin(35)? I get 17.468 and its telling me im wrong.
     
  5. Oct 16, 2008 #4

    LowlyPion

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    Isn't Cos the horizontal distance?
     
  6. Oct 16, 2008 #5
    I got the answer! thank you so much!
     
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