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Homework Help: Predicting the distance where torque should be applied?

  1. Nov 7, 2012 #1
    1. The problem statement, all variables and given/known data
    Finding the mass of the meterstick!!
    Distance was found through experimentation
    I am given a mass hanger of 50g to place on a 100cm ruler held up on its center of mass at 49.8cm
    I displace the clip holding the ruler by 15cm to the left. (assuming the ruler counts up from left to right) The experimental value in practice was 13.1cm for a balanced ruler. (How am I to calculate the assumed value?) With this information how am I to calculate the mass of the meterstick?

    If not sufficient information is given or hard to imagine I could draw a picture and upload it.

    2. Relevant equations
    Mass of ruler: 95.1 g
    τ = rfsinθ
    Ʃ F = 0
    Ʃ τ= 0

    3. The attempt at a solution
    I don't know what to plug in, I also think I am missing an equation. Trying to visualize the problem, I see how moving the clip to the left that I am increasing the torque in the problem, and to balance the meterstick, I must place the 50g mass at a certain distance to achieve equal amount of torque on both sides of the meterstick thus achieving equilibrium and a balanced meterstick. Am I missing something for finding the mass?
    Last edited: Nov 7, 2012
  2. jcsd
  3. Nov 8, 2012 #2


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    Hi JazzyJones ! :smile:

    i] what is the torque of the 50 g mass?

    ii] what is the distance of the centre of mass (from the pivot)?

    iii] so what must the meterstick's mass be? :wink:
  4. Nov 8, 2012 #3
    I'm taking a guess at this but what if I do this,
    .05kg*9.8m/s^2*.0217m = m*9.8m/s^2*.015
    right? because the net torque will equal 0
  5. Nov 8, 2012 #4


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    Hi JazzyJones! :smile:

    (try using the X2 button just above the Reply box :wink:)
    Where does the 21.7 cm come from? :confused:

    Apart from that, it looks right :smile:

    the LHS is the torque of the 50g mass, and the RHS is the torque of the unknown m,

    so now you just divide out to get m.

    btw, you should leave g as "g" (instead of 9.8), since it'll cancel out anyway,

    and you should leave everything in gram and cm (instead of converting to kg and m), since that'll cancel out also! :wink:
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