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Homework Help: Moments of force

  1. May 17, 2010 #1
    1. The problem statement, all variables and given/known data

    A uniform metre rule of mass 100 g is supported by a knife-edge at the 40 cm mark and a string at the 100 cm mark. The string passes round a frictionless pulley and carries a mass of 20 g as shown in the diagram.

    At which mark on the rule must a 50 g mass be suspended so that the rule balances?

    2. Relevant equations



    3. The attempt at a solution
    using the [tex]\sum F[/tex]=0
    therefore, R at the knife-edge + T at the 100 cm = W
    R = 0.8
    then take the moment at the c.g, which is at 0.5
    therefore R(z)=T(50)
    in the end,what i got is z=0.3125
    where T=0.5N
    wrong again....
     
  2. jcsd
  3. May 17, 2010 #2

    tiny-tim

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    Hi look416! :smile:

    (have a sigma: ∑ :wink:)

    I don't understand what you've done …

    there should be moments of three forces …

    and where does T = 0.5N come from? :confused:

    (btw, no need to convert to N … they're all weights, so just use the masses :wink:)
     
  4. May 17, 2010 #3
    lolz
    maybe im not good in explaining XD
    anyway
    if i take the head of the ruler as the moment
    which means
    RX + 50(100) = 50(100)
    which results in RX = 0
    =.=
    T = 0.5N is because the question demand for the X cm when the load is 50g
    for 50g x 10 x 10^-3 = 0.5N
     
  5. May 17, 2010 #4

    tiny-tim

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    uhh? :redface: where's the 40cm? where's the 20g? and what's R? :confused:
     
  6. May 17, 2010 #5
    =.=
    so, i have to find the value of R by taking moments as the head of the ruler
    if so,
    R(40)+20(100)=50(100)
    R=75g
    then??
     
  7. May 18, 2010 #6

    tiny-tim

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    Is R the reaction force at the knife-edge?

    If so, you don't find it by taking moments, you find it by adding the vertical components of force to zero …

    which I thought you'd already done, and found it was 80g (weight) ??
    You haven't included the 50 g mass in this moments equation. :confused:

    (btw, if you take moments about the knife-edge you won't need to find R anyway)
     
  8. May 18, 2010 #7
    but there said at which mark on the rule must a 50 g mass be suspended so that the rule balances?
    so shouldnt i have to included the knife edge in my calculation?
    not taking it as the moments ...
     
  9. May 18, 2010 #8

    tiny-tim

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    (i wish you wouldn't say "taking it as the moments" … you take moments of forces about a point :wink:)

    Yes, the 50g mass, and its unknown position, must be included (unless you take moments about that position).

    You can take moments about any point …

    it can be either end of the ruler, or the knife-edge, or the point where the 50g is, or indeed anywhere else, either on or off the ruler.

    But in this case it's easier to use the knife-edge, since that avoids working out the reaction force, R. :smile:
     
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