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Homework Help: Equilibrium and principle of moments

  1. May 14, 2016 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    Torque = F x D
    Manticlockwise = Mclockwise

    3. The attempt at a solution

    From what I understand, the moment on left hand side of the pivot point needs to be equal to the moment on right hand side, therefore...

    (Fx.2)+(5x.14) = (7xd)

    My initial thinking was to substitute 2N into F, so that both forces on either side of pivot point are equal to 7N. However, this was not correct because the resultant forces were not in equilibrium after checking.

    The answer in the book was this:


    Could someone explain to me why F wasn't factored into the working? The way I'm interpreting this is that there is no force acting at point F. Would this be a correct assumption?
  2. jcsd
  3. May 14, 2016 #2


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    Looks like a badly worded question to me.

    If you read Question (b) it says F replaces the 5N force. So F doesn't exist for question (a) ?
  4. May 14, 2016 #3


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    If F does exist for (a) then that's correct but you can't solve it. You have one equation and two unknowns so best you can do is write an equation for d in terms of F.
  5. May 14, 2016 #4


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    Staff: Mentor

    Part (b) explains that F acting at a distance 200mm is used in (b) as the substitute for the 5N force at a distance 140mm.

    So the diagram is doing double-duty, and F is relevant only to part (b).
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