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Homework Help: Momentsand Forces acting on a ruler

  1. Nov 15, 2006 #1
    I have a question about moments a forces acting on a meter ruler.

    "A meter ruler of weight 1.0N is pivoted on a nail passing through a hole drilled at the 10cm mark. A weight of 3.0N is suspended at the 30cm mark. A Newton meter supports the ruler at the 90cm mark so that it is horizontal.
    a) use the principle of moments to calculate the magnitude of force T needed to keep the ruler horizontal.
    b)the newton meter is raised until the rule makes an angle A with the horizontal. Without doing any further calculations copare the magnitude of the force provided by the newton meter in this new positionwith the force T when the rule was horizontal. Explain your answer."

    For part a I had (0.2 x 3) + (0.4 x 1) = 0.8T
    to give T = 1.25N

    For part b I am confused. Intuitively I think the force should have increased but I'm not really sure why and since none of the distances have changed and the weights are the same I don't how it can have changed.

    Please help!
  2. jcsd
  3. Nov 15, 2006 #2


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    Science Advisor

    Since it is still supporting the same weight, the vertical component of force must be the same. Now, the horizontal component is non-zero. what does that tell you about the magnitude of the force?
  4. Nov 15, 2006 #3
    If there is now a horizontal component the force must have increased to include this.

    Where does the horizontal component come from?
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