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Principle of moments

  1. Jul 14, 2006 #1
    Hello,
    This is probably really simple but i dont know if this is the right way to work out the answer
    i have attached an image containing the problem.

    I know how to get F2 :
    Taking moments about A:
    F2 Cos 60 = 1500
    F2 = 3000N

    But then it says to find F1 using the principle of moments, and the only way i can get the answer is:
    taking moments about B: (F1 x 1.5) = (1500 x 2.5)
    F1 = (1500 x 2.5) / 1.5
    F1 = 2500N

    is this right?

    Thanks alot
    Cain
     

    Attached Files:

  2. jcsd
  3. Jul 15, 2006 #2
    Well, you can check your answer by just using trig like you did earlier.
    [tex]F_1 = F_{2}*cos(30) = 1500*sqrt{3}[/tex]

    You fell into the unfortunate trap of notation. The definition of a moment is that M = B x r, which shows us that the moment is the cross product of the vector and the scalar where the vector is applied. You should go back and nail down the concept of moments, then try the problem out again.
     
  4. Jul 17, 2006 #3
    Your original equation is not right.

    To sum moments, you need a force AND a distance, i.e. F2 (cos 60) * 1.5 = 1500 * 2.5.
     
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