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How to calculate moments to find a force

  1. Feb 17, 2012 #1
    I'm looking at a problem and am having trouble finding the force using moments. To be honest i'm not sure am i going about it the correct way.

    Attached is a sketch of the problem,

    Is the cclockwise moment about A = .5m(1000N) = 500Nm
    and the anti clockwise moment about A = .75m(F)

    can these be equated to find F??


    Attached Files:

  2. jcsd
  3. Feb 17, 2012 #2
    Your image is too tiny.
  4. Feb 17, 2012 #3
    Sorry, attached is a bigger image in a word document.

    Attached Files:

    • FBD.doc
      File size:
      231.5 KB
  5. Feb 17, 2012 #4


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    Welcome to PF!

    Hi Sully1071! Welcome to PF! :wink:
    Yes, that's exactly correct :smile:

    (except that you've mixed up clockwise and anti-clcokwise! :rolleyes:)

    and yes, you now equate the clockwise to the anti-clockwise.

    Why is that worrying you? :confused:
  6. Feb 19, 2012 #5
    Im just unfamiliar with unfamiliar with these calculations but have to do them as part of a project.

    If i use the new free body diagram attached and the unit was to rotate about point a i assume my new moments would be calculated by:

    Anticlockwise moments = 1000(.65)
    Clockwise moments = F(.6)

    Attached Files:

  7. Feb 19, 2012 #6


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    Hi Sully1071! :smile:

    Is this a new problem? …

    sorry, I'm on an iMac, and it won't show images in .doc :redface:

    (but I could see your original .jpg)
  8. Feb 19, 2012 #7
    jpeg is attached

    Attached Files:

  9. Feb 19, 2012 #8


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    ah! nice and big! :biggrin:
    yes that's right :smile:

    generally, the moment of a force F is the dot product F.d,

    which you can do as either F times the perpendicular distance (your method)

    or as F times the whole distance times sine of the angle between :wink:
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