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Moment about arbitary point

  1. Sep 24, 2015 #1
    1. The problem statement, all variables and given/known data
    Why the moment isn't=15* 3Nm? Since we are taking moment about point O...it's 3m away from O

    2. Relevant equations


    3. The attempt at a solution
     

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  2. jcsd
  3. Sep 24, 2015 #2

    haruspex

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    Are you referring to the 15kNm moment that is applied? You don't multiply that by a distance. (It would give you something with units kNm2.) a force times a perpendicular distance gives a moment, but an applied moment is already a moment. Exactly where it is applied makes no difference, only its magnitude and direction matter.
     
  4. Sep 24, 2015 #3
    ya , i knew that . But , how can be moment be applied? only force can be applied , right?
     
  5. Sep 24, 2015 #4

    haruspex

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    Are you asking as a practical matter how it is possible to apply a moment as opposed to a force? There does not need to be a way to do that. Consider turning a nut using a spanner. One can think of it as applying a torque, or as applying two equal and opposite forces along parallel but different lines of action. If you are told a moment of some specified magnitude and direction is applied, you do not need to care about how it is applied.
     
  6. Sep 24, 2015 #5
    yes, this will only occur in the exercise , but not in daily life ?
     
  7. Sep 24, 2015 #6

    haruspex

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    I cannot think of a way to apply a torque to an object (in an inertial frame) other than by a combination of linear forces.
     
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