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Distance between the force and origin

  1. Sep 17, 2015 #1
    1. The problem statement, all variables and given/known data
    In this note , I was told that either vector OD , OC , AD and AC can be used as the distance between the Force and OA axis ... why ? To produce moment , the distance r must be originate from the origin of force ( point C ) , am i right ? So , IMO, only r AC can be used...

    2. Relevant equations


    3. The attempt at a solution
     

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  2. jcsd
  3. Sep 17, 2015 #2

    Doc Al

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    Note the presence of the cross product when calculating the moment. Would any of those choices give a different result?
     
  4. Sep 17, 2015 #3
    Sorry, I still don't understand. Can you explain further?
     
  5. Sep 17, 2015 #4

    Doc Al

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    Realize that when you take the cross product ##\vec{r} \times \vec{F}##, where ##\vec{r}## is a vector starting at pt O and extending anywhere along the line of ##\vec{F}##, that you get the same answer. Essentially you are finding the perpendicular distance between pt O and the line.

    In simpler terms: r X F = r sinθ F. Realize that when r changes, θ also changes so that r sinθ remains the same. Play around with this until you get it.
     
  6. Sep 17, 2015 #5
    from the figure , it show that the vector r _OD is not along the vector F ....
    By saying ##\vec{r}## is a vector starting at pt O and extending anywhere along the line of ##\vec{F}## , do you mean like this ? the r can be any vector ( in different colours ) along the vector F ?
     

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  7. Sep 17, 2015 #6
    Since Force at C is moved towards D , why not Vector r_CD is used ?
     
  8. Sep 18, 2015 #7

    Doc Al

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    Yes.
     
  9. Sep 18, 2015 #8

    Doc Al

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    Vector r begins at pt O.
     
  10. Sep 18, 2015 #9
    sorry, I'm still confused now . IMO, vector r _OD is not along the vector F .
     
  11. Sep 18, 2015 #10
    Can you explain further?
     
  12. Sep 18, 2015 #11

    Doc Al

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    That's true, it is not. Why would you think it would be?

    Are you confusing it with vector CD?
     
  13. Sep 19, 2015 #12
    Becoz I was told that the vector r must originate from anywhere along line of action ( force)
     
  14. Sep 19, 2015 #13

    Doc Al

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    The vector r originates at point O and terminates anywhere along the line of action of the force.
     
  15. Sep 19, 2015 #14
    the line of action refers to the line along F , right ? Vector CD isn't on this line , so i think vector CD is not suitable ...
     
  16. Sep 19, 2015 #15

    Doc Al

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    Right.

    Looks to me like F points along the line CD.
     
  17. Sep 19, 2015 #16
    Ya, it points along cd? Why vector cd is not used? But vector od is used?
     
  18. Sep 19, 2015 #17

    Doc Al

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    You seem to be mixing up ##\vec{r}## with ##\vec{F}##. The vector r must begin at O and end somewhere on the line of action of F. So r could be OD or OC or anything in between, but not CD!
     
  19. Sep 19, 2015 #18
    oh , yeah . I forgot that F can be longer . It doesnt necessarily ends at point D. that's why I'm confused.
     
  20. Oct 15, 2015 #19
    if i use MOA = uOA . (rOC X F) , then my ans = 108 , if i use MOA = uOA . (rOD X F) , then the ans = 100 , is it correct ?
     
  21. Oct 16, 2015 #20
    sorry , i didnt mean to spam , can someone help me with this ?
     
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