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Centre of gravity and force

  1. Apr 27, 2008 #1
    1. The problem statement, all variables and given/known data
    This is the case:

    We gonna decide the centre of gravity to a flagstaff with length 10 m
    We saved up the flagg staff in the thick end. We then need to use a vertical force F in the thin end to keep the flagstaff in horizontal balance. We then saves up the flagstaff in the thick end. Then we need to use a horizontal force on 4F in the thick end too keep the flagstaff in horizontal balance.
    i) Count out where the centre of gravity to the flagstaff is.
    ii) Count out the force F when the mass of the flagstaff is 250 kg.

    Can anyone get me started ?
     
  2. jcsd
  3. Apr 27, 2008 #2

    HallsofIvy

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    I have no idea what this means:
     
  4. Apr 27, 2008 #3

    tiny-tim

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    Hi Newton86! :smile:

    I'm confused. :confused:

    Can you give us the question in the original language … there'll probably be a lot of people on the forum who'll understand it! :smile:
     
  5. Apr 27, 2008 #4
    Ok:)

    bildr.no/view/190755
    Not sure i quite understand it myself :confused:
     
  6. Apr 27, 2008 #5

    tiny-tim

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    ah … got it … !

    We rest the thick end of the flagpole on something (a wall, maybe), and we weigh the other end, result F.

    Then we rest the thin end of the flagpole on something, and we weigh the other end, result 4F.

    Hint: take moments about the wall.:smile:
     
  7. Apr 27, 2008 #6
    one moore hint ? :P
     
  8. Apr 27, 2008 #7

    tiny-tim

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    ok … one more hint …

    there are three forces acting on the flagpole …

    draw a diagram …

    draw a force N where it rests on the wall, its weight W vertically through the centre of gravity, and the force F (or 4F) vertically at the other end.

    You don't know what N is, and you don't want to know, so you eliminate N by taking moments about the wall.

    Have a go … ! :smile:
     
  9. Apr 27, 2008 #8
    [​IMG]

    Like this ? Is there moore forces ?
     
  10. Apr 27, 2008 #9

    tiny-tim

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    (btw, it's "more", not "moore")

    Hi Newton86! :smile:

    Nooo …

    That's a really bad diagram … what are those two vertical lines on either side? … and you should have the arrows starting (or ending) at the point of application of the force … and you haven't marked the length of the flagpole or the postion of the centre of gravity.

    And you need two diagrams … one with the wall at the thick end and F at the thin end, the other with the wall at the thin end and 4F at the thick end.

    It is very very important that you get used to making clear accurate diagrams. If you don't, you may make mistakes, or even not be able to answer the question at all. :redface:

    ok … draw one of the diagrams, and then take moments about the wall. :smile:
     
  11. Apr 27, 2008 #10
    hmm Not sure i understand it quite but I tryed again.

    [​IMG]

    Do I know where the center of the gravity is ?
     
  12. Apr 27, 2008 #11
    I dont get this Iv searched for other examples too but no one uses forces =|
     
  13. Apr 28, 2008 #12

    tiny-tim

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    Hi Newton86! :smile:

    Now that's a much better diagram! :smile:
    No … but that's what the question asks you to find … so the standard trick is to give it a name, like x, and put x in the diagram.

    ok … now what, in terms of W and x, is the moment of the weight about the wall?

    and what is the moment of the F force about the wall? :smile:
     
  14. Apr 28, 2008 #13
    I would like to know that. I dont find any formulas ( used all day ) :cry:
    How can a noob get such a task :mad:
     
  15. Apr 28, 2008 #14

    tiny-tim

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    cheer up! :smile:

    Moment about a point = force times distance (that's the distance from the point to the line of the force).

    In this case, the distances of the three forces from the wall are 0, x, and 10 in the first diagram, and … ? in the second diagram.

    So the total moment about the top of the wall is 0 + x.W - 10.F in the first diagram, and … ? in the second diagram.

    When there is no turning, the total moment about any point must be 0.

    (is any of this familiar?)

    So … ? :smile:
     
  16. Apr 28, 2008 #15
    Ah This looks familiar too vectors. :smile:

    In this case, the distances of the three forces from the wall are 0, x, and 10 in the first diagram, and the same distance in 0, x, 10 ? in the second diagram.

    So the total moment about the top of the wall is 0 + x.W - 10.F in the first diagram, and 0 + x.W - 10.4F ( -40F? )? in the second diagram.

    ( 0, x, -10 )( 0,x,-40) ?

    wild guess is the the center of gravity is 7,07m from the thin end
     
  17. Apr 28, 2008 #16

    tiny-tim

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    Hi Newton86! :smile:
    oooh :cry: … you didn't put x into your diagrams, did you?

    If you had, you'd have seen that it's 10 - x in the second diagram … right?

    That's why good diagrams are so important!

    So the answer is … ? :smile:
     
  18. Apr 28, 2008 #17
    ye I was unsure about that. But I dont see how I can use this too find the center of gravity
    and What about the other diagram was it wrong too?
     
    Last edited: Apr 28, 2008
  19. Apr 28, 2008 #18

    tiny-tim

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    Hi Newton86! :smile:

    Your first formula was right.

    Your second formula would have been right, except that it shouldn't have had x.

    Get it right, put the two formulas equal to zero (you do understand why, don't you?), and then solve the two equations! :smile:
     
  20. Apr 28, 2008 #19
    Could you set them up correctly ?:redface: maybe I will understand a bit moore caus im a bit confused on this :confused:
     
  21. Apr 28, 2008 #20

    tiny-tim

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    No!

    Show us what you can do … then we can see where the problem is! :smile:
     
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