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Moment Question / inclined triangle

  1. May 13, 2016 #1
    1. The problem statement, all variables and given/known data
    In this I tried to resolve the components.

    So first thing, I converted the 12kg into newtons so it would be 117.72 newtons. Then found the perpendicular distance which is to g: cos(30)x1 then multiply the answer by 117.2N to give the weight down as 102N. As the moment principle state that clockwise have to be equal to anticlockwise the vertical of the Q have to 102N then i found the tension by 102/cos(30) which will give the final answer as 118N. But this answer is not one of the choices so it is wrong.


    Can someone explain to me where I have gone wrong.

    Thank You.
     

    Attached Files:

  2. jcsd
  3. May 13, 2016 #2

    haruspex

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    No, the weight down is mg. In what direction is there a force mg cos(30)?
    To balance the moments, pick an axis. Which axis did you use?
     
  4. May 13, 2016 #3

    Doc Al

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    You found the weight of the 12kg mass. Good.

    You found the anticlockwise torque (moment) due to the weight of that bonnet. Good. (Not the "weight" down, the moment.)

    Good.

    Not sure what you did here. What you need to do is find an expression for the clockwise moment due to the force F. (What's the definition of moment?)
     
  5. May 14, 2016 #4
    Hi there,


    This is where I am confused. Moment about a pivot is force multiplied by the perpendicular distance between the the pivot and the line of action
     
  6. May 14, 2016 #5
    We know the vertical component as 102 N so we can resolve to find the tension. This is where I got the expression as 102/cos(30)
     
  7. May 14, 2016 #6

    haruspex

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    No, the vertical force is mg, 117.72N.
    You can either take the component of that which is perpendicular to the car bonnet (that will be 102N) and multiply by the distance from the pivot to that force; or take the whole of mg and multiply by the perpendicular (i.e. horizontal) distance from the pivot to the line of action of mg.
    You will get the same answer either way. Choose one.
     
  8. May 14, 2016 #7
    Quite confused:

    is this what you mean 12x9.81xcos(30)x1=102N
    Then divide it by cos(30)x2 to give 58.86.

    but we are trying to find out the force of F which holds the bonnet equilibrium. By the way the answer is not 59N.
     
  9. May 14, 2016 #8

    haruspex

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    Explain why you divide by that.
     
  10. May 14, 2016 #9
    The principle of moment is the sum of clockwise moment is equal to sum of anticlockwise moment in an equilibrium position. the expression will be FxD=FxD
    So basically we are dealing with perpendicular distance so the expression with be [Cos(30)x1] x [9.81x12]=[Cos(30)x2] x F
    So making the F the subject will give us 58.86N.
     
  11. May 14, 2016 #10
    I am only dealing with resolved distance components
     
  12. May 14, 2016 #11

    haruspex

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    Yes, I understand that, but how do you come to 2 cos(30)? What is the perpendicular distance from the line of action of F to the pivot point?
     
  13. May 14, 2016 #12
    It is not from P to F, the distant from P to Q is 2 so resolved it do 2cos(30)..

    then I was going to find the value of F by doing 58.86/ cos(30). But i know I have done something wrong.
     
  14. May 14, 2016 #13

    haruspex

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    I'll be more specific. Why the cos(30)?
     
  15. May 14, 2016 #14
     

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  16. May 14, 2016 #15

    Doc Al

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    Realize that the applied force F is already perpendicular to the distance PQ, so there is no need to take any components.
     
  17. May 14, 2016 #16
    Make sense mentor
     
  18. May 14, 2016 #17

    haruspex

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    Look at the diagram. At what angle does F act? What is the perpendicular distance from the pivot point to the line of action of F?
     
  19. May 14, 2016 #18

    Nidum

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    F1 * d1 = F2 * d2 ?

    bonnet forces.jpeg
     
    Last edited: May 14, 2016
  20. May 14, 2016 #19
    Thank you for the consistent help.
     
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