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Finding the third force on a bracket when given the first 2 forces and the resultant

  1. Aug 26, 2012 #1
    Hi everyone, i came across this forum a while ago and have been lurking here for a while. Any ways, i have a simple problem but cant seem to find a solution to it the photo of the problem is the link just here

    http://i1053.photobucket.com/albums/s463/Popa911/1cc85138.jpg

    1. The problem statement, all variables and given/known data

    Three cables exert a force on a bracket (as shown in the photo) F1=300N, F2=500N and the resultant is 400N and 30° downward from x.



    2. Relevant equations


    the equation i used to solve was Fr=F1+F2+F3 rearranged to get F3= Fr - F1 - F2


    3. The attempt at a solution

    I got the i and j components and put them together as follows.

    F3= (346.41i -200j) - (180i + 240j) - (500i)
    F3 = -334i - 440j

    this answer is incorrect as it would mean that cable F3 is going through the wall? Any help would be appreciated, thanks.
     
  2. jcsd
  3. Aug 26, 2012 #2

    rude man

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    Re: Finding the third force on a bracket when given the first 2 forces and the result

    You sure you got the problem right?
     
  4. Aug 26, 2012 #3
    Re: Finding the third force on a bracket when given the first 2 forces and the result

    where do you think i went wrong? I even checked it back against the original Fr = F1 + F2 + F3 and it comes up with the correct Fr.

    Am i looking at the problem in the wrong way?
     
  5. Aug 26, 2012 #4

    NascentOxygen

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    Re: Finding the third force on a bracket when given the first 2 forces and the result

    Perhaps you copied that 400 value wrong? Maybe it should be 800 or so?
     
  6. Aug 26, 2012 #5
    Re: Finding the third force on a bracket when given the first 2 forces and the result

    i was thinking this as well, i've checked my figures though and re-checked the final answer against the resultant force and it seems correct but it doesnt fit with what the photo shows
     
  7. Aug 26, 2012 #6

    PhanthomJay

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    Re: Finding the third force on a bracket when given the first 2 forces and the result

    I'd chuck this problem as far as I could throw it and find another that makes more sense.
     
  8. Aug 26, 2012 #7

    rude man

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    Re: Finding the third force on a bracket when given the first 2 forces and the result

    I second that motion.
     
  9. Aug 26, 2012 #8
    Re: Finding the third force on a bracket when given the first 2 forces and the result

    There doesn't appear to be anything wrong with this problem apart from a terrible drawing...
     
  10. Sep 2, 2012 #9
    Re: Finding the third force on a bracket when given the first 2 forces and the result

    I talked to my lecturer and she said that the problem is either it is drawn wrong or the figures that were given in the question are wrong because there is nothing wrong with how i worked it out lol. Spent ages trying to figure out where I went wrong.

    Thanks for the help everyone as well!!
     
  11. Sep 2, 2012 #10

    NascentOxygen

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    Re: Finding the third force on a bracket when given the first 2 forces and the result

    Thanks for getting back, Popa91.
     
  12. Sep 2, 2012 #11

    nvn

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    Popa91: Theoretically speaking, there is nothing wrong with the given problem, nor diagram. You worked the problem correctly, and your answer in post 1 is correct. We can imagine ways in which the given problem is possible.
     
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