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Homework Help: Mechanics, Vector problem

  1. Sep 16, 2007 #1
    1. The problem statement, all variables and given/known data
    First of all I will have to apologize if this in in the wrong forum. It is a 1st year engineering mechanics problem.

    I am having an issue solving a secondary part of a question.
    The first part of the question has asked me to both graphically and numerically determine a resultant of three forces acting on an object, with magnitude and angles for all given. This was straightforward.
    I am now being asked to determine a fourth Force such that the resultant R of all the four forces will be vertical (90 degrees) and has a magnitude of 750 N.
    I have done this question graphically, but am wondering how I go about doing it numerically ..just buy using trig etc.
    thanks much,

    3. The attempt at a solution

    i have thus far determined that my previous resultant force was 821 N at 87 degrees North of East.
    I have therefore now said that my resultant is going to have to = 750 N so
    750 = (sum Fx + sum Fy)^(1/2).
    I am just unsure how I would go about getting two unkowns from this methodology, the X component and Y component of the unkown vector..
  2. jcsd
  3. Sep 16, 2007 #2


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    What is the x - component of your 821N force ? Add to that and the x-component of your new force and you get 0. solve for the x-component of the new force.

    same way, solve for the y-component.
  4. Sep 16, 2007 #3
    Well the last resultant at a magnitude of 821N had 50.0007N (X-component) and 819.181N (Y component). Direction was 86.5 degrees N of E
    Now I need to add a Force such the new resultant will be 750N and vertical (or 90 degrees).
    I guess what I am saying is, I don't know how to get the X or Y component for my new force, becuase I don't know what it will be.
  5. Sep 16, 2007 #4


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    50.0007N + Fx = 0, solve for Fx

    819.181N + Fy = 750, solve for Fy.
  6. Sep 16, 2007 #5
    of course.. how silly of me, I was trying to make a triangle and use law of cosines.. thanks much!
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