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Homework Help: Vector problem

  1. Sep 4, 2008 #1
    1. The problem statement, all variables and given/known data

    Please look at the first attachment, I need to solve question #15

    2. Relevant equations

    See the second attachment

    3. The attempt at a solution

    Well I got Ax = -75cm, Ay = 129.9cm but I just can't solve the problem because I got stuck at solving for Bx and By.

    Attachment 3 is my work.

    Please teach me how to solve the answer
    by the way, the answer is 196cm at 14.7 degree below x-axis

    also, if possible, may sketch the picture of this problem for me, please?

    Attached Files:

    Last edited: Sep 4, 2008
  2. jcsd
  3. Sep 4, 2008 #2


    User Avatar
    Homework Helper

    What you want to do is exploit the vector addition of the components.

    In this problem they have given you the initial displacement vector and the final result.

    Split the 3 vectors into their x,y components. And then write the equations for x adding to the result and then y adding to the result.

    Sure Vector 2 components will be unknown but you will have two equations and each equation will give you each component of Vector 2. Then solve and do the tan-1 as you did in your first try.
  4. Sep 4, 2008 #3
    hello lowly, thank you for your quick reply
    but i am really having problem with this
    so i did... (not quite sure everything you mean), but

    Ax = ACos = -75cm
    Ay = ASin = 129.9 cm

    Bx = BCos
    By = BSin

    Rx = Ax+Bx --> Rx = -75cm+Bx
    Ry = Ay+By --> Ry = 129.9cm+By

    R = 140 (resultant), and since R^2 = Rx^2+Ry^2, then i have
    140^2 = (-75cm+Bx)^2 + (129.9cm+By)^2
    but doesn't it turn into unknown solution? i can't solve for either component...
  5. Sep 4, 2008 #4
    i have tried this really hard...=.= still can't answer it
    it sounds stupid but.. i don't know why i got stuck

    Attached Files:

  6. Sep 4, 2008 #5
    anyone would like to give a hand?
    And then write the equations for x adding to the result and then y adding to the result.

    what do you mean?
  7. Sep 4, 2008 #6
    i actually figured it out (not yet to the right answer)
    but here is what i thought

    since R = sqrt of x^2 + y^2
    then the x = the sum of ax+bx
    and y = sum of ay+ay

    now the problem is, after many trial out, the closest i can get is 189.7

    i just need to confirm 2 things

    1) to find Vector A(1)'s component, is the angle 120, or 60?
    2) to find the R vector component, we use 35, -35, or 145 degree?
  8. Sep 5, 2008 #7
    A = (Ax,Ay) = (150 cos120, 150 sin120) = (-150 sin30, 150 cos30) = (-75, 75*sqrt(3))
    R = (Rx,Ry) = (140 cos35, 140 sin35) = ()
    A+B=R so B=R-A
    B = (Rx-Ax, Ry-Ay) = (140 cos35 + 75 , 140 sin35 - 75*sqrt(3))
    cos(35) = 0.819152044
    sin(35) = 0.573576436
    75 * sqrt(3) = 129.903811
    B = (Rx-Ax, Ry-Ay) = (114.681286 + 75 , 80.3007011 - 129.903811) = (189.681286, -49.6031095)
  9. Sep 10, 2008 #8
    ty all
    i figured out my calculation was wrong in the first place, so i didn't get a correct solution
    thank you very much
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