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Homework Help: Vector Calculation Problem

  1. Sep 8, 2008 #1
    1. The problem statement, all variables and given/known data

    Consider two vectors ~A and ~B and their resul-
    tant ~A + ~B . The magnitudes of the vectors ~A
    and ~B are, respectively, 12.1 and 7 and they
    act at 130◦ to each other.

    Use the law of cosines to calculate the mag-
    nitude of the resultant vector ~A + ~B.

    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 8, 2008 #2

    LowlyPion

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    How do you figure to apply the Law of Cosines?
     
  4. Sep 8, 2008 #3
    I did (A+B)[tex]^{2}[/tex]=A[tex]^{2}[/tex]+B[tex]^{2}[/tex]-2ABcos(130) and solved for (A+B). I got an answer of 17.444148, but that was wrong.
     
  5. Sep 8, 2008 #4

    Redbelly98

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    Draw a triangle using those vectors.

    edit:
    This will show you the angle you are supposed to use for the calculation.
     
  6. Sep 8, 2008 #5
    I'm a bit lost, why isn't 130 the correct angle to use?
     
  7. Sep 8, 2008 #6

    Redbelly98

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    Did you draw the triangle?

    A picture is worth 1000 words.
     
  8. Sep 8, 2008 #7
    Well yes, and I put the angle 130 between A and B, and if that's correct then I don't know how I got this wrong.
     
  9. Sep 8, 2008 #8

    Redbelly98

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    The correct picture is the 2nd one here.
     

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  10. Sep 9, 2008 #9
    Thanks, I see where my mistake was now, I didn't draw the vectors "tip-to-tail".
     
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