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Homework Help: Vector magnitude help

  1. Jan 20, 2010 #1
    1. The problem statement, all variables and given/known data
    Vector V1 is 6.4 units long and points along the negative x axis. Vector V2 is 8.9 units long and points at 60 degrees to the positive x axis
    1) find x and y components of v1 and v2?
    2) find magnitude of the sum v1+v2?
    3) find the angle of the sum v1+v2?


    2. Relevant equations



    3. The attempt at a solution
    I found the answers to part 1 (v1= -6.4,0 and v2= 4.5,7.7) and part 2 (8.0) but need help with part 3. thanks!
     
  2. jcsd
  3. Jan 20, 2010 #2

    rock.freak667

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    If you have |v1| ,v2| and |v1+v2|, then you have the 3 sides of a vector triangle.


    You can get the angle between v1 and v2, since the angles at a point add up to 180°.

    Then from there you can use sine or cosine rule.
     
  4. Jan 20, 2010 #3
    im having trouble with v1+v2. i am getting -1.9 for the x component and 7.7 for the y. could i use tan-1 (y/x) = angle in this case?
     
    Last edited: Jan 20, 2010
  5. Jan 20, 2010 #4

    rock.freak667

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    using that will give the acute angle measured clockwise. use tan-1(|y|/|x|) to get the magnitude of the angle and then 180- that angle to get it measured anti-clockwise
     
  6. Jan 20, 2010 #5
    will it matter if i use a pos 1.9 instead of a -1.9?
     
  7. Jan 20, 2010 #6

    rock.freak667

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    you will get a negative angle is all.
     
  8. Jan 20, 2010 #7
    so i got 76.1 degrees (using pos 1.9) so just 180-76.1= 104 degrees counterclockwise from the positive x axis?
     
  9. Jan 20, 2010 #8

    rock.freak667

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    Yes that should be correct.
     
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