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High school Grade 12 vectors

  1. Nov 3, 2013 #1
    1. The problem statement, all variables and given/known data

    Hi, for some reason I can't post my homework question (Grade 12 math)

    How do i fix this?

    Here's my homework question

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

    In Exercises 15-30, add the given vectors by using the trigonometric functions and the Pythagorean theorem

    19. A = 9.821,θA = 34.27
    B = 17.45,θB = 752.50

    2. Relevant equations


    θ=tan-1 Ry/Rx =

    3. The attempt at a solution

    So here's what I did. I copied exactly what my teacher did but, when I checked the answer at the back of the book, the answer was wrong =(. Can you guys point out how I'm not getting the correct answer? thanks.

    Here's what I have so far


    x component y component

    Ax= 9.821cos34.27 Ay=9.821sin34.27
    = 8.116 =5.530

    Bx=17.45cos752.50 By=17.45cos752.50
    = 14.72 =14.72

    =22.84 =20.25

    R=√22.84(2) + 20.25(2)= 30.52

    θ=tan-1 20.25/22.84=41.56



    R=27.27, θ=33.14
    Last edited: Nov 3, 2013
  2. jcsd
  3. Nov 3, 2013 #2


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    Dearly Missed

    Why have you added 180 degrees to your angle?

    Note that BOTH of your vectors lie in the first quadrant, so their sum must lie there as well.
  4. Nov 3, 2013 #3

    I don't get it =(
  5. Nov 3, 2013 #4

    Ray Vickson

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    Both of your given vectors point to the northeast, so their sum must also point to the northeast. When you added 180 degrees to the angle you forced your vector to point to the southwest. Why would you do that?
  6. Nov 3, 2013 #5


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    Your components for ##\vec{B}## are wrong. They shouldn't be equal.

    Even with your incorrect numbers, both components of ##\vec{R}## are positive, so you should expect the angle to be between 0° and 90°, which is what you got from the arctangent function. So why did you add 180° to your result?

  7. Nov 9, 2013 #6
    Update guys, I have actually had the right the answer the whole time rofl. The book just rounded it differently lol. Thanks for the help though.
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