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Homework Help: Adding 2 vectors. One is 20 degrees below x-axis

  1. Sep 9, 2008 #1
    I've done many other problems like this and understood them all but when it comes to 20 degrees below the x-axis it threw me off. I tried putting the angle to 340 degrees but that was incorrect. I know something has to be negative, i just can't keep guessing at getting it wrong. Hope someone can help!!

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

    A vector A has a magnitude of 51.0 m and points in a direction 20.0° below the x axis. A second vector, B, has a magnitude of 70.0 m and points in a direction 43.0° above the x axis. Using the component method of vector addition, find the x- and y- components of the vector C.

    2. Relevant equations

    Vector A + Vector B = New Vector C

    Cx = Ax + Bx
    Cy = Ay + By


    3. The attempt at a solution

    I'm really not sure. In the book it says to put vector B at the head of vector A. I just drew each vector as a separate triangle and solved for Ax, Ay and so on. Like i said before, i just need to know what to do with the "20 degrees below the X-axis. Is it -20? 340? I've tried both and they came out wrong so help! Thanks again.
     
  2. jcsd
  3. Sep 9, 2008 #2
    "I'm really not sure. In the book it says to put vector B at the head of vector A."

    This is one way of finding C, but is not the method you are asked to use

    "I just drew each vector as a separate triangle and solved for Ax, Ay and so on."

    This sounds like what they are asking you to do.



    "Like i said before, i just need to know what to do with the "20 degrees below the X-axis. Is it -20? 340? I've tried both and they came out wrong so help! Thanks again."

    You should be able to use either -20 deg or +340 deg. Make sure your calculator is i degree mode, if that's how you chose to do the calculation.

    In any case

    Ax = 51 cos(-20) = 51 cos(340) = 49.9

    Ay = 51 sin(-20) = 51 sin(340) = -17.4
     
  4. Sep 9, 2008 #3
    Thanks a lot for the help! This is a great forum. I'm glad I found it.
     
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