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Homework Help: Find the direction of the resulatnt vector

  1. Sep 10, 2009 #1
    Hello everyone, here is a question of which Part b is giving me a headache + tension! :) My teacher only gave us 2 days to do so much homework - one day left! :) Its impossible to take help from him......

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

    Consider four vectors ~F1, ~F2, ~F3, and ~F4,
    where their magnitudes are
    F1 = 50 N,
    F2 = 29 N,
    F3 = 27 N, and
    F4 = 58 N.

    theta1 = 150 degrees,
    theta2 = -140 degrees,
    theta3 = 24 degrees, and
    theta4 = -66 degrees, measured from the positive x axis
    with the counter-clockwise angular direction
    as positive.

    What is the magnitude of the resultant vec-
    tor ~F , where ~F = ~F1 + ~F2 + ~F3 + ~F4? Answer
    in units of N.

    I found the answer to his part:

    -17.259 i and -35.645 j , with a magnitude of 39.604

    My online homework accepted this answer :)


    (part 2 of 2)
    What is the direction of this resultant vector

    Note: Give the angle in degrees, use coun-
    terclockwise as the positive angular direction,
    between the limits of -180 degrees and +180 degrees from
    the positive x axis. Answer in units of degrees.

    2. Relevant equations

    I tried doing: tan θ = -35.645/-17.259 - θ=64.164 °

    3. The attempt at a solution

    I tried doing: tan θ = -35.645/-17.259 - θ=64.164 ° - The answer proved out to be wrong.....And thus I can now score a maximum of 93% on this question! :(

    I have NO IDEA as to how to solve it - I looked in google, but to no avail......All help would be appreciated! :)


  2. jcsd
  3. Sep 10, 2009 #2


    User Avatar
    Homework Helper

    Resultant is in the third quadrant. So theta should be -180 + 64 degrees.
  4. Sep 11, 2009 #3
    You are right!!! THANKS for the help!!!!!!! :)
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