1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Really easy vector addition.

  1. Jan 5, 2010 #1
    1. The problem statement, all variables and given/known data

    I have three vectors and I need to find the resultant of the three vectors and direction.

    2. Relevant equations

    *All angles are with the horizontal

    Vector 1 = 18 at an angle of 40 degrees (First Quadrant)
    Vector 2 = 10 at an angle of 72 degrees (Second Quandrant)
    Vector 3 = 22 at an angle of 55 degrees (Third Quadrant)

    3. The attempt at a solution

    In the X I got: 18cos(40) - 10cos(72) - 22cos(55) = -1.9
    In the Y I got: 18sin(40) + 10 sin(72) - 22sin(55) = 3.1

    √(-1.92 + 3.12) = 3.64 is the resultant

    tan-1(3.1/-1.9) = -58.5 degrees is the direction/angle

    I just need confirmation whether this is right or wrong. =)
  2. jcsd
  3. Jan 5, 2010 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    Looks good. Well, almost perfect. You should be a little careful with your terminology.

    The resultant is a vector, the sum of the three vectors, and it has a magnitude and direction. The 3.64 you found is the magnitude of the resultant, not the resultant itself.

    For the direction, you need to be a little careful because arctangent only returns angles between -90 degrees and +90 degrees. You should give a little more information to indicate exactly what the angle is, e.g. which quadrant, measured relative to what, etc.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook