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!

Understanding Trig In Force Diagrams

  1. Sep 18, 2012 #1

    I have the following problem part (b) which I already solved as you can see in the attached image. So I am not asking homework questions, I merely reviewing my homework for a better understanding for the test. I obtained the answer from a friend showing me his method. However, I am studying and a listed solution was the following:

    F_y = 2*(k*(q^2/r^2))cos(30)

    I've drawn out a force diagram but have no idea how, they have obtained the cosign. In the listed solution they state use oriented the y-axis such that it bisects charges q2 and q3.

  2. jcsd
  3. Sep 19, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    There are two forces acting on point 1. By symetry the result of these two forces will act along a line drawn through point 1 and a point mid way between 2 and 3. See diagram.

    So work out the component of the two forces pointing in that direction.

    Consider the triangle on the right.

    Cos(30) = Fr/F


    Fr = F Cos(30)

    That's not the whole solution obviously, just where the cos(30) comes from. The angle doesn't come from the direction of the result per se, it comes from the direction of the result in relation to the forces. eg If you rotate the triangle/problem drawing 30 degrees so the resulting force is vertical (on the y-axis) the answer will still contain cos(30).

    Attached Files:

    Last edited: Sep 19, 2012
  4. Sep 19, 2012 #3
    Thank you very much! And because of symmetry is why it's multiplied by 2?
  5. Sep 21, 2012 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Yes in this case.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Understanding Trig In Force Diagrams