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: Angle Calculation

  1. Oct 20, 2008 #1
    1. The problem statement, all variables and given/known data

    Can someone help me to find angle between 2 lines in 3D? I have 2 lines from (-x, 0, -y) to (0, 0, 0) to (0, x, y). I can use CAD to get the answer. But my shop guys will need some kind of formula. The easier way please, thank all.

    2. Relevant equations

    Example: (-10, 0, -6) to (0, 0, 0) to (0, 10, 6) is 105.3 degree.

    3. The attempt at a solution
  2. jcsd
  3. Oct 20, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi ikeoe! Welcome to PF!:smile:

    (what's CAD? :confused:)

    Hint: a.b = |a| |b| cos θ,

    so cos θ = a.b/√(a.a)√(b.b) :smile:
  4. Oct 20, 2008 #3
    Say you have three points, A, B, C. You want to find the angle between lines AB and BC.

    You can represent lines by vectors. Line AB would be the vector A - B and BC would be the line B - C. Then, to get the angle, you just need to use the dot product formula:

    x * y = |x| |y| cos(theta)

    Where * is the dot product, |x| is the length of the vector x, and theta is the smallest positive angle between the two vectors.

    I'll leave the algebra to you.
  5. Oct 20, 2008 #4
    Thank you, tiny-tim. CAD is Computer Aided Design mostly used from AutoDesk AutoCAD.
  6. Oct 20, 2008 #5
    Great, thanks to Tac-Tics.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook