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: Gamma angle

  1. Feb 24, 2009 #1
    For two arbitrary vectors in 3D space, subtending an angle, [tex] \gamma [/tex] , I know the cos relationship, but what's the sin relationship ? I ask because there is an ambiguity by only knowing the cosine form, since vector A can be either above or below vector B.

    cos\gamma = cos\theta_1 cos\theta_2 + sin\theta_1 sin\theta_2 cos( \phi_1 - \phi_2 )

    Sorry I ask a stupid question in this forum, but I didn't know what's the correct term I should type in the search engine to search in the internet.
  2. jcsd
  3. Feb 24, 2009 #2


    Staff: Mentor

    I'm not sure I understand your question. Is there some reason you need to know which vector is above or below which? If you know the cosine of the angle between the two vectors--which you can get using the dot product: cos(gamma) = (A dot B)/(|A|*|B|)--the sign of cos(gamma) tells you whether gamma is in QI/QIV (cosine > 0) or in QII/QIII (cosine < 0).

    I'm not familiar with your formula. What do theta1, theta2, phi1, and phi2 represent?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook