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: Simple cross product

  1. Oct 26, 2008 #1
    (This is part of a much larger problem)

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

    Find [tex]\hat{r} \times \hat{z} \times \hat{y}[/tex]

    2. Relevant equations

    [tex]x=rsin(\theta)cos(\phi)[/tex], [tex]y=rsin(\theta)sin(\phi)[/tex],[tex]z=rcos(\theta)[/tex] (cartesian->spherical)

    3. The attempt at a solution

    I decided [tex]\hat{r}=\hat{x}sin(\theta)cos(\phi)+\hat{y}sin(\theta)sin(\phi)+\hat{z}cos(\theta)[/tex]. Evaluating the cross product right to left, I got:

    [tex]\hat{r} \times \hat{z} \times \hat{y}=\hat{r} \times (-\hat{x}) = -cos(\theta)\hat{y}+sin(\theta)sin(\phi)\hat{z}[/tex], but the solution to the problem suggests this is not true. Am I wrong?
  2. jcsd
  3. Oct 27, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hi jhicks! :smile:

    Do you mean r x (z x y) or (r x z) x y? :confused:
  4. Oct 27, 2008 #3
    Hi tiny-tim,

    Well there're no parentheses in the problem, but somehow when I did this last night I concluded you evaluate cross products right to left, but I see the error of my ways.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook