# rotational symmetry

by adkinje
Tags: rotational, symmetry
 P: 11 I've been following along with Lenny Susskinds lectures on modern classical mechanics on youtube. at 34:30 he writes a few translation formulas on the board: delta X = - epsilon Y delta Y = epsilon X http://www.youtube.com/watch?v=FZDy_Dccv4s It's not obvious to me why these equations are true. I can't seem to find a derivation anywhere, nor can I work one out myself. Any help?
 P: 4 I haven't watched the video, but if you perform a rotation by an angle $\theta$ about the $z$ axis on the vector ${\bf r} = \left(x,y,z\right)$, you get $r' = r + \Delta r = \left(\cos \theta x - \sin \theta y, \sin \theta x + \cos \theta y ,z\right)$. For $\theta = \epsilon$ infinitesimal, this becomes $r' = r + \delta r = \left(x - \epsilon y, \epsilon x + y ,z\right) = r + \left(- \epsilon y, \epsilon x ,0\right)$, so that $\delta x = - \epsilon y$ and $\delta y = - \epsilon x$.
 P: 11 thanks, that's what I was looking for.

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