# Rotational symmetry

1. Jun 6, 2009

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

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?

Last edited by a moderator: Sep 25, 2014
2. Jul 9, 2009

### EricAngle

I haven't watched the video, but if you perform a http://mathworld.wolfram.com/RotationMatrix.html" [Broken] 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 $\delta y = - \epsilon x$.

Last edited by a moderator: May 4, 2017
3. Jul 14, 2009