1. The problem statement, all variables and given/known data Construct the [tex]S_r[/tex] operator, which is the spin operator in the r direction. 2. Relevant equations The pauli-matrices are useful, I would assume. 3. The attempt at a solution So, the way we constructed the other 3 operators by noting their effects on a state |s m> But I can't do that here because I have no idea what this operator [tex]S_r[/tex] will do to a state |s m> I thought about looking at the fact that: [tex] x = rsin(\theta)cos(\phi), y = rsin(\theta)cos(\phi), and z=rcos(\theta)[/tex] and just replace all the x's with [tex]S_x[/tex]'s and r's with [tex]S_r[/tex]'s etc. But I don't think this will work as it will give me 3 different answers. I need help on how to start with this.