code: def _rectForce(x,pot,t=0.):
      
      #x is rectangular so calculate R and phi
      R= nu.sqrt(x[0]**2.+x[1]**2.)
      phi= nu.arccos(x[0]/R)
      sinphi= x[1]/R
      cosphi= x[0]/R
      if x[1] < 0.: phi= 2.*nu.pi-phi
      #calculate forces
      Rforce= _evaluateRforces(pot,R,x[2],phi=phi,t=t)
      phiforce= _evaluatephiforces(pot,R,x[2],phi=phi,t=t)
      return nu.array([cosphi*Rforce-1./R*sinphi*phiforce,
                       sinphi*Rforce+1./R*cosphi*phiforce,
                       _evaluatezforces(pot,R,x[2],phi=phi,t=t)])
docs: NAME:
         _rectForce
      PURPOSE:
         returns the force in the rectangular frame
      INPUT:
         x - current position
         t - current time
         pot - (list of) Potential instance(s)
      OUTPUT:
         force
      HISTORY:
         2011-02-02 - Written - Bovy (NYU)