code: def _Rforce(self,R,phi=0.,t=0.):
          
          if R < self._rb:
              return -self._p*self._mphio/self._m*self._rb2p/R**(self._p+1.)\
                  *math.cos(self._m*phi-self._mphib)
          else:
              return -self._p*self._mphio/self._m*R**(self._p-1.)\
                  *math.cos(self._m*phi-self._mphib)
docs: NAME:
             _Rforce
          PURPOSE:
             evaluate the radial force for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             phi - azimuth
             t - time
          OUTPUT:
             the radial force
          HISTORY:
             2011-10-19 - Written - Bovy (IAS)