code: def _Rforce(self,R,phi=0.,t=0.):
          
          return self._A*math.exp(-(t-self._to)**2./2./self._sigma2)\
              /R*math.sin(self._alpha*math.log(R)
                          -self._m*(phi-self._omegas*t-self._gamma))
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:
             2010-11-24 - Written - Bovy (NYU)