code: def _zforce(self,R,z,phi=0.,t=0.,v=None):
          
          new_hash= hashlib.md5(numpy.array([R,phi,z,v[0],v[1],v[2],t]))\
              .hexdigest()
          if new_hash != self._force_hash:
              self._calc_force(R,phi,z,v,t)
          return self._cached_force*v[2]
docs: NAME:
             _zforce
          PURPOSE:
             evaluate the vertical force for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
             v= current velocity in cylindrical coordinates
          OUTPUT:
             the vertical force
          HISTORY:
             2018-03-18 - Started - Bovy (UofT)