code: def zforce(self,R,z,phi=0.,t=0.):
          
          return self._zforce_nodecorator(R,z,phi=phi,t=t)
docs: NAME:
  
             zforce
  
          PURPOSE:
  
             evaluate the vertical force F_z  (R,z,t)
  
          INPUT:
  
             R - Cylindrical Galactocentric radius (can be Quantity)
  
             z - vertical height (can be Quantity)
  
             phi - azimuth (optional; can be Quantity)
  
             t - time (optional; can be Quantity)
  
          OUTPUT:
  
             F_z (R,z,phi,t)
  
          HISTORY:
  
             2010-04-16 - Written - Bovy (NYU)