code: def rforce(self,*args,**kwargs):
          
          R,z= args
          r= numpy.sqrt(R**2.+z**2.)
          kwargs['use_physical']= False
          return self.Rforce(*args,**kwargs)*R/r+self.zforce(*args,**kwargs)*z/r
docs: NAME:
  
             rforce
  
          PURPOSE:
  
             evaluate spherical radial force F_r  (R,z)
  
          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)
  
             v - current velocity in cylindrical coordinates (optional, but required when including dissipative forces; can be a Quantity)
  
          OUTPUT:
  
             F_r (R,z,phi,t)
  
          HISTORY:
  
             2016-06-20 - Written - Bovy (UofT)