code: def omegac(self,R):
          
          return nu.sqrt(-self.Rforce(R,use_physical=False)/R)
docs: NAME:
          
              omegac
          
          PURPOSE:
          
              calculate the circular angular speed at R in potential Pot
  
          INPUT:
          
              Pot - Potential instance or list of such instances
          
              R - Galactocentric radius (can be Quantity)
          
          OUTPUT:
          
              circular angular speed
          
          HISTORY:
          
              2011-10-09 - Written - Bovy (IAS)