def vphi(self,*args,**kwargs):
        """
        NAME:
           vphi
        PURPOSE:
           return angular velocity
        INPUT:
           t - (optional) time at which to get the angular velocity
           vo= (Object-wide default) physical scale for velocities to use to convert
           use_physical= use to override Object-wide default for using a physical scale for output
        OUTPUT:
           vphi(t)
        HISTORY:
           2010-09-21 - Written - Bovy (NYU)
        """
        thiso= self(*args,**kwargs)
        if not len(thiso.shape) == 2: thiso= thiso.reshape((thiso.shape[0],1))
        return thiso[2,:]/thiso[0,:]
NAME:
           vphi
        PURPOSE:
           return angular velocity
        INPUT:
           t - (optional) time at which to get the angular velocity
           vo= (Object-wide default) physical scale for velocities to use to convert
           use_physical= use to override Object-wide default for using a physical scale for output
        OUTPUT:
           vphi(t)
        HISTORY:
           2010-09-21 - Written - Bovy (NYU)