control force plus the external forces (drag, etc)

def force(self,c):
        '''control force plus the external forces (drag, etc)'''
        
        # bring instance variables into local scope
        x = self.x
        v = self.v
        b = self.b
        k = self.k
        x0 = self.x0
        
        F = self.control_force(c)
        
        return F - b*v - k*(x - x0)