It is proved that because the object A is at rest, the pulling force and gravity of the object A in the vertical direction are a pair of balanced forces, so the pulling force of the spring dynamometer on the object is equal to the gravity of the object, that is, f = g；;

The force f of the dynamometer to the object A and the force F' of the object A to the dynamometer are a pair of acting force and reaction force. According to Newton's third law, F = F', so f' = g.