> you're saying is that the spring can't distinguish between the setup on the right and the setup on the left. Correct, because according to the balance, both

> you're saying is that the spring can't distinguish between the setup on the right and the setup on the left. Correct, because according to the balance, both setups are indistinguishable. The dark secret of spring balances is that they measure (and display) force in only one direction. In stasis, the net forces on the balance are exactly zero N since there are equal and opposite forces acting upon the balance (true in each setup), yet the needle moves because it is measuring the movement (stretching) of the spring inside the balance (which at one end is anchored internally to the balance casing, and the other end is anchored to the string/mass). So, as for the setup on the right, the left-hand weight is pulling on the balance with weight 1kg (9.8N), and the right-hand weight is pulling on the balance with weight 1kg (9.8N), but since the balance only displays forces in one direction, it will display 1kg (in this case it doesn't matter which direction it is measuring because the forces are 180 degrees from each other).