I think that's reading a bit too much one-directionality into it. While true, it's the amount of force that's at issue here - not the direction. If a second

I think that's reading a bit too much one-directionality into it. While true, it's the amount of force that's at issue here - not the direction. If a second 2kg weight were added to the "anchor" end of the spring balance - it would either register -1kg (if it's built to do that) or break or lock up givinging invalid or useless readings. Of course - assuming the balance isn't anchored anywhere else, this would set the whole system in motion, and it would all fall apart ratherquickly.