Quote from the question source:
In times past or in other countries, people purchase fresh produce from piles. The vendor puts a mass on one side of the scale - same 1 kg - and some produce on the other side to weigh out 1 kg of produce for the customer. When there is a little less than 1 kg of produce, the scale tilts downward on the 1 kg but stops at some point rather than continuing to rotate. If the weights/torques are not balanced, why doesn't the scale continue to rotate? Why do you want the scale fixed directly parallel to the ground or perpendicular to the force of gravity to know you have just the right weight? Why not just have the scale stationary - no longer rotating? Do NOT consider friction because it is not relevant to the answer.
None given, but maybe:
Torque = lever arm x force
The Attempt at a Solution
I drew a diagram which you can see here: http://i.minus.com/ibzw9bF4vAnaJ.png (The weight differences are probably very exaggerated.)
But I am not sure of the answer: I thought that the scale stopped because the lever arm decreases as the scale becomes more vertical, but that happens to both sides so both torques decrease at the same rate (right?).
So why do scales stop when slightly unbalanced? It seems like friction is not the answer, because sometimes the scales tip backwards and then forwards until it stops.