How does a simple balance scale work?

[f todd baker]

Visualize a simple balance, a pan at either end of a beam which balances on a fulcrum. The way it works is that if each pan carries the same weight, it balances horizontally. If one pan carries slightly more weight, it tips slightly off horizontal. If one pan carries more weight yet, it tips farther off horizontal. But that is not the way it would work if simple physics is applied to my simple model: for equal weights it would balance horizontal or not, and for unequal weights it would rotate fully until it came to a stop, regardless of how unequal they were. There must be some simple explanation, but I have not been able to find it.

 

[jbriggs444]

The pivot point is slightly above the attachment points for the pans. As the beam tilts right, this means that the rotation moves the pans very slightly left. Similarly, if the beam tilts left, the rotation moves the pans very slightly right.

 

[DrStupid]

The pivot point is slightly above the attachment points for the pans.

The weight of the index needle also results in a counter torque.

 

[f todd baker]

Actually, I found the answer in an earlier thread. It is really quite simple. The center of gravity of the unladen scale is below the point of support (fulcrum) so when the scale tips, there is a restoring torque. I think this thread can be closed.