Why does a Scale Balance Horizontally?

[Freespader]

This may seem like a really dumb question, but I can't figure out what the answer is, so please just bear with me.

Why does a balance that is equal come out straight? If one side is tilted slightly lower than another, shouldn't it just stay that way since the masses are the same on either side, so the force down is too? I feel like the answer probably has something to do with torque, but I honestly don't know how it happens - although I also get the feeling it'll be pretty obvious once I hear it. Thanks.

 

[mfb]

You need an additional mass below the anchor, which acts a bit like a pendulum - if you raise one side, you shift the "pendulum" mass to that side, and get a restoring force in the opposite direction. If both sides are in balance, the stable position is horizontal. If one side is lighter, it goes up until the "pendulum" mass balances this.

 

[Freespader]

To make sure I get you: there's actually 3 masses in the whole thing? I guess that does make sense. And that would also explain why on a Triple Beam Balance a small amount of extra mass won't push it down all the way. Thanks!

 

[russ_watters]

There doesn't need to be three masses -- you just need to shape the beams so the masses are below the fulcrum. Then it has positive stability.

 

[Philip Wood]

The lower the centre of gravity of the beam, the less sensitive the balance (the less the beam deflection from the horizontal per unit excess mass on the weighing pan).

 

[Freespader]

Alright, to see if I get this: when the beam is bent, this would provide for greater torque, right? (I diagrammed it out, and that's what it seemed to be.) So if you have a straight rod, and no pendulum at the bottom, then the smallest amount extra on either side (neglecting friction, of course), would push the whole thing down to the ground?

 

[mfb]

Alright, to see if I get this: when the beam is bent, this would provide for greater torque, right? (I diagrammed it out, and that's what it seemed to be.) So if you have a straight rod, and no pendulum at the bottom, then the smallest amount extra on either side (neglecting friction, of course), would push the whole thing down to the ground?

Right.