Understanding the Balance: Equal Weight, Unequal Position

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Discussion Overview

The discussion revolves around the mechanics of a common balance scale, specifically addressing why the balance achieves a horizontal position despite equal weights on both arms. Participants explore concepts of equilibrium, torque, and potential energy in relation to the balance's behavior.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why the balance chooses a horizontal position when both arms have equal weight, suggesting that it should be stable in any position due to zero torque around the center.
  • Another participant asserts that the arms of the balance must be in a straight position for equilibrium when the masses are equal.
  • A different participant expresses confusion about the potential energy in a tilted position, arguing that the increase in potential energy from one mass moving up is balanced by the decrease from the other mass moving down, questioning the source of restoring torque.
  • Another contribution discusses the mechanics of the pointer and restoring torque, introducing variables such as the weight of the pointer and its distance from the pivot, while noting that scales are typically used in a closed-loop manner.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there are multiple competing views regarding the mechanics of the balance and the sources of restoring torque. The discussion remains unresolved with ongoing questions and clarifications.

Contextual Notes

Some assumptions about the balance's design and behavior are not fully explored, and there are unresolved aspects regarding the mathematical treatment of torque and potential energy in tilted positions.

krishna mohan
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Confused about a simple thing...

For a common balance, since both arms have equal weight, what causes the pole to choose a horizontal position?
Shouldnt it be stable in any other position as the torque around the centre is zero for any other position too?
 
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The arms of the balance move up and down, leaving the only position for equilibrium as exactly straight, for equal mass arms.
 
I am a little confused here...

Even if you move the balance to some other position, the potential energy is not going to change..as the increase in PE due to one mass moving up is compensated by the decrease due to the other moving down..

In a slightly tilted position, where does the restoring torque come from? The two masses seem to contribute equal and opposite torque leading to zero torque...
 
If the pointer points below the pivot point then if the scale is unbalanced the pointer provides a restoring torque = W*l*sina where W = weight of pointer, l = distance from pivot to pointer's c.g. and a = offset angle. Since scales are always balanced with known weights, the "open-loop" quantity da/dm is of no first-order importance. Scales are always used in a closed-loop manner.

I don't know much about scales but I would think that's how they're designed.
 

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