How does the scale become horizontal when each side gets same weight?

Question

I am a newbie in physics.

now I am reading 'Six Easy Pieces' by Richard Feynman. and I was reading the part of gravitational potential energy. while I was reading Feynman took an example of scale(lifting machine) like this.

And I came up with very silly question; 'why is the scale balanced when both sides gets same weight?'

the reason I was confused is because I thought scale would stay just they were when both sides get same weight at the same time.

My thought process is like this this ;

1.suppose the scale is unbalanced because only left side of the scale got 1kg of weigh and there is nothing on the right side. like this.

2.is this state, left side are down to the floor because the energy to the earth on the left side is 1kg bigger than right side. so left side keeps going down but stopped because of the floor.

3.and then let's put another 1kg to the right side in this situation. then both sides gets same amount of energy downward. Now, the scale should be stay same(unbalanced, left sides down) because the bar on the scale gets same energy while It is unbalanced state.

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But I know in the reality it doesn't work as I imagine. if I put another 1kg of weigh on the right side while the scale is unbalanced with 1kg on the left side, I know it will get balanced; ride side will go down and left side go up.

I don't think I understood how the balance of energy(power) works.

what am I missing?

Answer 1

You missed the part of the scale design that puts the centre of gravity of the system below the pivot point. This is perhaps a result of representing the balance beam as a dimensionless straight line.

Consider the situation in your second diagram.

The uniform beam is balanced on a central pivot point, that is collocated with the centre of gravity of the beam. Absent any weight on either end, the total potential energy of the beam is independent of angle; the beam should remain at rest in any position where it is placed.

In the diagram, the masses are above the beam; each of their C of G are above the beam. This move the centre of gravity of the system as a whole to a new location Specifically, it is now located somewhere on a line passing through the pivot point at right angles to the beam, and above the beam

As the beam rotates, it should flop back and forth, slowing and speeding up. If you place the beam exactly horizontally, it will randomly "fall" to one side of the other.

Consider the case where the masses are located below the beam. as in a chemicals lab balance with hanging pans.

Now the C of G of the system is below the pivot point, and will swing to a stop with the system C of G directly below the pivot.

A perfectly balanced beam will line up horizontally from any starting position. If one weight is slightly larger than the other, the C of G moves towards the larger mass, and the beam will rotate slightly in that direction.