helium scale weight

[charlie frank]

If you flip a scale upside down and hook it to the ceiling, then release a helium balloon under it, such that the balloon pushes up into the scale...

 

A. What will the scale reading indicate? Will it really reflect the (negative?) weight of the balloon?

Edit: Will it reflect the difference in weight between the balloon and an equal volume of air?

 

B. Will the mechanics of how a scale operates prevent it from working in an upside down position? What if you "zero it" first?

Edit: What if the moving parts of the scale were weightless?

Edited July 20, 2012 by charlie frank

[Iota]

No it would not reflect the true negative value. In terms of this being a practicable experiment, no:

 

1) Normal scales do not measure negative values, therefore zeroing the scale will not make a difference in terms of recalibrating.

2) According to point 1, above, additional force must be applied to the upside-down scales to achieve the same gram for gram value, to overcome/compensate for the gravity affecting the scale's strain gauge technology.

[PaulS1950]

Using a perfectly balanced beam which pivots at its center on a frictionless pivot, one side of the beam resting on the scale and the other end attached to the balloon filled with helium your scale will measure the net lifting force of the balloon.

The net lifting force will be equal to the pressure density of the atmoshere (under test conditions) minus the pressure density of the helium minus the mass of the balloon.

 

Paul

Edited July 20, 2012 by PaulS1950

[Iota]

Edit: What if the moving parts of the scale were weightless?

 

I'm using an educated guess... so, physics professionals out there, if I'm talking rubbish, please warn OP.

 

 

I suppose in that case (assuming the balloon material has negligible weight), a very sensitive set of scales could give you a value for the upwards force exerted by the helium in the balloon, given in grams. The same way if you had Xenon gas in a balloon (material of negligible weight), rested on a set of scales correct way up, it would give you it's weight.

 

So I guess that could work yeah. I personally can't see the flaws.

 

I'm not sure what it would be worth, having weight in grams, though I guess that depends on the experiment's purpose, because gravity is still affecting the He atoms in the balloon; meaning it's not measuring some form of reversed mass.

[swansont]

If you had a scale that worked when flipped over, you'd measure the buoyancy force from Archimedes principle (which is the weight of the air the balloon displaces), less the weight the balloon has. That difference is the net force on the balloon, not including the force from the scale.