Query for Scepti:
Ok, so I was thinking about the balloon situation as a means of possibly getting numbers, as many people ask for, for a concrete value of the pressure exerted. However, there's one detail I can't get my head around.
If we take an uninflated balloon, and weigh it, it weighs slightly less than an inflated and tied off balloon. Initially I put that down to it displacing more air, but the more I thought about it that wouldn't seem to be the case, as volume of the material of the balloon is constant, and given how porous objects behave, it doesn't seem like it should be the case that the air within the balloon is counted as displaced. So, there are two cases:
If the air within the balloon isn't counted as displaced, which feels right to me, then why would the weight measured of a balloon increase when inflated? The amount of displaced air doesn't alter.
Ok, let's see if I can make a fist of explaining this.
Analogy time.
Imagine you have a container of various grains of slightly different densities. You know, sand, sugar, salt,baking powder, etc.
Ok, now as you know, the more of something there is, the more densely packed it will be due to more smaller particles.
Think of mud against sand as an instance.
I know you get this.
Anyway the container is a substitute analogy for atmosphere.
Now we place a ping pong ball at the bottom as a balloon analogy. Now, if you leave it at that, we will simply see a ping pong ball that is covered by various layers of grain (atmosphere) and yet it stays put. Start to tap the container and you see the ping pong ball start to rise. It is being pushed up because the denser particles are surrounding it but will find their place back at the bottom and in doing so will push up the ping pong ball by squeezing, except the ball doesn't crush, it merely resists the squeeze by sheer expansion inside of it - and rises.
Hopefully this analogy might be enough to trigger your thoughts. If not, I'll try again.
Second case: if, instead, the air within the balloon counts as displaced, we'd need the inside of the balloon to essentially be an isolated system, separate from the stack outside it, in which case shouldn't there be an internal denpressure system? (eg: if you slide something like a paperclip inside and inflate, and turn the balloon around, the paperclip should stay pressed to one side independent of the external pressure).
Actually there is another force acting on the paper clip but it's a near equal force inside and that force will only change upon acceleration against an outside force, such as the obvious atmosphere working against that acceleration.
This requires a more in depth but still simplistic explanation which I'll provide if you wish, because I know you're taking notice.
First case is what feels like should be the case, from my understanding of denpressure, but I can't reconcile it with the fact an inflated balloon, despite displacing an equal amount of air, weighs more.
It might be that I've forgotten to take something into account, in which case please could you let me know?
Basically think of this.
When a balloon is deflated it's mass lies on a scale plate as simply a near airless flat rubber that displaces a tiny amount of atmosphere as we know.
Add atmosphere to that balloon and you make that balloon rise into the atmosphere by placing that external atmosphere into the internal balloon and now have a standing balloon being gripped around by external atmosphere.
This would make it ever so slightly less in weight, because a near buoyancy has been sort of made.
Edit: only solution I can see is that the scale's measuring the air pressure inside the balloon, which adds a bit, but as it ought to do so to all sides there might be a simple experiment we can do on this, if this is indeed the explanation.
The air pressure put into the balloon is only the air pressure taken externally.
If I'm not being too clear then just say and I can try and use another thought on it.