h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I don't know...

If all the balls were at the top of the box, then they were all let go, so they fell to the bottom of the box, when they're half-way down, the box should be lighter than when they're all at the bottom.

This is the idea that means that I can't quite accept that the weight of the box is the same when they're all bouncing around. You probably are right, I just don't quite know how to convince myself that it is right

Hmm... Ok, good, so thinking about it, the balls bouncing around is equivalent to them being at rest at the bottom of the box... Although they're not all in contact (like they would be as a solid lump) they're equivalently all in contact, just that there's a "delay" in transmitting their weight towards the bottom of the box...

So... This implies that the air pressure is greater than the (equivalent of the) weight of air, because the pressure will be a superposition of the air's weight-factor, plus any extra momentum they posess from kinetic energy due to their temperature.

Hmm... It's good to talk to someone who *doesn't* have their head up their own arse.

... no it wouldn't... The thermal energy would cancel out in any vertical axis... Ok, lol, atmospheric pressure is entirely due to the air's own weight

Just out of curiosity, does anyone know how much helium one would have to put in a balloon before it would be heavy enough to sink instead of float in air?

Traveller in Time confused
"Just a little less then needed to let it float.

Think better question would be how much to compress helium to get it equal weight to surrounding air ?

again wrong question, container to keep it under pressure would be too heavy to get airborn . "

I think you nailed it T in T.

" It's good to talk to someone who *doesn't* have their head up their own arse."

I'll take that as a hint never to bother trying to help someone here again. Bye bye.

SoRB

>> I'll take that as a hint never to bother trying to help someone here again. Bye bye.

First you try and imply that I'm a moron for even having an interest in understanding the process, and then that I'm a moron for even daring to *try* and understand the process. Then you tell me I'm ridiculous for not immediately 'getting' the 'obvious' details of the problem, while at the same time admitting that you don't understand how it works? Sorry, but that's just absurd.

Traveller in Time on his head
"Do not take it too personal, he is having a difficult Real Life time. "

Ok, fair enough.

I always wonder how conversations about fairly simple natural phenomena manage to turn into obtuse discussions of Newtons and complicated mathematical formulas in this forum.

Think of air molecules as cheerleaders on a human pyramid... forgetting the need for stability for the moment. The kid on the very top of the pile has zero children on top of him, so she's free to move around however she likes. The two kids she's on are sharing the weight of one individual, so if they needed to move around, they would be physically able to on a slightly limited basis. The three kids below them are really under pressure, because of the three kids above them, so they're basically bearing the weight of one full kid each. Moving around with that kind of weight on them would be very difficult. The four below them would not be able to move around at all.

Molecules like to move around, but the more molecules are on top of them, the less their energy will allow them to, because it can't overcome the resistance of all the other molecules crowding in on them. Without increasing their available energy (temperature), they will be confined to moving within a smaller space.

The original post asked why the pressure would not be different inside and outside the house. The reason is that air pressure is a force which acts in every single direction at the same time... air molecules are trying to move, and as soon as they find a new direction that'll let them move more freely, they go for it, and in between they're pushing against each other to try to find some more room. So even if the house were a total vacuum, as soon as the door was opened they air molecules near the door would detect the extra space and go rushing into it. Others would follow until they're just as jam-packed inside as they were outside. The pressures balance.

Now, let's say you close the door on a hot day and turn on the air conditioner. The temperature goes down and the energy of the air molecules decreases. They become sluggish and stop pushing each other around so much, so the air pressure decreases. Someone opens a door again, and more air comes rushing in to fill the extra space. This is why you can be in the back bedroom and hear a window rattle when the front door is opened.

Okay, I don't know whether this is necessary, but I'd like to apologize for the more idiotic parts of my question, especially the house thing. I'm not quite as incompetent as I expect I made myself appear-- at least I like to think I'm not-- but I wrote the original post at four in the morning, so I was not exactly at my intellectual peak.
Have I already explained this? It's possible that I have.
Anyway, the point is that we can from now on ignore the more obviously ridiculous questions that I now rather regret posing, and possibly address the helium question. I had hoped that someone would calculate it based on molar mass and molar volume, because I lack sufficient motivation to figure it out myself, but perhaps tonight at four...
So! Again, this is possibly not necessary, but I have moments of temporary stupidity sometimes, and I hope this doesn't require further explanation.

Well Traveler in time aswered the question. But to elaborate. You couldn't put enough in a balloon. The balloon would burst long before you put enough Helium in it.

Well, duh! This is hypothetical. So, apparently, this falls to me to answer myself.

The density of air at STP is about 1.29 g/L ; the density of helium is .1785 g/L. Therefore, air is about 7.23 times as dense as helium.

V1/V2=T1/T2

V1=22.4L
V2=22.4/7.23=3.10
T1=273K

22.4(T2)=3.10(273)
T2=37.8K

So! One would need to cool helium to about 37.8 Kelvin to make it as dense as air. That's -235 Celsius, I think. Anyway, I should hope this marks nearly the end of a very long and overly drawn-out discussion.

"I should hope this marks nearly the end of a very long and overly drawn-out discussion."

Well, it did, and then I arrived. But then, I have a habit of reawakening sleeping conversations.

First, don't forget that there are many lurkers here. We even have a smiley for them. So, even if you don't need a question answered for yourself, it's no harm to allow people to discuss it.

Second, I just want to put in a quick word about how pressure works.

Take a wellington boot. Fill it with water. Punch a hole in the toe. Watch the ensuing fountain.

The water is pushing up, not down. Pressure is a function of depth, but not of direction. Bear that in mind. The pressure is coming from the weight of the water above the hole (and of the column of air above that), but it can be equally expressed in downward, upward, or lateral velocity.

Air pressure is exactly the same. SoRB was actually wrong when he said that the pressure would be equal at the top and the bottom of the box. Go back to your welly, and put two holes, one above the other, about five inches apart, in the vertical side of the boot. The jet from the top hole will have noticeably less force than the jet from the lower hole. In the case of air and a reasonably sized box, though, the difference will be miniscule.

TRiG.

Traveller in Time expecting egg shaped soap bubbles
"< A317107 > 'soap bubbles'
>'with the same force'

The difference in pressure for air is equal inside and out, whether you measure at the bottom or in the top. "

well I think you're both right. There is a difference, but for air it's so small as to not be appreciable in measurements or observations of soap bubbles.

<>

This is only the case in situations where gravity is a factor. If you put water in a boot, the pressure is caused by gravity pushing the water down. However, if the water inside changes temperature, that increase in pressure will be distributed throughout, and you'll see the water level go up.

This is not the case in a helium balloon, a tire, a CO2 cannister, or many and sundry applications for pressurized materials where gravity is the cause of the pressure. If you punched a hole in your water pipe, it would spray the same whether you punched it in the top or the bottom.

"... Here gravity is [not] the cause of the pressure ...," I assume.

Um. In the open atmosphere, air pressure is caused by gravity. In a box at normal atmospheric pressure ... thinking about it, the pressure is due to the sides of the box, not to gravity. But surely gravity still has an effect, even on a pressurised box, no?

In a hosepipe, we are dealing with a flowing fluid. So that's different. Ish.

TRiG.

see my post above. Then you can stop arguing. Or carry on, if that's what you'd rather.