[Penguin (OP)]

Just recently we build a party  boat for our pond. We used 8 plastic oil drums for the floats. All we did was seal the lids on them with window sealing caulk. I asked my dad if we compressed them if the would be more buoyant. He said probably not, because air has a weight, although we never think about it. So if we compressed the barrels with air and seald them, would they be more buoyant, or less? And how about compressing them with hydrogen? Although it's the lightest element, it still has weight, would it be the same as air? And if comrpressing the barrels makes them less buoyant, what if we had made a vacume, and the barrel didn't implode. Would this make it float better?

[Quantumcat]

Um .... hmm ... interesting question, but I think if you compressed gasses they'd be denser and wouldn't float so well (and would sink when you compressed them enough to be denser than water) If I'm wrong correct me; perhaps things get less dense when they're compressed but I think they do get more dense. As to the air has a weight thing, the reason we don't feel it (the fact that there are tonnes and tonnes of the stuff pushing on us on every square centimetre) is because we are evolved to NEED it, if we didn't have it we'd like explode (like what happened in Mission to Mars I think it was called, and the guy pulls his helmet off and his head explodes)

Am I dead? Am I alive? I'm both!

[Penguin (OP)]

so would the vacuume in the barrels help it float better?

[Ylide]

Well...density is mass divided by volume, so in an evacuated barrel, where the mass (apart from that of the barrel) is theoretically zero, I'd say that would be more bouyant.  

But consider this....the mass of the barrel is going to be orders of magnitude greater than that of the mass of the gas inside, barring lots and lots of compression.  The mass difference between a barrel filled with air that is at atmoshperic pressure and a barrel that contains a vacuum is going to be negligible.  50 gallons of air at 1 atm. pressure..let's see, approximating with ideal gas laws, n=PV/RT and n=m/MM.  The average molar mass of the atmosphere is about 29 g/mol  (70% N2, 20% O2, 5% Ar, 5% misc, averaged together by %)  So m=(MM*P*V)/(R*T)   50 gal is approximately 12 L (very rough estimate), R is 0.0820575 L atm/mol K, and we'll assume 298 K for temp.  (about 25 deg Celcius)  the mass of the air is about 14 grams.  Compare that to the mass of the barrel, which is probably about 15 kg, (45-50 lbs) and you'll see that removing the air doesn't change the total mass very much.  (15000 g vs. 15014 g)

Now, if you're really bored, given that the density of water at 25 deg C is about 0.9917 g/mL and the rest of the assumptions I made above, calculate the pressure of air inside that would increase the total density of the barrel + air to that of the water so that it no longer floats.  

Back to my phys. chem. lab report now.  See what this class is doing to me?  =P  





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[chris]

Cannabinoid - I'm very impressed !

But what about the other suggestion, which was to fill the barrels with hydrogen or helium ? How would this affect buoyancy ?

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[Quantumcat]

It would be more bouyant I assume ... since hydrogen and helium have a smaller density than air. (at least, i think)

Am I dead? Am I alive? I'm both!

[Ylide]

depends on how much gas you put in.  (i.e. the pressure, which is a function of the total mass of the gas you put into the barrel)  If you put in 1kg of helium, you'll be less bouyant than 0.5 kg of air...but that's an awful lot of gas.  

As far as gas density goes, a gas will occupy the full volume of its container, so the molecular weight of the gas divided by the volume of the barrell is the density.  Thus, yes, you're absolutely right, in the same container hydrogen or helium would be less dense than air.  (but the effect on bouyany due to this weight difference is going to be really small)





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[tweener]

A vacuum would be best if you could keep the barrels from collapsing.  In reality, as cannabinoid has shown, it really doesn't matter what gas you put in the barrels.  Just make sure they don't have holes in them.  Water would be detrimental to the bouyancy!


----
John

[chris]

In marine recovery operations, heavy components are often lifted off the seabed using huge bags of 'oil' like diesel or paraffin. Being less dense that the water they exert a force surfaceward. Presumably liquids are used because a gas would expand too much as the bags rose from the depths and potentially rupture ?

Also, going back to the "what to put in the barrels" argument, presumably hydrogen at room temperature and pressure would be the best (albeit tiny) contribution to buoyancy ?

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx