Lifting a Man with 100m^3 of Helium

[physicsforumsfan]

Hi,

Reading about different ways to lift a human being, I've come to realize the easiest is simply manipulating density as follows:

Denisty of Air at STP: 1.20 kg/m^3
Density of He at STP: 0.000179 kg/m^3

Therefore for every cubic metre (m^3) of air displacement, lift achieved is ~1.2 kg/m^3 (deducting air density from helium density).

Thus to lift 100 kg person, one would need to displace: 100 kg/1.2 kg/m^3 = ~83 m^3 of air

Therefore if a material of negligible weight with dimensions of 4*4*6 metres (equalling 96 m^3) was filled with helium, a net upward lift of approximately 13 kg would be achieved.

Can anyone confirm this to be correct. If yes then what lightweight material could you use to make this?

Thanks everyone in advance

 

[ZapperZ]

Hum... you've never seen a helium balloon?

Zz.

 

[Charles Link]

Try looking at this thread: https://www.physicsforums.com/threads/helium-for-flying-buoyancy.866819/#post-5443653 particularly posts 6 and 12. $\\$ (Post 12 shows the details of the density of air calculation, using $PV=nRT$, which also gives the buoyancy per unit volume, (treating kilograms as "weight"), since that is the weight of the air that gets displaced ( the weight of the helium per unit volume needs to be subtracted along with any weight of the balloon material, to get the net buoyancy of the helium per unit volume)). $\\$ See also: https://en.wikipedia.org/wiki/Larry_Walters $\\$ I wouldn't recommend doing this, but it is amusing. :)

 

[jbriggs444]

Denisty of Air at STP: 1.20 kg/m^3
Density of He at STP: 0.000179 kg/m^3

One of those has to be incorrect. Helium has a molecular weight of 4. Air has a molecular weight of 28, give or take. The ratio should be roughly seven to one, not seven thousand to one.

 

[Charles Link]

One of those has to be incorrect. Helium has a molecular weight of 4. Air has a molecular weight of 28, give or take. The ratio should be roughly seven to one, not seven thousand to one.

The helium should be $\delta_{He}=.18$ kg/m^3.

 

[sophiecentaur]

Some years ago (I guess it must have been about 18) my wife and I went to the Millennium Dome in London (now the O2). In the dome, there was an impressive show of absolute nutters doing daring things, suspended from the top of the dome Also, there were young women (probably very light) and they were freely buzzing around slowly, over everyone's heads, supported in harnesses by surprisingly small helium filled balloons. The lifting power of He was really brought home to me and I couldn't help comparing those little balloons with the massive hot air balloons that are used to lift a few people in a basket.
I have done the sums and it's no real surprise - just impressive.
I looked all over for a photo on line, Couldn't find one.

 

[Charles Link]

Some years ago (I guess it must have been about 18) my wife and I went to the Millennium Dome in London (now the O2). In the dome, there was an impressive show of absolute nutters doing daring things, suspended from the top of the dome Also, there were young women (probably very light) and they were freely buzzing around slowly, over everyone's heads, supported in harnesses by surprisingly small helium filled balloons. The lifting power of He was really brought home to me and I couldn't help comparing those little balloons with the massive hot air balloons that are used to lift a few people in a basket.
I have done the sums and it's no real surprise - just impressive.
I looked all over for a photo on line, Couldn't find one.

It is possible they were also suspended with wires. A light 100 lb. person, (about 44 kg), would need about 44 m^3 of helium balloon volume to keep them afloat.

 

[sophiecentaur]

It is possible they were also suspended with wires. A light 100 lb. person, (about 44 kg), would need about 44 m^3 of helium balloon volume to keep them afloat.

Unlikely that they were suspended because they were very mobile and it would have been something I was looking for. But 44m³ would only require around 5m diameter for the balloon and that wouldn't have looked too enormous in a place that size. A bit of perspective at work there with the girl only just above head height and the balloon some way above. I wish I could find some pictures because they were really impressive. I might have some 'prints' in a box somewhere. (Stone age photography)
Edit: I seem to remember now that they were moving around with a small propellor. That could have provided some lift as well as propulsion. Battery life (even in those ancient times) would have been enough for a short performance. Health and safety must have been considered in that sort of project.

 

[sophiecentaur]

I rummaged around in some old photos and I found this. Pretty well agrees with what we were thinking and I think it's pretty impressive. Not something to try outdoors!

 

[Charles Link]

@sophiecentaur The photo is so neat. And it agrees very much with our calculations. It looks like a somewhat heavy balloon material, so the combination of person plus balloon material may weigh a couple hundred pounds. That would mean a helium balloon volume of 100 m^3 or more. Thank you very much. :) $\\$ And a suggestion would be to enter the photo in one of the photo contests that Physics Forums has, where physics principles are being demonstrated.