Will a Blow Up Doll filled with Helium Float?

Assume the doll is about the size of a 50kg human.
Since we're mostly water, this means that the doll will contain about 50l helium.

Now, a litre of air weighs about 1.204g, whilst a litre of helium weighs about 0.178g. This means that for each l of helium, we can add 1.02g to put it back in equilibrium.

You'd have to fill it up to a fair bit beyond atmospheric pressure to get tension though, so you'd probably only get about 0.8g back.

So the answer is that if the blow-up doll weighs about 40g or less, then yes, it would float.

Reply 21

spocckka

14

Original post by juliusb

Guess you don't do Chem. Helium won't ignite however hard you try.
The breathing is dangerous you are right.

But yeh this is my question, is the entire system lighter than its equivalent air volume?

Breathing it is fine...

Reply 22

LETSJaM

16

Unlikely to. But have fun playing with it.

<3 x

Reply 23

juliusb

OP

2

Those last two were very useful, although doesn't look good for our prank. Maybe we need to attach extra balloons.

Reply 24

LeeC

15

Original post by juliusb

You're right, you reckon if we tied on a few balloons it would make a difference? or would that be too many? I mean kids fly off with helium balloons all the time..

You can easily work it out by representing the doll as a spherical head, 4 cylinders for arms and legs and a rectangular body. This should give you a close enough answer to work out whether it's possible all not, assuming that you know the mass of the doll.

I'm guessing that it's not going to happen since the plastic the doll is made from is too heavy..this guy used 62 balloons apparently.

[video="youtube;t_eZM6Cp07Q"]http://www.youtube.com/watch?v=t_eZM6Cp07Q[/video]

Reply 25

juliusb

OP

2

Original post by spocckka

Breathing it is fine...

Breathing small ammounts from balloons is fine because it is mixed with air in balloons. We were thinking about getting industrial helium which is more dangerous. But its not toxic or anything, just doesnt have enough oxygen.

Reply 26

username451352

11

Original post by LeeC

You can easily work it out by representing the doll as a spherical head, 4 cylinders for arms and legs and a rectangular body. This should give you a close enough answer to work out whether it's possible all not, assuming that you know the mass of the doll.

I'm guessing that it's not going to happen since the plastic the doll is made from is too heavy..this guy used 62 balloons apparently.

For the future, you can estimate the volume of the human body by just assuming it's all water, since we just about float (in a swimming pool, empty your lungs and you'll probably sink, fill them again and you'll float). So for a rough estimate,

V/l = m/kg

Reply 27

marcusfox

16

No. It won't. The mass of the doll will be too much and will overcome the lifting force it contains. You will need about 8 cu ft of helium to lift the weight of a doll - about half a pound in weight.

Mythbusters did an episode (season 4, episode 6) where they filled a raft with helium to see if they could make a 'flying carpet'. It didn't work. Turned out they needed over 5000 cu ft of helium to lift a person.

They actually built the thing out of 50ft long balloons, but it was too flimsy and fell apart.

Oh, and look at Lawnchair Larry.

And the one about the priest who took a (one way) trip to meet his boss. Google translate if you don't read Portuguese.

(edited 12 years ago)

Reply 28

Rennit

Original post by juliusb

This is a prank me and my friends want to pull off. I want to work out whether its feasible using buoyancy, but I have no idea what to use as the mass of the doll and the volume. Any ideas?

The buoyancy force is equal to the weight of the air displaced by the balloon. You find this by multiplying the volume of the balloon by the density of the air and then multiplying by 9.81. To simplify the situation, just calculate the volume of helium you put in.

If this buoyancy force is greater than the weight of the balloon, then it will float.

Reply 29

spocckka

14

Original post by juliusb

Breathing small ammounts from balloons is fine because it is mixed with air in balloons. We were thinking about getting industrial helium which is more dangerous. But its not toxic or anything, just doesnt have enough oxygen.