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Will a Blow Up Doll filled with Helium Float?

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?

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Reply 1

Besakt

Prank with you and your friends....sure. :wink:

Reply 2

Mr Ben

I wouldn't have thought so, the material used to make them is much heavier than a balloon.

Reply 3

multiplexing-gamer

Well the weight of the doll multiplied by gravity is the force acting downwards. So say it 0.5kg heavy, its force downwards would be the mass, 0.5, timsed by the force of gravity (9.8 in Mechanics or 9.81 in Physics) which gives a downward force of 4.9N, I think.
I'm not sure how you'd find the upward force considering it's helium, look on the internet for some research.

Hopefully, this has been some sort of help.

Reply 4

alexlduffy

No harm in trying. After all, you've probably got spares lying around, eh?

Reply 5

oh_adele

Someone clearly missed Myth Busters this evening.

Reply 6

juliusb

Original post by multiplexing-gamer

Downward force is easy, dont worry i'm taking physics. you also need the weight of the helium.
Upward force is called buoyancy, for which i need the volume... any ideas?

Reply 7

juliusb

Calm down with the sex gags. If i was doing a blow up doll, i wouldnt want it to be hard to get. :wink:

Reply 8

juliusb

I think so too. but my friends are doubtful. Apparently 'balloon' helium isn't pure, we are thinking about buying some industrial He to be sure.

Reply 9

Kiss

Of course it will float; they all float.

Reply 10

hollywoodbudgie

So this is why they're calling physics a 'boys subject' nowadays. :holmes:

Reply 11

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?

Reply 12

Tobedotty

Original post by juliusb

Downward force is easy, dont worry i'm taking physics. you also need the weight of the helium.
Upward force is called buoyancy, for which i need the volume... any ideas?

fill it up with air and measure the displacement of water from a bath. Easy way would be to fill up the bath, mark a level on the side, then submerge the thing, which will raise the water level and then remove the water to get it back to it's original level. I doubt it'll have enough volume though.

Reply 13

Joinedup

Pretty unlikely as they're made of fairly thick plastic to take the strain of *ahem* their intended use. also Baloons are a pretty efficient shape for holding a volume with the least surface area, Therefore material and mass are minimised. Thats not really true for dolls which are a pretty inefficient shape imo.
Suppose you could submerge the inflated doll in a bathtub then find the number of baloons you need to hold under to raise the level by the same amount. If that number of balloons weighs much more than your doll you're in trouble.

Reply 14

juliusb

Original post by Tobedotty

That would work well, but we haven't bought the doll yet...

Reply 15

juliusb

Original post by Joinedup

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..

Reply 16

username951095

want the doll to go on top for once?

Reply 17

Keckers

In reality the variance in field strength is going to be the least of his problems

Reply 18

juliusb

Original post by Keckers

In reality the variance in field strength is going to be the least of his problems

That made me laugh :h:

Reply 19

goodfellow

Basically for the doll to float it's density needs to be less than that of air.

It's density will depend on what it's made of (Helium & Plastic).

So the density of plastic varies but generally it looks like it has an average of about 1200 kg/m^3.

The density air is 1.2 kg/m^3 so obviously if theres no helium in the doll it sinks.

You need to get the average density to below 1.2 kg/m^3.

The density of Helium is 0.1785 kg/m3.
----

I'm too tired right now to calculate how much helium you will need.

Edit - I just read that helium is a 'lifting force' if 1 litre per gram, so if your doll weighs 2 kg, you need 2000 litres of helium. Sounds about right. And I don't think 2000 litres of helium will fit into a doll that size =P

So no, a blow up doll filled with helium won't float.