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• # Floating, but Heavy, Sphere

Discussion in 'The main mechanical design forum' started by nitay, Jul 7, 2014.

1. ### nitayNew Member

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Jul 2014
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Hello,

I'm working on a project for which I need a sphere, about 40-50mm diameter, which will be heavy enough to turn a turbine (100-200g +) but will still float on water.

I've checked hollow steel spheres, but couldn't find a thick enough sphere (most spheres are 0.5-1.5mm thick). Styrofoam balls (polystyrene) are too light, and I couldn't find a supplier that gives freedom in choosing the foam density. Fishing buoys are too big.

Does anyone has an idea on where can I get spheres as described?

Thanks
Nitay

2.
3. ### LochnagarWell-Known Member

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Hi Nitay,

If you take a solid sphere that is 50mm diameter - that will displace 65450mm^3 of water. Therefore the mass of water displaced is 65.45grams. So if you want it to float - then it has to be less than 65.45grams.

If you have a hollow sphere - where the internal cavity is not open to be flooded - then the displaced volume of water is still 65450 mm^3. The weight in air of a 50mm steel sphere - with a 2.5mm wall thickness is 140 grams - so that will sink.

However, (you could puncture the hollow sphere) - to let it flood - and below are some 2.5mm wall thickness spheres - which I think is what you said you wanted - but as I said above they will sink.

What in my opinion you need is a 50mm OD x 1mm wall thickness - which will weigh 60 grams - and so will "just" float.

http://www.fhbrundle.com/iron-spheres.htm

Hope this helps.

4. ### ErichWell-Known Member

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Feb 2012
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Lochnagar's analysis shows that what you are asking for is impossible.
A 50mm sphere that weighs more than 65 grams will not float.
You want 100-200 grams. Not going to work.

To get on the right side of physics you have to change your requirements.

1. Live with less mass.
2. Go to a larger diameter so the density of a 200 gram ball is less than 1 gram/cc
3. Replace water with Mercury.

The last one is not serious but a much denser item will float in a denser fluid than water.

Since we have no real idea what you are up to, only you can decide how to modify your requirements.

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