# Calculating bouancy force

1. Apr 15, 2015

### mido

I have a hollow cylinder float as seen attached, trying to work out the buoyancy. I work out the inner volume for the cylinder, but do I use air density or cylinder density ?

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2. Apr 15, 2015

What is the fluid it will be immersed in? If it is air, then you should use air density ρ=1.225kg/m3 at 15° to 20°C at sea level.

Buoyancy depends on the density of the fluid and the volume of the object,
FBfluidVobjectg

3. Apr 15, 2015

### sophiecentaur

As Archimedes is reputed to have said: "the weight of the displaced fluid"

4. Apr 15, 2015

### mido

it will be immersed in water, it is a hollow cylinder with air trapped in

5. Apr 15, 2015

### sophiecentaur

The solution to this depends upon how accurate you need the answer. The air/water interface at the bottom is under pressure ρgh and the volume will be less than the cylinder volume, so the upthrust will be less. You may need to account for the volume of air being under water pressure of around 0.3m depth (about 1/30 atmospheric pressure) - which means that the original volume of air will be compressed additionally by about 3% (back of a fag packet calculation). Is that relevant for you? A more accurate answer can be obtained if you are bothered about greater accuracy than a percent.
Otherwise you can say the upthrust will be the volume of the cylinder times the density of water. (minus the actual weight of the material of the cylinder, of course.)

6. Apr 15, 2015