What density do I use to calculate buoyancy for a hollow cylinder float?

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In summary, the conversation discusses the calculation of buoyancy for a hollow cylinder float. The volume of the cylinder is determined, but there is a question of whether to use air density or cylinder density. It is clarified that the fluid the cylinder will be immersed in is air, so air density should be used. However, there is a potential for the air to be under water pressure, which could affect the calculation. It is suggested that for a more accurate answer, this should be taken into account, but for a rough estimate the volume of the cylinder can be multiplied by the density of water. There is also a mention of the accuracy needed and the potential impact of the cylinder's weight and depth it will be submerged. Ultimately, the numbers will
  • #1
mido
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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
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,
FB=ρfluidVobjectg
 
  • #3
As Archimedes is reputed to have said: "the weight of the displaced fluid"
 
  • #4
it will be immersed in water, it is a hollow cylinder with air trapped in
 
  • #5
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
sophiecentaur said:
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.)
that is correct, but i don't think he would want to go for THAT accuracy, involving the extra compression of air by 3%. He could simply obtain the volume, plug in the value of ρwater= 1000kg/m3 along with g=9.8m/s2 and voila.
ALTHOUGH yes, i don't think the upthrust would be enough to keep the cylinder afloat
 
  • #7
It would all depend upon how far under the cylinder would go. on that vertical rod. and the mass of the cylinder. My lungs collapse enough to make me neutral at less than 3m depth.
AS with all Engineering matters, the Numbers Count.
 

1. What is buoyancy force?

Buoyancy force is the upward force exerted by a fluid on an object that is partially or fully submerged in the fluid. It is a result of the difference in pressure between the top and bottom of the object.

2. How is buoyancy force calculated?

The buoyancy force can be calculated by multiplying the density of the fluid, the volume of fluid displaced by the object, and the acceleration due to gravity (9.8 m/s²). This is known as Archimedes' principle.

3. What factors affect the buoyancy force?

The buoyancy force is affected by the density of the fluid, the volume of fluid displaced by the object, and the acceleration due to gravity. The shape and density of the object also play a role in determining the buoyancy force.

4. How does the shape of an object affect the buoyancy force?

The shape of an object affects the buoyancy force by determining the volume of fluid that is displaced. Objects with a larger volume will displace more fluid and experience a greater buoyancy force.

5. Why is buoyancy important in science?

Buoyancy is important in science because it helps explain why objects float or sink in fluids and it is a key concept in understanding fluid mechanics. It also has practical applications in fields such as marine engineering, architecture, and transportation.

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