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If a hollow cylinder(something like a plastic cup)is partially submerged in water, am I correct in thinking that the only three forces acting on the cylinder are the buoyant force, the weight of the cylinder and the force exerted by the air at the water air barrier in the cylinder? Also is the resultant force due to the pressure distribution acting on the container and the air within it the buoyant force subtract the force a due to the air pushing at the air water barrier(inside the cup). The cylinder has a column of air inside it, so I thought the buoyant force would push against the air in the cylinder, and the force would be equal to the volume of water displaced. The height of the water is at a distance d below the water level in contact with the atmosphere.

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  • $\begingroup$ by a hollow cylinder do you mean a plastic cup with the bottom cut off? Or just a plastic cup 'as-is'? $\endgroup$ – mogelbuster Oct 25 '16 at 23:13
  • $\begingroup$ Hi a perfect cylinder that you could fill with water to drink out of(usual hole in the top so you could drink out of it), but upside down and in equilibrium within the water. $\endgroup$ – G2357 Oct 26 '16 at 5:12
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The force of ATMOSPHERE at the water barrier inside the cup......and also outside the cup cancel each other out so that is not a factor. The vertical column of water inside cup will always balance out with the water outside the cup minus the buoyancy or weight difference by area of the "cup plus the unfilled area of the cup" verses the water outside the cup until full. When it is full it is just the buoyancy of the cup or the weight difference between the cup itself only and water. There is a oh so small of a difference of atmospheric pressure of course with a different distance from sea level but with a cup you would play heck to measure it. literally undetectable.

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  • $\begingroup$ Google Archimedes principle. $\endgroup$ – william deets Oct 26 '16 at 16:36
  • $\begingroup$ Thanks for the reply. When I drew the diagram it made sense that inside everything cancelled out! Very frustrating that I didn't see this straight away. $\endgroup$ – G2357 Oct 28 '16 at 19:12
  • $\begingroup$ Thanks for saying so. What I know in physics I know from 35 years of repetition and real world experience, I want to help and I really know what I talk about when I talk about it. It's really hard to get an attaboy it seems with out a doctorate or professorship on this sight and your little thanks is the best. $\endgroup$ – william deets Oct 31 '16 at 2:13

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