SITEMAP   School-college Physics Notes: Forces Section 7.3 Upthrust in liquids

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Pressure and upthrust in fluids: 7.3 Other cases of floating versus sinking in fluids (gases or liquids)

Doc Brown's Physics exam study revision notes

Explaining submarine depth control, iron floating on mercury, why an iron ship floats, why a helium balloon floats in air, various cases of floating versus sinking in gases and liquid fluids

7.3 Other cases of floating versus sinking in fluids (gases or liquids)

(noting the density of water is 1000 kg/m3)

Floating metals!

Iron nails will float on the much more dense liquid metal mercury.

Group 1 metals: Lithium - density 535 kg/m3 floats on water, caesium - density 1870 kg/m3, sinks in water.

By changing the upthrust submarines can sink below the surface of water or rise back to the surface.

Sea water can be pumped into tanks to increase the weight of the submarine to that greater than the weight of the volume water it displaces i.e. the weight of the submarine is greater than the upthrust from the sea water.

Far less dense compressed air can be pumped in to the water tanks, displacing the water, and decreasing the weight of the submarine to less than the weight of the volume of sea water displaced by the submarine. The upthrust force is now greater than weight of the submarine which can rise to the surface.

By controlling the air and water levels in these buoyancy tanks you can stabilise the submarine at various depths.

You can also get material with a density greater than the fluid to float by changing its shape e.g.

You can build a ship of steel (density of ~8x water) that floats on water. The shape of the hull, filled with very low density air, allows the displacement of water equal to the weight of the ship. If you consider the whole of the volume of the ship, the average density will be less than that of water. Any individual steel plate of the ship will of course sink, as will the ship if it is holed and water flows in!

Why steel floats!!!

Don't forget materials float or sink in gases !!!

Gases are a fluid and any object less dense than the gas will rise up in the gas due to the upthrust of the more dense fluid.

e.g. a helium filled balloon rises upwards through the more dense air of the atmospheres - used by meteorologists for data measurements in the higher atmosphere.

Carbon dioxide is more dense than air and will initially sink in air - though eventually, the random movement of particles mixes everything quite fast - which can't happen with the helium trapped in a rubber balloon.

Keywords, phrases and learning objectives for upthrust in fluids

Be able to discuss flotation in terms of weight and upthrust e.g. submarine depth control, why iron ship can float, why helium balloon floats - explaining cases of floating versus sinking in fluids like  gases liquids. Explain why an iron nail floats on mercury!

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