Why objects feel lighter in water, why ships float and ice floats low.
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A body in a fluid feels an upward force called upthrust or buoyancy, because pressure is greater at the bottom of the body than at the top. Archimedes' principle: when a body is wholly or partly immersed in a fluid, it feels an upthrust equal to the weight of the fluid displaced. Apparent weight = real weight − upthrust.
A stone weighs 8 N in air and 5 N in water. Find the upthrust.
Upthrust = 8 − 5 = 3 N, equal to the weight of water displaced.
Why does a body seem lighter in water?
The upthrust partly balances its weight.
Find the upthrust on a body that displaces 300 cm³ of water (1 g per cm³, g = 10).
Mass of water = 300 g = 0.3 kg; weight = 0.3 × 10 = 3 N.
A body floats when its weight equals the upthrust, so the weight of the body equals the weight of the fluid displaced. It sinks if its weight exceeds the upthrust (density greater than the fluid) and floats if its density is less. Relative density = density of the substance ÷ density of water. A floating body displaces fluid of weight equal to its own, so the fraction submerged = density of body ÷ density of liquid.
A block of density 800 kg/m³ floats in a liquid of density 1000 kg/m³. What fraction is submerged?
800 ÷ 1000 = 0.8, so 80% is under the surface.
Ice has density 900 kg/m³. What fraction of an iceberg is under water (1000 kg/m³)?
900 ÷ 1000 = 0.9, about 90% is submerged.
Why does iron sink but a steel ship float?
The ship is hollow, so it displaces water whose weight equals the ship's weight even though steel is denser than water.
A ship floats because its hollow shape displaces a large volume of water. A submarine dives by letting water into ballast tanks to increase its weight and rises by blowing the water out. A hydrometer floats lower in a lighter liquid and higher in a denser one and reads relative density. A lactometer is used to test milk. Floating in the Dead Sea is easy because its water is very dense.
How does a submarine sink and rise?
It fills ballast tanks with water to sink and expels the water with compressed air to rise.
Why is it easier to float in sea water than in river water?
Sea water is denser, so it gives a greater upthrust for the same volume.
What does a hydrometer measure?
The relative density (specific gravity) of a liquid.
1. State Archimedes' principle.
2. Why does a cork float on water?
3. A body weighs 10 N in air and 6 N in water. Find the upthrust.
4. State the condition for a body to float.
5. Define relative density.
6. Why does an iron nail sink while a ship floats?
7. What is the use of a hydrometer?
8. A body has density 600 kg/m³ and floats in water. What fraction is submerged?
9. Why does a fish bladder help the fish?
10. Why does a person float more easily in the Dead Sea?