Floating, Sinking & Buoyancy
Why does a huge iron ship float when an iron nail sinks? Drop cork, ice, iron and gold into different liquids and watch the push from below fight their weight, just as Archimedes worked out.
Class 8 & 9 Floating and Sinking · Class 11 FluidsTry this
0 of 5 doneMake something float with less than half of it under the surface.Hint: try cork or wood, or a heavier liquid
Float an iceberg: how much of the ice hides under water?Hint: pick Ice, then Water or Sea water
Make ice sink!Hint: find a liquid that is lighter than ice
Make a block of iron float.Hint: it needs a liquid denser than iron
Weigh iron on a spring balance under water. How much lighter does it feel?Hint: Iron, Water, Spring balance, then lower it all the way in
What's happening
The push from below
Liquid pushes on everything dipped in it, and it pushes harder the deeper you go. So the push on the bottom of an object is bigger than the push on its top: the difference is an upward force called buoyancy, or upthrust.
Archimedes' principle
The upward push equals the weight of the liquid the object pushes out of the way. A 1-litre block fully under water pushes aside 1 litre of water, which weighs about 9.8 N, so it feels 9.8 N lighter, whether it's made of wood or gold.
Float or sink? Compare densities
If an object is less dense than the liquid, it sinks only until the liquid it displaces weighs as much as it does, then floats. The fraction under the surface is the ratio of the densities. If it's denser than the liquid, the push can never catch up, and it sinks.
Who figured this out?
Archimedes
about 250 BCE, SyracuseWorked out that a floating or sunken object is pushed up by the weight of the water it displaces. Legend says he realised it in his bath and ran out shouting 'Eureka!'
Simon Stevin
1586, NetherlandsExplained why water pressure depends only on depth, and used it to show where the upward push on a submerged object comes from.
Samuel Plimsoll
1876, BritainCampaigned for a load line painted on every ship's side, showing how deep it may safely sit in the water. Ships still carry the Plimsoll mark today.
Learn more
- Archimedes' principle · Wikipedia
- Buoyancy · Wikipedia
- Densities of common materials · Wikipedia
Check yourself
1. A block floats with exactly half of it under water. What is its density?
2. An iron block weighs 77 N in air and 67 N fully under water. What is the buoyant force?
3. Why does a ship float in the sea a little higher than in a river?
4. A solid iron ball sinks in water. Which liquid could make it float?
Common questions
What is Archimedes' principle?
Archimedes' principle says that when an object is wholly or partly immersed in a fluid, it feels an upward force equal to the weight of the fluid it displaces. It explains why ships float, why things feel lighter in water, and how hydrometers measure density.
What is buoyant force?
Buoyant force, or upthrust, is the upward push a liquid or gas exerts on an object in it. It comes from the pressure being greater at the bottom of the object than at the top. Its size is density of fluid × volume submerged × g.
Why do some objects float and others sink?
It depends on density. An object less dense than the liquid floats, settling until the liquid it displaces weighs as much as the object. An object denser than the liquid sinks, because even fully under, the liquid it displaces weighs less than it does.
Why does a heavy iron ship float when an iron nail sinks?
A ship is a hollow shell full of air, so its average density, iron plus all the air inside, is less than water's. It sinks in until the water it displaces weighs as much as the whole ship. A solid nail is denser than water, so it can never displace its own weight.
How much of an iceberg is under water?
Ice is about 917 kg/m³ and sea water about 1025 kg/m³, so roughly 917 ÷ 1025 ≈ 89% of an iceberg is under the sea and only about a tenth shows above it. That is where the phrase 'tip of the iceberg' comes from.
Why do we feel lighter in water?
Water pushes up on the part of your body that is under it, with a force equal to the weight of the water you displace. Your mass doesn't change, but that upward push takes some of your weight off your feet, so you feel lighter.
Solid 10 cm cubes moving only up and down, with typical densities at room temperature. Real objects also tilt and are slowed more by the liquid.
Play next
All simulationsProjectile Motion
Launch at any angle and speed, then find the angle that throws the farthest.
Forces and Friction
Push, pull and change the surface to see which forces win and why things stop.
Levers and Pulleys
Slide the fulcrum and add pulleys to lift a heavy load with less effort.