1Upthrust
Water pushes harder on the bottom of an object than on its top, because the bottom is deeper. The difference is an upward force called upthrust. Air gives upthrust too.
upthrust
Lesson 21.3 notes
Floating and sinking
Upthrust, why things float or sink, density, and why a steel ship floats.
2Weighing in water
A stone that pulls 8 N on a newtonmeter in air pulls only 5 N in water. The water pushes up on it with an upthrust of 8 − 5 = 3 N.
3Floating
A floating object sinks in until the upthrust equals its weight. Then the forces are balanced, and it floats. In denser salty water, it doesn't need to sink in as far.
floats
4Sinking
If an object's weight is bigger than the upthrust, even when it is completely under water, the resultant force is downwards and it sinks.
sinks
5Density decides
Objects less dense than water (1000 kg/m³) float, and denser ones sink. The closer an object's density is to water's, the more of it is under: ice (920 kg/m³) floats nine-tenths under.
density
6Why ships float
A steel ship floats because its hull is hollow. The steel and the air inside together have an average density less than water's.
average density
7Submarines
A submarine dives by letting water into its tanks, making its average density bigger than the water's. To rise, it blows the water out with air.
Explore: Float or sink lab
Choose an object and a liquid, then drop it in. Compare its weight with the upthrust, and its density with the liquid's.
Common mistake: Heavy things can float
It is density, not weight, that decides. A ship weighing thousands of tonnes floats, and a small pebble sinks. The ship's average density, with the air inside, is less than water's.
Remember
- Upthrust is the upward push of a liquid or a gas, because the pressure on the bottom of an object is bigger than on its top.
- An object floats when the upthrust equals its weight, and sinks when its weight is bigger than the upthrust.
- Objects less dense than a liquid float in it; denser objects sink.
Going further at GCSE