Archimedes' principle
That the two columns of the table in activity 15.4 came out the same is no accident. It is a law of nature, and it was first introduced by the scientist Archimedes.
Archimedes' principle
When an object is immersed in a fluid partly or wholly, the upthrust acting on it is equal to the weight of the fluid it displaces.
Three phrases worth dwelling on
“a fluid”
Not water alone — it holds for any liquid or gas. It is why a gas balloon rises.
“partly or wholly”
The object need not be fully immersed. When only part of it is under, the volume displaced equals the volume of that part.
“the weight of the fluid displaced”
The weight, not the volume. The same volume weighs differently according to the density of the fluid.
How the principle is used
If the weight in air is 20 N and the apparent weight in water is 5 N
upthrust = 20 N − 5 N = 15 N
By Archimedes' principle, the weight of the water displaced is also 15 N.
That is, two balance readings are enough to measure an upthrust — the displaced water need not be collected and weighed at all.