Three objects in water
With Archimedes' principle in hand, we can now compare three objects that behave in three different ways.
Figure to be added
A — floats partly immersed
Part of the object stands above the water surface. A piece of wood or a ship floats like this.
B — floats fully immersed
The object is entirely below the surface but has not settled on the bottom. It hangs, as it were, within the water.
C — sinks
The object sinks through the water and comes to rest on the bottom of the vessel. A stone behaves this way.
What makes the difference?
All three objects are made of different materials. On each of them the weight acts downwards and the upthrust acts upwards. So the behaviour can only be settled by comparing those two.
The next step
In all three cases we must measure the weight and the upthrust and set them against each other. Activity 15.5 on the next page does exactly that.
From the three conditions that comparison yields, we shall be able to say in advance whether any object will float or sink in any fluid.