The three conditions
We can now account for all three behaviours with a single rule. It turns on how the upthrust acting when the object is fully immersed compares with the object's weight.
(a) Upthrust less than weight — it sinks
If the upthrust when fully immersed is less than the weight of the object, the object sinks in the fluid. A stone behaves this way.
(b) Upthrust equal to weight — it floats submerged
If the upthrust equals the weight of the object, the object floats fully immersed in the fluid — hanging, as it were, within the water.
(c) Upthrust greater than weight — it floats partly immersed
If the upthrust is greater than the weight of the object, the object floats partly immersed, at the point where the fluid produces an upthrust equal to its weight.
(c) is the subtle one
In case (c) the object does not stay fully immersed. It rises, the immersed part shrinks, and it settles at the level where the upthrust is exactly equal to the weight. So there is something true of every floating object, in both (b) and (c):
The law of a floating object
When an object floats in a fluid, the weight of the fluid it displaces is equal to the weight of the object.
A common mistake
It is wrong to think there is no upthrust on a sinking object like C. There is — in activity 15.5 the upthrust on C was 1.9 N. It is simply less than the weight of 2.4 N, so the object sinks.