Final answer:
A floating solid block in water has a mass equal to the mass of the water displaced, as indicated by Archimedes' principle. The principle explains buoyancy depending on the density of the material relative to water, affecting whether an object floats or sinks.
Step-by-step explanation:
When a solid block is floating in water, according to Archimedes' principle, the mass of the block is equal to the mass of the displaced water. The principle states that any object, wholly or partially submerged in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object. This is why a block of wood floats while a block of brass sinks, despite having the same mass; the wood has a lower density than water, while brass has a higher density.
For two blocks with the same volume, the one with more mass has a higher density. This concept is evident when a polystyrene cube, which has a significant volume, floats on water with 90% of it above the surface, indicating a low density. If additional weight is added to the polystyrene block, more of the block submerges because the overall density has increased, and it displaces more water to provide the necessary buoyant force.