Final answer:
The question pertains to the physics concepts of density and buoyancy, focusing on how a polystyrene block behaves in water and how adding weight changes its buoyancy.
Step-by-step explanation:
The student is asking about the density and buoyancy of a plastic block in different fluids, which relates to physics principles. To answer the questions related to the polystyrene cube:
- To calculate the density of the polystyrene, we would use the fact that if 90% of it floats above water, only 10% is displacing water. The weight of the polystyrene is equal to the weight of the displaced water. Using the volume of the submerged part (10% of the total volume), and the density of water, we would calculate the polystyrene's density.
- With a 0.5 kg mass added, the new total weight would increase, causing more polystyrene to be submerged. Using the principle of buoyancy, we can find the new percentage of the block above water by calculating the volume of water now displaced by the combined weight of the polystyrene and the additional mass.
- If the water is replaced with ethyl alcohol with a density of 790 kg/m3, the polystyrene would be more buoyant, possibly floating differently or even more completely above the surface.