Final answer:
The water level remains unchanged when a piece of wood floating on top of another is removed and placed into the water, due to the volume of water displaced being equal to the total weight of both pieces of wood combined.
Step-by-step explanation:
When the top piece of wood is removed from the floating position on the first piece and placed in the water, the water level in the tub will remain the same. This is due to the fact that the bottom piece of wood displaces a volume of water equal to the weight of both pieces of wood. When the second piece is placed in the water, it will displace a volume of water equal to its own weight. Since the weight of the two pieces of wood hasn't changed, the total volume of water displaced remains constant, hence the water level does not change.
This can be understood through the principle of buoyancy, which states that the buoyant force on an object in a fluid is equal to the weight of the fluid displaced by the object. The removal of the top piece does not alter the amount of water being displaced by the combination of the two pieces when they were together.
Furthermore, the concept of density helps to explain why materials like wood float while more dense materials such as brass would sink when placed in water. Objects with a density lower than that of water will float because they displace a volume of water equal to their weight before being fully submerged.