Comment by modeless
2 years ago
Calling the air inside the duck (which is not sealed inside) part of its "material" would be misleading. That's not how most people would interpret the statement and I'm confident that's not the explanation for why the statement was made.
The air doesn’t matter. Even with a vacuum inside it would float. It’s the overall density of “the duck” that matters, not the density of the plastic.
A canoe floats, and that doesn't even command any thought regarding whether you can replace trapped air with a vacuum. If you had a giant cube half full of water, with a boat on the water, the boat would float regardless of whether the rest of the cube contained air or vacuum, and regardless of whether the boat traps said air (like a pontoon) or is totally vented (like a canoe). The overall density of the canoe is NOT influenced by its shape or any air, though. The canoe is strictly more dense than water (it will sink if it capsizes) yet in the correct orientation it floats.
What does matter, however, is the overall density of the space that was water and became displaced by the canoe. That space can be populated with dense water, or with a less dense canoe+air (or canoe+vacuum) combination. That's what a rubber duck also does: the duck+air (or duck+vacuum) combination is less dense than the displaced water.