Sinking & Floating
“Things float because they have air in them.” “Heavy things sink to the bottom.” These are just a few of the ideas that children regularly offer when asked the simple question, “What makes some things float while other things sink?”
None of the ideas offered above is entirely wrong, And all are reasonable guesses, probably grounded in direct (but incomplete) experience with rubber bath toys or playthings used in swimming pools, or on poorly understood information picked up at school or elsewhere. But none of the statements above closely matches the accepted scientific explanation of the world of sinking and floating (buoyancy). Each is what is called a naive conception (or less accurately, a misconception) about the topic.
Buoyancy is not a simple topic. Most people know that they do not have a clue about electronics or magnetism, but many children and adults have well-formed and sometimes firmly held beliefs about why their rubber ducks float but the soap does not, or why ocean liners (made of steel) seem so solidly placed on the surface of the ocean. In fact, the scientific explanation for all these phenomena is quite sophisticated and rather abstract. It requires a good understanding of the concept of density and the concept of the equilibrium of opposing forces, not to mention the seemingly strange notion proposed by Archimedes more than 2,000 years ago, that displaced fluids exert an upward force on the object displacing them.
This is not a complete school curriculum about buoyancy and density and opposing forces. It is a program of exploration that can be used, as written, in an informal educational setting such as an afterschool program, or as a supplement to a formal course of study in school. The principal intentions are for children to experience the sinking & floating phenomena in action, in such a way that they question their old (naive) certainties; that they notice another example of a predictable relationship in the physical world; that they uncover a scientific relationship through disciplined yet enjoyable exploration; and that they lay down the intellectual groundwork for a more formal study of this and other topics in school or elsewhere.