|MadSci Network: Physics|
Good question. That is indeed a heavy (pun intended) topic for 3rd graders.
A relatively simple answer is that "density" depends on not just the mass (weight) of the object, but the volume (size) of the object as well. The problem is that density is not very useful unless you are comparing two different densities. It may be easier to start with comparing different objects to water (1 gram/mL). Very light objects tend to float, very big objects tend to float, and very light AND big objects tend to float. Once you have the idea that less dense objects materials float on more dense materials, you can expand to other comparisons.
I would suggest rooting the information in familiar terms. Ask the students to name things that float in water. Boats, people, beach balls, etc. Ask the students to consider and list the properties of the objects and materials. Guide the discussion so that they start thinking about both the shape and the material. If you think they'll get guided where you want them to go, ask the students to devise an experiment that might prove or disprove their ideas. Whatever they come up with, you'll have to get them to convince themselves that it's not just the weight but also the shape of the object they're trying to float that counts. "Displacement" might be too much for them to handle, so the "volume of the submerged part" might work.
There's a bunch of good experiments online. A good one is to give each team an equal mass of modeling clay and ask them to make it float. A sphere of clay will sink, but a boat shape will float. Try to get them to figure out why.
Another one is a competition to build a boat out of a sheet of aluminum foil. The one that can hold the most pennies wins. (If you want to cheat, zinc filled pennies weigh less than solid copper ones (around 1960)).
There's a good demo where you drop unopened cans of soda in a fishtank. Typically, the diet ones will float and the standard ones will sink.
I hope this helps.
Arizona State Modeling
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