Density
Forces and Motion | Properties of Matter

Why do some things float and others sink?

Teaching Guidance for 11-14 Supporting Physics Teaching

Heavy or dense?

Wrong Track: Well that's easy, heavy things sink and light things float.

Right Lines: Floating and sinking depend on the density of the material.

Explaining density

Thinking about the learning

For equivalent volumes of substances the statement heavy things sink and light things float is close to being true. However, pupils faced with a heavy block of wood, perhaps too heavy for them to lift, and a lightweight paper clip, will make the wrong predictions. They will predict that floating and sinking depends on the gravity force and not on an intrinsic property of the material, which is the density.

Children struggle to make sense of the ideas of floating and sinking:

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Thinking about the teaching

Tangled up in this misunderstanding is the issue of density and also the language we use to compare things. What do larger or bigger mean? To say that a block of metal sinks in water because the mass is greater is only part of the explanation.

Its mass is greater than the mass of the same volume of water. Density is the key concept behind these ideas and it is not an easy concept. It involves the relationship between two quantities, mass and volume. Each of these is hard enough to understand on its own. Here we are not only expecting such an understanding but also an appreciation of how the two ideas combine.

The critical observation is to see what happens to two blocks. One should be a low density material such as wood, but with a large mass (i.e. a large lump of wood.) The other should be a metallic object with a smaller mass than the wood. A coin would do. Holding the two objects will give pupils a clear and immediate sense of a difference in mass. Following a prediction about which will float, the two objects can be lowered into a sink or bowl full of water.

For materials, it is some feature of the material, as opposed to the object itself, that determines if it will float or sink. Mass is a characteristic of a particular object. Volume is also a characteristic of a particular object. Density is an intrinsic property of a material.

For deciding about objects, again the target concept is the compound property of density, not the contributing concepts of mass or volume alone.

Density
appears in the relation m=ρV
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