A formula with mass appeal

16th September 2005 at 01:00
My students enjoy squashing bread! I tell them that they are going to learn how to calculate density, and at the end of the lesson they will be asked to discuss what density is.

They all need a slice of bread each, as well as a balance, ruler, and calculator.

I write the formula for density on the board: density = massvolume.

* Step 1: use the balance to find the mass of the slice of bread.

* Step 2: use the ruler and the formula "length x width x height" to calculate the volume of the slice of bread (estimates have to be allowed).

* Step 3: calculate the density of the bread using the formula: density = massvolume. Don't forget the units.

I ask students to hold the bread flat on one hand and judge whether it's heavy or light. Then I demonstrate how to repeatedly fold and squash the bread into a compact cube. Students carry out the same squashing and again hold the cube on a flat hand to judge whether it's heavier or lighter than before. The bread should be judged to be heavier.

Students should now independently calculate the density of the cube using the same procedure as before. They should notice that although the mass has not changed the density of the bread has increased.

The ensuing discussion should provide a reasonable definition of density.

The spin-offs can include thinking about: how to measure the volume of unusual-shaped objects; how fantastic their brains are to be making these "density judgments"; which organs judge volume; how mass is sensed, and how the two are combined without thinking.

Tony Keane, teacher of science, The Blue Coat School, Oldham

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