Measuring and weighing solid blocks
Practical Activity for 14-16
A group of blocks, all the same size but made from different materials, is used to introduce the concept of density.
Apparatus and Materials
For each student group
- Ruler marked in centimetres (not millimetres)
- Lever-arm balance
- Blocks of materials (see Technical notes)
Health & Safety and Technical Notes
Be careful that larger blocks of dense materials do not drop onto feet.
Materials should be provided in the form of blocks with edge lengths equal to an integer number of centimetres. There should be two types:
- Blocks of standard size (for example, 5 cm x 4 cm x 3 cm), allowing direct comparison;
- Blocks of various other sizes. Suitable materials:
- Soft wood
- Hard wood
- Polystyrene foam (Styrofoam)
- Paraffin wax
You could also supply polystyrene foam sheet (5 cm x 30 cm x 40 cm), marked to show that it is the equivalent of 10 x 10 or 100 of the smaller polystyrene blocks.
- Compare the masses of two standard-sized blocks of different materials, first by ‘weighing’ them, one in each hand, and then using a balance to find their masses.
- Draw up a list, putting the materials in order from lightest to heaviest.
- Investigate blocks of other materials which are not of the standard size. Devise a method for including these materials in your list.
- The approach suggested introduces the concept of density. The aim is for students to develop a familiar feeling for density as something we know about a material; a useful qualitative concept rather than a quantitative definition and a scheme of measurement. With an average or slow group of students this might simply take the form of comparing equal-sized blocks of several materials; first by holding them (perhaps blindfold}, then by weighing them with a direct reading balance. Even if you go no further, this will establish general knowledge such as iron weighs three times as much as aluminium, for the same chunk, and many times more than plastic foam.
- A blindfold guessing game will make a very good start. Ask students to guess which block of material they have been given and maybe even to say which block is the heavier.
- It will help if the rulers are only marked in centimetres and not also marked in millimetres. If the rulers are only marked in centimetres, students will measure to the nearest centimetre. If there are millimetres as well, some will measure a width as 1.9 cm and thereby ruin the whole experiment in a mass of complicated arithmetic in a misplaced attempt to achieve
- The difficulty of weighing the small foamed polystyrene block (5 cm x 3 cm x 4 cm) should lead the students to the need to weigh a large number of these. The large polystyrene sheet is 5 cm x 30 cm x 40 cm (i.e. 100 small blocks). The sheet should be supplied ruled with markings to show that it is the same as 100 small blocks, which can then be weighed all at once.
- There should be sufficient balances so that there is one for every four students. These should read up to 1000 g. Chemical balances (or other equal-arm balances) should not be used. For some students it may help considerably if the second scale on the balance is covered over with black masking tape.
- You could lead the teaching on to a discussion of weighing different sizes and working out the mass of some standard size, such as a unit cube (cm 3) but this may create barriers for some students. Students can calculate volumes, particularly when the measurements are simple, whole numbers. They can divide mass/volume but many do not see the point; they do it but do not see the necessity. Finding the mass of a unit cube seems to have more purpose when it can be used to compare different materials.
This experiment was safety-tested in July 2007