# Stretching rubber

##### Class practical

To look at a material that does not obey Hooke’s law.

#### Apparatus and materials

Retort stand, boss and clamp

Mass hanger plus masses (100 g)

Metre rule

Selection of rubber bands, elastic cord

Marker pen

#### Health & Safety and Technical notes

Students must wear eye protection.

Fishing elastic (used by anglers to give a bit of stretch to their lines) comes in a range of thicknesses. Students could investigate the stiffness and breaking strength as a function of thickness.

#### Procedure

a Take a rubber band and mark it across its width at two points, one close to each end.

b Hang the rubber band from the clamp.

c Hang the mass holder on the lower end of the band.

d Measure the distance between the two marks on the band.

e Gradually increase the load on the band, recording the distance between the marks each time.

f Gradually reduce the load, recording the distance between the marks each time.

#### Teaching notes

1 Rubber bands provide an interesting contrast to springs. On stretching, they do not obey Hooke’s law very precisely. On unloading, they show hysteresis.

2 The experiment must be done with care. Hang a rubber band or length of elastic vertically and attach weights to the lower end. The load must be increased in even steps; as the load is increased, care must be taken to ensure that the rubber is not allowed to slacken. Then the load must be gradually reduced, again ensuring that the rubber does not slacken too much and that it is not stretched more as the load is removed. Unless these precautions are taken, the non-Hookean behaviour may not show up.

3 How Science Works Extension: Students can investigate the effects of a range of factors, including width and thickness of the rubber band. Using a hairdryer, they could raise the temperature of the band and observe the effect.

The worksheet could be used as the basis of an investigation. It includes some questions intended to test students’ understanding of the design of the experiment.

4 Table 1 shows some typical data. If students plot a graph of extension against load, the points will appear to fall close to a straight line. However, ask them to pick up the paper and squint along the line; they should be able to see a clear S-shaped curve. The rubber stretches slowly at first, then roughly linearly, then more and more slowly as it becomes stiffer. Emphasise the need to carry out the experiment with care and to plot the data accurately if the detail is not to be lost.

Table 1 Stretching a rubber band (original dimensions: 95 mm x 6.0 mm x 0.85 mm)

 Load /N Length /mm Extension /mm 0 95 0 1.0 112 17 2.0 137 42 3.0 168 73 4.0 207 112 5.0 242 147 6.0 275 182 7.0 306 211 8.0 328 233