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.

Read our standard health & safety guidance

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

1. Take a rubber band and mark it across its width at two points, one close to each end.
2. Hang the rubber band from the clamp.
3. Hang the mass holder on the lower end of the band.
4. Measure the distance between the two marks on the band.
5. Gradually increase the load on the band, recording the distance between the marks each time.
6. Gradually reduce the load, recording the distance between the marks each time.

Teaching Notes

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