# Investigating simple steel springs

##### Class practical

The behaviour of springs provides a topic through which students can learn about simple relationships between pairs of variables, in a practical context. Seventeenth-century scientists, like Robert Hooke and Robert Boyle, helped to lay the foundations for physics and for other sciences by working in this way.

#### Apparatus and materials

Extendable steel springs, 2 or 3

Metre rule

Mass hanger and slotted masses (100g)

Eye protection for each student

G-clamp

Rubber bands OPTIONAL

Set square OPTIONAL

#### Health & Safety and Technical notes

Students should clamp their stand to the bench to prevent it from toppling.

Students must wear eye protection. Eyes may be at the same level as clamp and the nail. Also, steel springs store more energy than copper springs and can fly off their supports.

Provide spare springs. Students will stretch springs beyond their elastic limit and replacements will be necessary. This is not willful destruction but, rather, good science.

If the springs are supplied close-coiled it is better to have the coils separated before issuing them to the students. Hanging about 500-600 g gently on the tightly coiled springs will do this.

#### Procedure

a Fix the nail horizontally, with its point in the boss on the stand. Hang a spring from it and secure it so that it does not fly off.

b Hold the metre rule vertically in the clamp, alongside the spring.

c Record the metre rule reading level with the bottom of the spring. The number of masses hanging from the spring is 0 and the extension of the spring is 0 cm.

d Hang a mass hanger from the bottom of the spring. Record the new metre rule reading, the number of masses (1) and the extension of the spring.

e Add a mass. Record the new metre rule reading, the number of masses (2), and the total extension of the spring from its unstretched length.

f Repeat this until after the spring has become permanently stretched.

g Describe the pattern in the results. To do this fully, you will need to plot a graph. Plot the number of masses on the horizontal axis, since it is the input (or independent) variable. The extension of the spring is the output (or dependent) variable and you should plot it on the vertical axis.

#### Teaching notes

1 This is a more formal variation of the Home-made springs experiment. There is benefit in doing both, since it will invite discussion and thought on the nature and use of graphs.

2 You could discuss whether doubling the load on a spring sometimes or always doubles the extension. This relates to the shape of the graph, whether it is sometimes or always a simple straight line passing through the origin. It thus leads to the concept of proportionality. Proportionality, or linearity, describes a simple form of relationship between variables. This relationship is common in nature.

Much of physics is devoted to seeking such simplicity. Hooke's law states that, up to a limit, extension is proportional to load. (When the load is doubled then the stretch is doubled.) Robert Hooke noticed this very simple pattern in 1676. Since he was worried that others, maybe even Newton, would steal the credit for this he wrote in code at first, and created an anagram: ceiiinosssttuv. This is taken to mean ut tensio sic vis, which is Latin for: as the stretch, so the force. The fact, though, that Hooke's law is only obeyed by materials up to a limit highlights the fact that nature does not always offer simplicity.

3 Invite students to think about applications of springs, in systems from door catches to vehicle suspensions. Point out that engineers must understand the behaviour of springs.

4 Extension activity can include investigation of other springs, elastic bands and any other elastic materials (e.g. polythene strips). Comparison of graphs provides opportunity for discussion.

5 How Science Works extension: Include among the equipment available for this experiment a second boss and clamp as well as a set square for each student group. Either prompt a discussion initially or leave the students to work out how these extra items might be useful.

Students can improve the accuracy of their measurements by clamping the metre ruler in place and then using the set square to make the length/extension measurement. They can also use the set square to make sure that the clamped ruler is vertical in relation to the bench. Students might set the clamped ruler at 0 cm when no masses are added and so read the extension directly. This procedure helps them avoid simple mistakes that arise when measuring lengths and then calculating extensions. These refinements provide good illustrations of improving an experimental method.

Further ideas:

• Give students access to extra springs so that they can try series and parallel arrangements. You could also ask them to predict what they expect to happen qualitatively and perhaps even quantitatively.

• Investigating whether the same results are obtained when a materials is loaded and unloaded, particularly if rubber bands are used. Stretched rubber exhibits elastic hysteresis.

This experiment was safety-checked in January 2007