Forces between particles in solids

Exerting forces; changing the shape of things

Civil engineers, dressmakers, dentists and fishermen are just some of the people for whom a knowledge of the strength and flexibility of materials is important. For pupils, stretching materials is an area of science where they can do some real investigating on everyday items, such as T-shirts, carrier bags and party balloons, and learn about controlling variables, data collection and resource management.

The forces evident in such systems are invariably in equilibrium. When hanging masses on a spring, gravity provides a downward force that is balanced by an upward force from the stretched material. We often say this upward force is due to the tension in the material. As the force of gravity increases, the tension increases – often accompanied by a change in shape of the material – it stretches. It is this change in shape that might be investigated. For example, what happens to the length of a rubber band as we hang more masses on it? Squashing as well as stretching can change shape. Loading masses on a piece of foam cushion material provides the basis for an interesting investigation.

Relationships between force and change in shape

The relationship between force and change in shape isn't always simple. For a spring the extension is directly proportional to the force applied up to a limit beyond which the spring is permanently deformed. The force / extension graph showing this behaviour is a straight line up to the elastic limit.

For an elastic band the pattern is more complex and the force-extension graph is a curve.

There is more potential for fruitful reasoning as pupils explore the subtleties of stretching behaviour and the resulting graph than in the cruder breaking limit investigation. The latter poses a greater safety concern with heavy objects falling to the ground near tender toes. Stretching without breaking is a process that can be repeated without having to replace springs or rubber bands constantly.