All materials stretch when we hang a load on them but some stretch more than others. It is easiest to see this when the material is shaped in the form of a thin wire. The amount of stretch depends on the load we use and the shape of the material – a long wire will stretch more than a short one and a thin wire will stretch more than a thick one. The material matters too - rubber will stretch more than steel for example. Knowing the amount of stretch in different situations can be important for designers who might need to know how much the cables supporting a suspension bridge or the cable of a crane would stretch. We can also use the stretch of a wire (or a spring) to measure a load.
With most materials (especially metals) a large load is required to give an extension that we can easily measure, even if the material is in the form of a long, thin wire. This is why this experiment is easier in the virtual laboratory that in the real one, where a micrometer has to be used to measure the extensions even of thin wires several metres long. When using extension to measure a load (in a spring balance, for example) we need a large extension for a moderate load. This can be done by winding the material into a spiral coil called a spring. In a spring the extension is caused mostly by the coils of the spring separating rather than by stretching of the material itself. This means that there are more variables to be controlled when doing experiments on springs – the number of coils per centimetre, for example, and the diameter of the coils.
Scientists use a number of words in a special way when talking about stretchiness. These include:
Load. This is the mass (the amount of stuff in) the object we hang on the wire or spring. It is usually measured in grams or kilograms.
Force. The effect of the Earth’s gravity in the load causes a downward pull that makes the spring stretch. This pull is an example of a force.
Extension. This is the amount by which a wire or spring gets longer when it has a load on it.
Now answer the following questions:
Now go back to the intro and think about the questions there.