BuiltWithNOF
Stretching
stretching kit w1

This type of investigation is ideal for encouraging pupils to design their own experiment. The starting point could simply be…

How stretchy is an elastic band?

After discussion as to what we mean and the introduction of useful vocabulary and examples, pupils are given a range of simple items which they can use in their experiment.

In the simplest form this experiment can be performed with the following items:

  • an elastic band (several different colours but all the same size for class exercise)
  • ruler
  • pieces of dowel rod
  • cellotape
  • marbles or 2p coins as masses
  • plastic cup

Various practical problems arise and pupils are challenged to overcome them as they improve their experiment to the point where it can be performed with great accuracy and a set of results obtained showing mass added and length (extension) of the elastic band.

A graph can be drawn from the results which shows that the extension of the elastic band is proportional to the mass (number of marbles) added. (in the early stages with low mass this may not quite be the case and if too great a mass is used the relationship varies also as the elastic becomes damaged). Most elastic bands will not return to their original length when all of the mass is removed! This is an excellent point to ‘stretch’ the more able and can be related to the choice of a material for certain types of clothing eg. Cycling shorts.

Studying how much a material stretches is very important and lots of precise experiments are conducted on textile materials used in clothing and in construction eg. seat coverings, clothing for athletes eg. cycling shorts and climbing ropes and tapes, bandages, tents etc.

A fun scenario for this simple experiment would be to find out which of a selection of textile materials would be best for use in making some clothing for a cyclist or athlete (of course a series of other criteria for selection would also come into play but stretchiness is very important).

Pupils are divided into groups of 2 or 3(max) and are given a coloured elastic band. Each group is given a different colour if possible and the elastics should be of approx the same size and thickness. They are asked to think of ways to find out how stretchy their elastic is compared to other groups and to draw a diagram of their experiment. Group discussion follows in which the essential considerations are brought out. Eg. We have to measure something. We could measure the length of the elastic before and after stretching. We have to find a way of stretching the elastic. A force is needed. We could stretch the elastic by hand but how do we know how much force we are using? Different groups might use different forces so how could we compare? Is there a way to stretch the elastic without pulling on either end by hand? What else could provide the force needed?

Various suggestions are made by pupils and someone usually suggests hanging a heavy object on the end of the elastic then measuring with a ruler how much it stretches. If time permits then this can be tried – possibly as a demonstration.

More information can be obtained if we gradually stretch the elastic band. Eg. We could add one marble at a time and record the new length each time up to say 50 marbles.

At this stage the basic equipment could be given out and pupils asked to work out how it can be used to give us our measurements.

Fact: the paper used in making bank notes has to pass a very strict stretch test!

stretch0004 w1

Clamps and stands aren’t needed for a basic stretching experiment. Simply hang the elastic band or strip of cloth over the end of a bench using a wooden dowel which is taped at the back end to stop it flipping over.
A plastic cup with two holes punched near the rim to hold a wooden dowel for attaching the elastic band is ideal for holding masses such as marbles.
The ruler is taped to the table leg to keep it still so that measurements can be made of the increase in length of the elastic.
In this example the dowel in the rim of the cup has a point to make it easy to read the ruler - it points at the ruler scale and started at zero when the cup was empty.
Adding the marbles in fives up to a maximum of about 40 marbles gives a reasonable extension.

The ‘twist in the tail’ is that when we have finished adding all the marbles we should really do the measurements in reverse so as to unstretch the elastic to see if it goes back to its original length!

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