Choosing a material:

Hooke's Law: You will need the following equipment: A metre rule A spring Some weights and holder to place the weights on A retort stand to hold the ruler and the spring. First measure the length of the unstretched spring and record it. Then you will need to set up your apparatus as in the diagram below: Attach the holder to the end of the spring and gradually increase the amount of weight acting on the spring. As you do so record the length of the spring and the weight acting on it. Now draw a graph of your results, weight vs length. Do you notice anything about your results? Try repeating the experiment with a strip of polythene and a rubber band. What do you notice about your results now? How does the molecular structure of these materials help explain how the material behaves when a force acts on it?

A material scientist will often want to test how much force can be applied to a material before it changes shape. To do this they test the amount of stress and strain a material can take.

STRESS = FORCE / AREA

Stress tells us how much force per unit area a material will take before it breaks.

STRAIN = ORIGINAL LENGTH / EXTENSION

Strain tells us about the change in shape produced when you apply a force to a material.

It is often useful to plot a stress-strain graph for a material:

The gradient (or steepness) of a stress-strain graph allow us to find out the Young Modulus of a material. This tells us how much force a material will take before it changes its shape permanently

YOUNG MODULUS = STRESS / STRAIN

Young Modulus: You will need the following equipment: Approximately 2 metres of copper wire A holder and some weights A metre rule A large G-clamp A pulley which can be attached to a bench Some safety glasses to protect your eyes if the wire snaps Firstly set up the apparatus as in the diagram below: You will need to find some way of measuring the amount of extension the weight acting on the wire produces. You could make a vernier scale if you are feeling clever, or simply attach a piece of tape to the wire to act as a marker. You will also need to measure the diameter of the wire using a micrometer screw gauge, so that you can find its area. You should also measure the length of the wire. Gradually increase the amount of weight acting on the wire and note the extension produced. Record your results in a table. Now draw a graph of your results. Use the Young Modulus Formula to find out the Young Modulus of your wire. Check your answer with one published in a textbook to find out how accurate your experiment is.