Outcomes

Pupils can sketch the shape of Force verses Extension graphs for helical springs and metal wires.
Pupils can identify and recall that the initial linear region of a force-extension graph is associated with Hooke's law.
Pupils associate elastic behavior with the ability of a material to recover its original shape after the forces causing deformation have been removed.

Specification References

1.27 describe how extension varies with applied force for helical springs, metal wires and rubber bands
1.28 recall that the initial linear region of a force-extension graph is associated with Hooke's law
1.29 associate elastic behaviour with the ability of a material to recover its original shape after the forces causing deformation have been removed.

Holding a spring scale in your hand begin to ask questions about how a spring behaves when it is stretched. Looking at the scale how do they know that it stretches evenly? (You could remind them about the spring scales that they made in the 3rd year) Draw a force against extension graph on the board. What would this graph look like? They should recall from the 3rd year that the plot will be a straight line.
Hand out to each pupil a piece of thin wire
Now have the pupils put on goggles. Then they wrap the thin wire several times around one of their pencils, then take the other end of the wire and wrap it several times around another pencil. This should leave at least 5 cm of wire between the two pencils. Now ask the pupils to pull carefully and slowly on the two pencils to try and stretch the wire. The wire should be difficult to stetch and then with additional force it should yield a bit and then break.
Using the small whiteboards have the pupils draw a force extension graph for a piece of wire. How is it different to the spring? Why?

Before both practicals have been done it would be worth going through the three main graphical relationships they need to know about, namely linear, non-linear and directly proportional. They can sketch these in their notes
The students will carry out a force extension graph for a a spring. Here is a Hooke's Law powerpoint to go along with it. They are to take the spring beyond its elastic limit and so the necessary safety precautions should be taken. Eg. safety glasses and the top loop should be wound with string to stop it from unwinding. The pupils could look out for the elastic limit by checking to see if the spring returns to its elastic limit each between readings. The use of a pointer and secured vertical ruler may improve results.
As a class the force extension graph for a metal wire should be done. Here the vernier scale could be introduced.

To plot graphs of force vs. extension for the spring and wire and write conclusions for both.
If you have finished all of the graphs in class then they can read pg 60-61 and answer the qs on pg 61

Starter
thin copper wire cut into 50cm lengths (class set) Newton meters (class set), mini white boards
Main lesson
Class set - experiment on force vs extension of spring and elastic. The will need to support a ruler vertically and will need some sort of pointer - springs, rulers, clamp stands, masses.
Class Demo - force vs extension of wire + large vernier callipers to help explain vernier scales, digital micrometer to measure diameter could also be supplied

Goggles should be worn during any stretching activity due to the chance of something pinging off
Mass may fall off and fall on feet - ensure pupils stand during practical
Weigh down the clamp stand to prevent it tipping