4-FAD-04

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.

Starter

• 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 and a rubber band.
• Have the pupils pull on the rubber bands. Ask them to describe how it feels. Does it get easier or more difficult to stretch as you pull on it? Ask them what rubber is? They may know that it is made of long chain molecules. Draw their attention to a pile of string. Ask them what would happen if you start pulling of opposite ends of the pile. What will happen to the behaviour of the pile when all the string is evenually pulled straight? Does this help them explain how and why rubber bands behave as they do when stretched?
• 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. Ask them how this differed to rubber. How is metal and rubber different on a molecular level?
• Using the small whiteboards have the pupils draw a force extension graph for a rubber band and a piece of wire. How are they different to the spring? Why?

Main Body of Lesson

• The students will carry out a force extension graph for a a spring. Here is a 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.
• The students need to carry out a force extension graph for an elastic. Here, they could take readings as they load and unload. The introduction of what the area of a force extension graph represents could be discussed and even measured. To help them 'experience' the energy difference from loading and unloading you could have the class repeatedly stretch the elastic band while it is in contact with their top lip. They should feel that it heats up.

Additional Information

Textbook References

• None

Website References

• None

Skills Addressed

• Obtaining

Notes

• None