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 of 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 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?

Additional Information

Textbook References

• None

Website References

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Skills Addressed

• Obtaining

Notes

• None