4-DFAD-04

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Outcomes

  • Pupils can sketch the shape of Force verses Extension graphs for helical springs, metal wires and rubber bands.
  • 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.

Starter

  • Hand out to each pupil 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?
  • 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 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.
  • Ensure that the pupils have sketches of the 3 types of stretching material in their notes. The board tend to plot force against extension despite force being the independent variable. For the more abloe you could ask them what the graph would look like if the axes where reveresed. Identify the Hooke's law regions and the elastic limit. Ask pupils to describe what the gradient means (i.e. steep = stiff)
  • Go through Hooke's law qualitatively as they need to be able to quote it


Plenary

Homework

  • If they did not do the textbook questions last week then they can this week (read pg 60-61, qs on 61)
  • Here are four Questions on Hooke's Law

Additional Information

Resources Required

  • Starter
  • Large pile of string with cross bonds between strands (velcro)? Cooked very long spagetti?, elastics (class set),mini white boards
  • Main lesson
  • Class set - experiment on force vs extension of elastic. The will need to support a ruler vertically and will need some sort of pointer - elastics, pen to draw lines on elastics, rulers, clamp stands, pointers, masses.

Textbook References

  • pg 60-61

Website References

  • None

Skills Addressed

Safety/Hazards

  • Goggles should be worn during any stretching activity due to the chance of something pinging off
  • Ensure pupils do not flick elastic bands around the room!
  • Mass may fall off and fall on feet - ensure pupils stand during practical
  • Weigh down the clamp stand to prevent it tipping

Notes

  • None