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- How far can we stretch things?
How far can we stretch things?
Levels: 3-5
NoS achievement aims:
Investigating in science
Contextual strands:
Physical world
Topic: Forces in action
Rationale
The amount of distortion of a material increases in proportion to the force applied (within limits).
By conducting experiments to gather data about the stretch in a rubber band, students will model scientific investigations and explore scientific laws.
What you need
- A range of different sized rubber bands.
- Sets of hanging weights, about 250 g each, totalling about 3 kg per set (or 25 N force meters).
- Two small double ended hooks per group.
- One 30 cm rule per group.
- A selection of springs and appropriate weights (for the activity extension – see below).
Focus
- How heavy a load do you think rubber bands can hold? How far can they stretch before they break? Do all rubber bands stretch the same amount? Why/why not?
- Can you suggest a pattern in what happens to rubber bands as we slowly increase the load on them? If we hang twice the load, do we get twice the stretch?
- How could we find out if we are right? What could we measure? What could we change?
- Do all materials behave the same as rubber bands? If we tried pulling steel wires, how could we measure very small stretches?
- Is there a ‘law of stretching’ that might apply to anything?
- What experimental evidence would scientists need to confirm such a law?
Exploration
- Discuss with the class any patterns they might predict regarding stretch and load.
- What sort of evidence would support your predictions? What variables could you measure and how could you record your data systematically?
- Show them the equipment available then get them to discuss in groups how they could set up an experiment to test their predictions.
- Invite each group to set up their equipment and record their measurements. Ask them to make a note of any problems they have with their method.
- Have each group compare their data with the other groups, and discuss as a class any patterns about the relationship between stretch and load.
Reflection
- Did you obtain any data that supported your predictions? Why/why not?
- Did you find any limits to your predictions? If you did, what do you think the reason was?
- Why might you find it hard to test the stretching of watch springs?
- What new evidence would you need to gather to formulate a law about all elastic materials?