A spring is attached to a retort stand and a pointer is attached to the very bottom. A ruler is attached to the stand so the pointer points to the ruler's scale.
Equal weights are added to the bottom of the spring one at a time and the extension (how far the pointer moved between weights) is recorded. Slotted weights with hangers are perfect for this practical as they can easily be attached to the spring loop.
Equipment required (per set):
- 1 x spring
- 10 slotted 100g masses or similar on hanger
- If you have no hanging masses, a paper cup to hold the weights
- 1 x ruler
- 1 x retort stand, boss and clamp
- Sticky tape or blu-tak to hold the ruler
Pupils should find that the extension of the spring is directly proportional to the weights added (the graph they plot will have a straight section) This means for example that if 100g is added, the extension may be 2cm, if 200g is added then the extension may be 4cm and so on.
This is called 'Hooke's Law' and this principle can be applied to the stretching of wires and metalwork in bridges and architecture.
Hooke's Law only works up until a point though. The spring will only stretch so far and the point at which it will no longer return to its original length is called the 'elastic limit.' Beyond this point, the spring will be overstretched and become deformed permanently.
Some teachers may ask if they can deform a spring in this way to show the elastic limit. Springs are fairly inexpensive and are available from most lab suppliers in packs.
The same equipment can be used to investigate the stretching of thin copper wire and elastic bands. In each case, Hooke's Law can be applied. Eye protection should be worn for all these practicals.