What it shows
The stretch of a rubber band grows with the load. A scale marked with known weights turns it into a simple balance that measures weight.
The science
For a spring, the extension is proportional to the force applied, which is Hooke’s law. A rubber band follows this only for small loads and then stretches more, so the scale is not perfectly even. Weight is a force measured in newtons, while mass is measured in kilograms. A spring balance really measures weight, and the scale is marked in grams as a convenience.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A cardboard strip 30 cm long
- A strong rubber band
- A drinking straw as a pointer
- A paper clip as a hook
- A set of known weights such as 50 g coins or small bags of rice
- Pins, tape and a marker
Safety
For this project: An adult must cut the cardboard. Keep eyes clear when the rubber band is stretched.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fix the rubber band at the top of the cardboard strip with a pin.
Attach the paper clip hook to the lower end of the band.
Tape the straw to the hook as a pointer along the cardboard.
Mark the zero line where the pointer rests with no load.
Hang 50 g and mark the pointer position, then 100 g, 150 g and so on.
Draw the scale and number it.
Weigh unknown objects like an apple and a pencil box and check with a kitchen scale.
Plot extension against load and note where it stops being a straight line.
What you should see
A working balance that weighs small objects within about 10 percent, and a graph that is straight at small loads.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is Hooke’s law?
The extension of a spring is proportional to the force.
2What does a spring balance measure?
Weight, which is a force.
3Why does the graph bend at higher loads?
The band goes beyond its elastic limit.
4What is the unit of weight?
Newton.
Take it further
Make a second band balance from a thicker band and compare the two scales.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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