What it shows
It is much easier to roll a load on rollers than to slide it over the surface. A rubber band stretches less when the load rolls, showing that rolling friction is smaller than sliding friction.
The science
Friction is a force that opposes the motion of two surfaces in contact. When one surface slides over another, tiny bumps catch on each other and give a large friction. When an object rolls, the surface under it is not dragged but is lifted off and put down again, so the force needed is much smaller. This is why wheels, rollers and ball bearings are used to move loads. The stretch of a rubber band is proportional to the pulling force, so the length shows how large the friction is.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A thick rubber band, 15 cm
- A heavy book or a bag of rice, 1 kg
- Six round pencils or marker pens
- A table
- A ruler
- A strip of cloth to put under the book for a rough surface
- Marbles or a set of ball bearings, if available
Safety
For this project: Keep the rolling load away from the table edge so it does not fall on feet.
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.
Tie the rubber band to the heavy book with string so that you can pull the book along a table.
Measure the natural length of the rubber band with no pull.
Pull the book slowly and steadily along the table, and read the length of the stretched band while the book moves. Subtract the natural length to find the stretch.
Repeat three times and write the average stretch for sliding.
Place the pencils as rollers under the book, side by side, and pull again. Read the stretch while it moves.
Repeat three times and write the average stretch for rolling.
Repeat both tests on a cloth to see the effect of a rough surface.
Draw a bar chart of the stretch for each case and say which has the least friction.
What you should see
The band stretches a lot when the book slides and only a little when it rolls on pencils, so rolling friction is much less than sliding friction.
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 friction?
A force that opposes the motion between two surfaces in contact.
2Why does rolling need less force than sliding?
The surfaces are not dragged against each other, so the bumps do not catch.
3Why does the rubber band measure the force?
It stretches in proportion to the pull.
4Where are ball bearings used?
In bicycle wheels, fans and machines, to reduce friction.
Take it further
Try rollers of different thickness and a glass bottle and see which gives the least stretch.
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.
Free ebooks for this project
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