What it shows
Friction is the force that stops a shoe from slipping. On wet mud, the grip depends on the sole pattern, because studs and spikes dig in and rubber flat soles slide on a film of water.
The science
Friction depends on how two surfaces interlock and on the force pressing them together. On dry hard ground a flat rubber sole has high grip. On wet mud, a film of water lowers friction, and studs penetrate it and push against solid soil. This is why footballers wear studs and sprinters wear spikes.
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.
- Three old shoes or sandals with different soles, or cut pieces of sole fixed to wooden blocks
- A spring balance (0 to 10 N) or a kitchen scale
- A shallow tray of damp soil and one of dry soil
- A string and a small hook
- Weights for adding load
- A notebook
Safety
For this project: Wash hands after handling soil. An adult must cut shoe soles or nails for studs, and no one should wear spiked shoes indoors.
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 each sole to a block of wood so the three test pieces are the same size.
Add the same load of 1 kg on top of each block.
Tie a string to the block and a spring balance to the string.
Pull the block slowly and steadily along the dry soil tray and read the force just as it moves.
Repeat three times and take the average for each sole.
Wet the soil in the second tray until muddy and repeat the test.
Make a table and bar graph of pulling force for each sole on dry and wet soil.
Explain the results in terms of friction and the way each sole grips.
What you should see
On dry soil the flat sole grips well and the studs and spikes grip better, while on mud the studs and spikes hold much better than the flat sole.
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 sliding between surfaces in contact.
2Why does mud reduce grip?
A wet film separates the two surfaces and lets them slide.
3How do studs help?
They dig into the ground to push against firmer soil.
4Why keep the load the same?
A heavier load increases friction, so it must be a fair test.
Take it further
Measure friction against the number of studs, such as 0, 4, 8 and 12.
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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