Science corner

17th March 2006 at 00:00
Ray Oliver explores the effects of friction

Many people have investigated the effects of friction, often in unusual ways. Sir Humphry Davy found that rubbing two ice blocks together produced enough frictional heat to melt them. An early heating system in the US used water power to rotate iron plates together, giving frictional space heating.

Children may have noticed that bicycle brakes get very hot when used to slow them when going downhill.

Many products are designed to reduce friction. Non-stick frying pans and the smooth polished surfaces of skis are good examples.

Children can investigate the effects of static or limiting friction, the force needed to just start an object sliding over a surface.

Use a large unpolished wooden board or plank that can be raised on blocks to give a sloping surface.

Fix a drawing pin at the top edge, with a string and weight attached. This acts as a plumb-line and will show how the angle of the slope varies as the board is raised.

Use a selection of shoes of the same size, but with a variety of sole designs. The choice should include different types of trainers, ballet or other similar flat shoes, football boots with studs or running shoes with spikes, and walking shoes with a ridged sole.

Since each shoe must be pressed down to make good contact with the surface, weights are also needed. One easy way is to use a plastic bag filled with a kilogram of sand. This will shape itself to the design of any shoe.

Start with the smoothest sole and place the weighted shoe at the top of the slope. Gradually raise the board, increasing the angle until the shoe starts to slide.

Compare the other shoes in the same way, finding the minimum angle needed to overcome static friction. Use the data to set up a ranking order of shoes, showing how well they grip the surface.

Children may be surprised to see the result for a football boot or running shoe. They both slide quite easily. See if anyone can explain the design of the sole, noticing that these shoes are to be used to grip soft turf rather than a hard surface such as wood.

Ray Oliver teaches science at St Albans Girls' School, Hertfordshire

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