Science corner

30th June 2006 at 01:00
Ray Oliver explains how to test the limits of air resistance

The passage from first bicycle, fitted with stabilisers, to reckless feats on two wheels can be a surprisingly short one. Children soon discover the exhilaration of riding fast downhill and so are made aware of the limits imposed by air resistance.

By changing from a traditional upright cycling posture to a more streamlined one, they realise that greater speeds become possible. The hi-tech bikes used by Olympic champions are ultra-streamlined, as are the riders' helmets and clothes.

Children can investigate air resistance by making and testing model parachutes. The first recorded parachutist was the French aeronaut M Garnerin who jumped from a balloon over Paris in 1797 (although it is claimed that an Andalusian Arab made a successful parachute jump in 852, see page 18). Although Garnerin's parachute was probably made of oiled silk, just like the balloons, children can experiment. Ask them to cut out circles from a range of materials, about 25cm in diameter. First try paper, polythene and cotton. By securing threads around the circumference they can make a simple model.

After attaching a weight to represent the person, pupils can try dropping the parachutes down a stairwell or from a stepladder. Using a rubber eraser as the weight will reduce the chance of injury to anyone below. The problem with most simple designs, as they will soon discover, is that either the canopy fails to open fully or it collapses just after release. Ask for design ideas to overcome this problem - eg, attach a framework of drinking straws, rather like the spokes of an umbrella.

Extend the investigation by making parachutes of unusual shapes, such as crescents or rectangles. Pupils can also try cutting holes in the canopy, centrally or in a pattern, to see how this affects the speed of fall.

Test the final designs in a parachute competition, with points for the slowest or quickest descent, or for the parachute that falls closest to a target, just like sky divers.

Talk about other examples of using parachutes to increase air resistance.

Include the controlled return of spacecraft to earth or slowing down planes as they land on the deck of an aircraft carrier.

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

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