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Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format. New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address. THE LAST WORD Piles of reason Why do rugs placed on carpets appear to move of their own volition? How can a bunch of inert fibres exert the force needed to move my rug 15 centimetres in just two weeks when there is a substantial armchair on it? n A carpet has a pile direction. The pile does not sit upright but points a little in one direction, giving a slightly smoother feel when stroked with the pile rather than against it. When something like a rug is placed on a carpet and walked upon, the pile bends in that direction, carrying the rug with it. With the release of foot pressure there is no longer the same amount of traction between rug and carpet, so as the pile returns to an upright position it does not drag the rug back with it. Traditionally, carpets are laid with the pile towards the door. This is from the pre-vacuum cleaner days when carpets were brushed clean and it was easier to brush the dirt out of the door in the direction of the pile. Max Lang Northampton, UK n I too have various rugs which travel rapidly on a cut-pile carpet, eventually pushing up against the skirting board. After a period of careful observation I noticed that the rug always travels in the direction that the pile leans in, and only when someone walks on it. When the foot is removed, the flattened pile returns to its rest position but, like a ratchet, does not move the rug back to its starting point. The movement is noticeable on cut-pile carpets, but hardly at all on loop-pile carpets, suggesting the effect depends on the pile structure, the length and stiffness of the fibres, the pressure of the footstep and perhaps the structure of the underside of the rug. Incidentally, it is easier to drag a rug – with or without chair on top – across a carpet in the direction of the pile rather than against it. I have not calculated the forces involved but they must be fairly large to move an armchair. I feel an Ig Nobel award approaching. Donald Brown Kellas, Angus, UK Tyre strategy Why can’t you mix cross-ply and radial tyres on a car? n You could mix cross-ply (called bias-ply in the US) and radial tyres, but the steering may feel odd and the car would become unsafe at high speeds, especially around corners. This is because the tyres are designed to handle road stresses in different ways. All tyres are reinforced with cord, made of nylon or another textile. A cross-ply tyre has its layers of cord laid at an angle, or bias, to the centre line of the tyre, creating a herring-bone pattern beneath the tread. When the car goes around a corner, the whole tyre, including the tread surface, leans outwards. This lessens the contact pressure with the road on the inside edge of the tyre, reducing traction. A radial tyre’s cord layers lie at right angles to the tyre centre line. Look at the wheel from the side and the cords run radially from its centre, which is how the tyre gets its name. The tyre also has belts, usually of steel, beneath the tread. In a corner, the radial’s tread tends to stay flat on the road, while the body of the tyre flexes towards the outside of the bend. There is less lifting of the tread from the road, giving better traction. When a car with all radials or all cross-ply tyres loses traction in a corner, all four tyres theoretically break loose at the same time, allowing you to control the slide with steering to prevent spinning. This is what racing-car drivers are doing when you see them sliding around a corner. With radials on the front and cross-ply on the back, the back wheels will lose traction first, causing the rear of the car to spin and resulting in loss of control. With the tyre mounting reversed, the situation is worse: the front will break loose first, causing immediate loss of steering control. The car would veer off to the outside of the corner. Rick Dieckmann Fort Jones, California, US This week’s questions HAPPY FEET The blue-footed booby (pictured) is an extraordinary-looking bird. It has fairly dull plumage but strikingly coloured blue legs and feet. What could be the evolutionary benefit of such a conspicuous feature? Both sexes have blue feet so they don’t seem to be for impressing potential mates. Sam Moore London, UK DON’T CALL ME GINGER Why are orang-utans orange? It doesn’t seem to be a camouflage mechanism. And why are they so hairy? They live in tropical forests after all. Peter Webb London, UK Last words past and present, plus questions, at www.last-word.com The new collection: witty, brilliant, intelligent and packed with insight Available from booksellers and at www.newscientist.com/elephants Why can’t elephants jump? “It is easier to drag a rug across a carpet in the direction of the pile rather than against it”
