The last word–

ALARMING MICROWAVEIf we use our microwave oven for

longer than about 30 seconds, our

car’s alarm goes off. Why? The car is

at least 20 metres away through two

walls. The inside of the microwave is

a little corroded and the car has a

remote central locking/alarm system.

Certain car alarms, such as those fitted to recent Mazda 6, Toyota Rav4 and Mitsubishi Shogun models, transmit a continuous signal at 2.45 gigahertz at powers of up to 500 milliwatts. The microwaves are picked up by sensors inside the vehicle, which detect changes in intensity to signal the presence of intruders. Microwave ovens also operate at 2.45 GHz. While the power radiated within the oven is typically in the range 600 to 800 watts, the amount radiated outside the appliance will typically be less than a watt. When your oven is in operation, the microwaves reaching your car may be powerful enough to trigger the sensors inside it, which the alarm system interprets as a disturbance within the vehicle.

It is possible to set a car alarm so that the internal signal generator is disabled. You might also want to have your microwave oven serviced in case there is a serious leak of radiation. If your microwave has damaged shielding the radiated power could be higher than the values above.Joel Smith

Pateley Bridge, North Yorkshire, UK

It is odd that your microwave is leaking enough radiation to trigger the car’s alarm, considering the legal

limit – in the US, at least – for leaked radiation from a microwave oven is 1 milliwatt per square centimetre at a distance of 5 centimetres. Perhaps your microwave has a serious leak, or you have an unusually sensitive car. You could try parking the car in front of a friend’s house and running their microwave oven to see what happens. If it appears to be solely your problem, consider getting the microwave replaced.Alex Reinhart

Boerne, Texas, US

LIKE FALLING OFF Why is it easy to balance on a moving

bike, but almost impossible to stay

upright when it stops?

(Continued)

I was pleased to see that the importance of the gyroscopic effect on the stability of conventional bicycles has been downgraded in previous answers, but was surprised that the most important design aspect that makes bicycles easy to ride has been overlooked. Every cyclist knows that above a certain speed it is possible to ride hands-free. Most will also be aware that once a bicycle without a rider has been given a push to a little over walking speed it will stay upright for quite a while. This is because a conventional bicycle is designed to be inherently stable.

This stability is mainly due to the trail. Trail is the distance that the point of contact of the front wheel with the road lies behind the point of intersection of the steering axis with the road (see Diagram). This comes

from the built-in tilt of the steering axis and the rake of the front fork. The effect of trail is similar to the action of castors. If the bicycle leans to the left, the contact force at the road will turn the front wheel to the left. This allows the hands-free rider to achieve a degree of steering control by leaning slightly one way or the other. Dynamic analysis shows that trail, together with a gyroscopic effect, can produce stability above a critical speed, trail being the most important factor. H. R. Harrison

Loughton, Essex, UK

The usual notion is that to steer to the left you push the handlebars to the left. However, experiment by holding the handlebars with your fingertips and you find that pushing the handlebars to the left by a couple of centimetres makes the bike turn right, rather than left. This counter-intuitive effect arises because turning the handlebars a little to the left makes the bike lean to the right, which then “turns” the bike to the right.Mark Pettigrew

Sheffield, South Yorkshire, UK

For those who would like to see a

bicycle that cancels out gyroscopic

effects, Hugh Hunt of the department

of engineering at the University of

Cambridge has posted some images

at www2.eng.cam.ac.uk/~hemh/

gyrobike.htm. In 1987, New Scientistreported the work of Tony Doyle,

then at the University of Sheffield,

UK, who built a bike that not only

cancelled out gyroscopic effects but

also had no trail, and so no castor

effect (30 April 1987, p 36). “Once

[riders] had overcome their initial

impulse to scream, they could ride

the destabilised bike easily,” ran

the article. “But whereas a normal

bicycle stabilises itself almost

instantly, when the riders were left

to make the corrective movements

for themselves, they could do so only

after a delay.” Doyle also describes

the sequence of events needed to

turn a bicycle: to begin turning right

when travelling at a fair speed, cyclists

do indeed push the handlebars

to the left, and continue doing so

throughout the turn. – Ed

Last Words past and present, plus a full list of unanswered

questions, are available on New Scientist’s website at

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THIS WEEK’S QUESTIONSFog warning

On cold mornings I notice that train windows mist up on the inside. Once the outside temperature reaches about 6 or 7 °C the windows normally clear. If it is raining, however, the windows mist up at these temperatures or even higher. Why does rain on the outside of the train, even at higher temperatures, cause the inside of the windows to mist up?Klaus Forroby

North Harrow, Middlesex, UK

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Steering

axis

Direction of motion Trail

Front

fork