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A PHYSICIST WRITES . . .
(September 2002)
One of the basic laws of physics which drivers make use of all the time, even if they don’t know it, is the connection between force and acceleration: if a force acts on an object, then it will accelerate. In other words its speed will steadily change (unless there is an equal force on it in the opposite direction, such as friction). Indeed, you can’t have acceleration without a force.
No surprises for us in this, surely? You put your foot on the accelerator pedal, the tyres grip the road and your car speeds up. Or you touch the brakes and the car decelerates — though physicists prefer to think of this as “acceleration backwards”, because the force is acting the other way.
But there is a third possibility: acceleration sideways, which always happens on a bend and which doesn’t itself change your speed. Suppose you are negotiating a roundabout at a steady speed. Believe it or not, you and your car are actually accelerating sideways, towards the centre of the roundabout, all the while. And the force causing this acceleration is once again the grip of the tyres on the road.
If we guess that the roundabout is 40 metres across, then whatever your speed happens to be, I can give you an idea of how large your continuous sideways acceleration is: it’s as if you had to brake to a halt, from the same speed but on a straight road, in just ten metres. And therefore the amount of tyre grip you are relying on to get you round is the same as if you were trying to stop in this 10 metre distance.
No wonder that advanced drivers are told not to put even more demands on the tyres by accelerating or braking on bends, and no wonder some other drivers’ tyres distort and squeal in protest at the change in direction. Having recently acquired power-steering, I miss the ‘feel’ for how much stress my tyres are under when cornering.
Changing the subject, here’s how you can estimate the speed of vehicles as they pass by. All you need is a slightly musical ear. Stand at the road-side and listen for an approaching vehicle with a distinct engine note. If it is coming at a steady speed, the ‘pitch’ of the note will be steady too. Then as it passes, the note will suddenly drop a bit (this is called the Doppler effect).
When the speed of the vehicle is about 40 mph, the change in the note will be like the start of Three blind mice. At 80 mph it will be twice as big, from the Three down to the mice.
A drop in pitch like the beginning of Why are we waiting indicates about 110 mph. The end, wait-ing, clocks the vehicle at 150 mph. And if you hear an even bigger change in the engine note as in There’s a hole in my bucket, you are being passed at about 190 mph.
Not very likely, you may say. OK then, just sit in your armchair on a Grand Prix Sunday and listen to the cars passing the track-side microphone. Also, when you are on the road yourself you might hear quite a large drop in the engine note from an oncoming car or bike as it passes by. You can then estimate its speed by subtracting your own speed from the appropriate number above.
If instead you are being overtaken by a noisy vehicle, then you need to add on your speed to the correct number above, to calculate the speed of the overtaker. But think twice before volunteering to give evidence against him or her in court!
Peter Soul
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