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A PHYSICIST WRITES . . .

(November 2003)

The five headings of the System of Car Control are written on every advanced driverís heart (or maybe on the back of the hand at least): Information, Position, Speed, Gear, Acceleration. Probably everyone thinks about this list in a slightly different way. Hereís what a physicist thinks.

Letís get the textbook stuff out of the way first: your speed is the rate at which you are changing your position along the road (Position in the SCC list also means where you are sideways, i.e. which lane you are in). Speedometers might be even more useful than they are if, as well as miles per hour, they were marked in metres per second ó indicating more clearly how fast you are following the road.

Similarly, your acceleration is the rate at which your speed is changing (upward or downward) in metres per second, each second. Would it be helpful to have an accelerometer next to the speedometer? Probably not, as you can see how quickly the speedo needle is rising or falling, and anyway you are controlling the acceleration all the time by how hard you are leaning on the accelerator or the brake pedal.

Which brings me to the main point: strictly speaking, the two pedals directly control only the acceleration of the car ó not its speed, and still less its position along the road! If you want to move smartly up from 30 to 40 mph, for example, you have to apply some acceleration until you reach 40 and then (the harder bit) lift the pedal just enough to cut the acceleration to zero.

Or you might be approaching a junction. Can you judge exactly the amount of constant braking force needed to bring the car to a halt in the distance available? A GCSE physics student ought to be able to work this out on paper, but on the road it is not easy to avoid having to readjust the brake pedal as you slow down.

Here is a more complicated situation: you are on a fast, clear approach to a large and busy roundabout, and there is an useful gap in the traffic coming round from the right. How much ó and when ó should you brake, in order to cross the line at exactly the right time and at the right speed so as to slot neatly into the gap? This is an A-Level question at least! But Iím sure that most drivers can learn how to solve it instinctively (if this is not a contradiction), instead of always stopping at the line and waiting for a larger gap.

Getting this sort of thing right, whether you are slowing down or speeding up, is what they mean by having acceleration sense. The reason this is so valuable to possess is that you do only have immediate control of acceleration, as I said. Your speed depends on what your acceleration was previously, and your position now depends on what you did even further back along the road.

You might want to argue with me that a small shift of the accelerator pedal often has the direct effect of raising your speed a bit and holding it there. I agree this is true, and it happens because the air resistance and other drag-forces grow with the speed and come into balance again against the increased push from the engine. The same (or rather, the opposite) can happen if the pedal is lifted slightly. But this topic will have to wait for another time ó and so will Information and Gears, from the System of Car Control.

Peter Soul

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