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A PHYSICIST WRITES . . .
(November 2002)
A couple of months ago we looked at force and acceleration, and how much you rely on tyre grip to push the car forwards when you are accelerating, or backwards when braking, or even sideways (into the bend) when cornering.
While all this is going on, what is happening inside the car? Anything that is loose and free to slide about will appear to move backwards when you accelerate, or forwards when you brake, or sideways (outwards) when you corner. This might seem rather odd behaviour — but each time the item is actually trying to continue on in a simple straight line at a constant speed, as far as it can (this is Newton’s First Law of Motion in action).
Now let’s think about the ‘items’ that are strapped in: you and me. Suppose I am driving and you are the passenger. When I speed up or brake or steer or change gear, I hardly notice the varying acceleration and forces on me. This is not so much because of my seat belt, more because I have a firm grip on the steering wheel — also, I know what the car is going to do next!
In contrast, your body is free to move about (within the constraint of your seat belt) and so is trying to behave like the loose item I mentioned above. You therefore have to make continual muscle adjustments in an attempt to stay still in your seat, hence you are more aware of the changes in acceleration than I am.
But then I notice your discomfort out of the corner of my eye, and I remember the advanced driver’s trick of moving the pedals and the wheel slowly every time, instead of quickly. Now, I hope, you barely notice each new force even though the change is as large as before, because it comes more slowly and your body has time to readjust to it.
Another trick out of the same bag helps to prevent the final jerk when braking smartly to a halt. The jerk happens because the deceleration drops suddenly from a large value to zero at the moment the car stops. The secret is to lift the brake pedal smoothly in the final second — the deceleration then falls gently away, and you come to rest more restfully.
Stopping quickly and comfortably on a slope (either uphill or downhill) requires even more delicate footwork, because now gravity is interfering. After letting up on the brakes just before you stop, immediately you have to apply them again to catch the car before it takes off downhill. The trouble is, like riding a bicycle, if you think about this too hard your performance may go downhill too...
Peter Soul
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