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A PHYSICIST WRITES . . .
(November 2010)
Before this year runs out (our newsletter doesn’t appear in December), here are some more follow-ups to previous columns. Early on in the year I discussed runaway acceleration. Later I was forwarded a personal account of such an event: “I’ve just suffered a spectacular uncommanded acceleration in my Volvo V70. I was in a multi-storey car park, reversing slowly into a space between two cars, using my mirrors either side, when suddenly my car accelerated backwards in a great smooth surge.
“My right foot hit the brake pedal — but it didn’t slow the car, so I hauled on the hand-brake. Even then the acceleration continued, but only for a second and then the car seemed to stop instantly. I took stock: my feet were correctly on the brake and clutch pedals (both buried in the carpet). The handbrake was nearly vertical. The car was parked perfectly. What had happened was that the cars on either side had driven off simultaneously. I hadn’t noticed they were occupied, and as they were my main visual reference in the poorly lit car park, I thought I was accelerating backwards.”
Well, I’ve written often enough before about how the brain can deceive you (or do I mean itself?) at the wheel, but this is as dramatic an example as any. And it reinforces the message of a BBC2 Horizon programme last month (available to watch on iPlayer until the end of November: click here), in which what you see was demonstrated to override almost every other sense, affecting what you think you are hearing, or tasting, or feeling, and so on. And on top of this (as I’ve described also), much of what you ‘see’ is invented by the brain anyway!
In February I mentioned that during a collision, when your seat-belt has automatically pulled tight (assuming yours does) and locked up, it doesn’t have much ‘give’. I was thinking of the bruising the belt can cause. But a reader pointed out to me the advantage of a tight seat-belt: your own deceleration starts early. It then continues while the front of the car is concertina-ing, and hence it lasts the maximum possible length of time — which means that the (average) force on you from the belt is minimized.
In my May column I compared journeys on foot and by car. I said it’s curious how much detail I can remember about a long walk afterwards, in comparison with a long drive. Since then my mathematical mind has thought of another oddity. Suppose you go on a roughly circular six-mile ramble. Then the area you’re walking around works out at just under three square miles. If instead you take a circular 60 mile drive, the area your route encompasses is nearly 300 square miles (not that you get to see much of it, of course). This huge difference is a basic property of geometry: increasing the dimensions of any shape by a factor of 10 (as in this case) makes its area go up by 10 squared, or 100 times.
Now consider a car, parked alongside a scale model of itself. If the real thing happens to be ten times longer than the model, then (as above) there will be 100 times more surface area to be painted. The volume of the car, however, will be 1000 (or 10 cubed) times larger than that of the model! What I’m leading up to is this: if you see a statement that one object is so many times “the size of” or “bigger than” another, then be suspicious, because these ambiguities might be referring to dimensions, or to area, or to volume. And depending on which it is, the objects could be vastly different to look at or not very different at all. [Just testing: if a snowball weighs eight times more than another one, how do their diameters compare? The answer is below.]
Also in May I presented you with stereo-pair photos of my Corolla and then (in the website version of the column) of the S of France. Later I read a report that the recent popularity of 3D cinema and TV has led to a significant number of children and adults discovering that they are unable to see the stereo effect, or else it makes them feel dizzy — the reason being that they have undiagnosed eyesight defects or problems. OK, not every 3D film is properly made for comfortable viewing (I’m told), but if generally they make you suffer, it would be a good idea to visit an optician. Best not to base this just on my Corolla photos and holiday snaps, though.
In September I talked about keeping to the speed limit. I’ve since come across another good reason for doing so, which is that if you’re over the limit and tailgaters catch up with you, they are likely to be mouthing: Why aren’t you going even faster? Or words to that effect. Whereas if you are on the limit they can only complain that you’re sticking to it, which is easier to justify (to yourself, anyway). You will also be safer at the lower speed, when being tailgated...
More about the hired Kia I mentioned last month: once I had got used to the indicator stalk being on the right, I thought how silly it is for any manual car to have it on the left, preventing you from changing gear and nudging the stalk with a finger at the same time. On top of which there’s the irritation, when you’re driving a car differently configured from the one you are used to, of finding yourself indicating with the wipers.
Lastly, a follow-up to my medical tale in March: in this month of Movember, I and other men have been growing moustaches. We’re doing it to increase awareness of prostate cancer, which kills 10,000 men every year because it was not diagnosed early enough for successful treatment (are you male and over 50?: see your GP about checking your blood-PSA level annually — also about any water-works trouble you may be having!). And to raise funds for the Prostate Cancer Charity, which supports both sufferers and research into better treatments. To find out more, and to see the result of my best efforts at cultivation, go to my special webpage: http://uk.movember.com/mospace/1120260. Many thanks for your interest.
[The answer to the snowball test: one diameter is twice the other.]
Peter Soul
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