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A PHYSICIST WRITES . . .  

(February 2011)

Life is too short (and the tasks that I enjoy, such as writing columns, too long) for me to want to watch TV just casually in case something of interest turns up. So I am indebted to the Editor of our Thames Valley Group newsletter for drawing my attention to an edition of Top Gear (BBC2) from last month.

I was surprised at the advice he said it contained, so I searched through the programme on iPlayer to check. First I encountered a filmed, mile-long race between a Porsche and a Beetle ó won by the latter (just), as the Porsche was being driven on a salt-flat, whereas the Beetle was released from a mile high (without a driver, I should add).

Then I found what I was looking for. Jeremy Clarkson said: ďTo keep your fuel costs down, never brake! Every time you do, you are turning your money into pointless heat. If instead you simply take your foot off the accelerator as you slow down for a roundabout or whatever, in a modern car youíre not using any fuel at all.Ē

Such calm common sense is the last thing you might expect to hear on Top Gear, but all around me on the roads Iím sure I have been seeing drivers trying to follow his advice ó and then discovering at the last moment that they still need to screech to a halt.

So I would like Mr Clarkson to have explained more clearly to the motoring masses that in order not to have to brake, they need to lift the foot a lot earlier than they normally would: thatís how the fuel is saved. But probably nothing would convince the majority of drivers that thereís any difference between braking and rolling to a halt (or at least to a lower speed), in fuel terms.

So many driving actions are automatic, for me, that I have no idea how much I actually do use the brakes. Iím fairly certain, though, that my Corolla is on the same sets of front pads and rear shoes as when I bought it nearly 70,000 miles ago!

Another clue lies in something that I am conscious of while driving: the feeling of satisfaction I get when I arrive at a roundabout, or traffic lights on red, or some other hold-up, at just the right time and speed to make immediate progress through instead of having to wait, simply by having lifted my foot at the optimum moment, way back (hence also not needing to brake and waste fuel-energy, see above).

The curious thing is that this trick seems to work best on familiar roads. But Iím having to think hard about why this should be so ... I do believe itís a matter of gentle road gradients.

I mentioned Ďgradient senseí a few years ago as a useful thing to have ó though then I was talking about adapting your driving to obvious slopes, uphill or downhill. However, even a slight gradient (up or down) can make quite a difference to the distance your car will travel after youíve taken your foot off the accelerator. And often itís difficult to detect such a gradient by eye. Iím reminded of certain places in the world where flowing water seems to be moving uphill, because the surrounding landscape distorts your sense of the horizontal.

Anyway, my theory is that as you get to know certain stretches of road, you subconsciously learn how far your car will coast along them, without realizing that this depends a lot on a gradient that is too small to see.

And yet (where is this column leading me? I have no idea!), think of the almost invisible information that the eyes do take in while youíre driving. It astonishes me that I can travel a straight road at 70 mph holding the wheel almost motionless, perhaps nudging it a millimetre or two, though I really canít consciously see why, each time. Itís as if the car is locked into the middle of the lane, or else steering itself. Actually, Iím repeating myself here from six years ago.

But it puts a new thought into my head: you can lay out a road thatís perfectly straight (to the eye), but you wonít be able to drive along it for more than a fraction of a mile without readjusting the wheel as I described. However close you think youíre holding the straight-ahead position, you will never be quite on to it. If you could keep the wheel fixed while you transferred your vehicle to Top Gearís salt-flat or similar, you would inevitably find yourself driving in a wide circle.

Circles ... roundabouts ... when you approach Bracknell from Reading on the A329, you arrive at a large roundabout now being put in alongside the dual carriageway. A temporary 50 mph limit applies for about half a mile in each direction from it, reinforced by average-speed cameras at both ends. Going east, itís still a straightish road and so you must watch your speed. But driving west, because of having to negotiate the roundabout you wouldnít get your average above 50 even if you did most of the rest of the stretch at 90, probably! Whatís the point, then, of the cameras on that side of the road? [Later, signs went up saying ďCameras not in useĒ (crazy or what?), then the signs and cameras were removed, and now apparently the speed limit is permanent, as there are painted roundels on the road.]

Roundabouts ... cyclones ... Iíve just discovered a Met Office webpage, www.metoffice.gov.uk/satpics/latest_IR.html, showing a satellite view of the clouds in a region of about 4000 by 3000 miles, centred on the UK. You click a button to replay the past 24 hours of weather, and the results (this week, anyway) are fascinating: within a general drift from west to east are countless swirls and reversals of cloud ó what causes them all?

And hereís a question I have never dared ask an expert directly, for fear of seeming stupid: why do winds not blow straight from high-pressure regions to low, as physics ought to demand, instead of circling them all the time? [A meteorologist reader of the newsletter sent me an immediate response, which I will try to summarize in a later column.]

Admittedly, anything seems plausible to me now meteorologically, after the sight of that Beetle pirouetting through the air.

Peter Soul

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