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Post by Twoddle on Jun 28, 2008 19:44:40 GMT
A very lucid explanation by Pete. I would be grateful if he could expand it to explain the phenomenon whereby when travelling on the highway at a constant speed, which is slightly higher than another vehicle also travelling at a constant speed, the rate of drawing away from that vehicle is - or appears to be - faster than that when closing the gap. I understand the concept in my mind, but have been unable to transmit it to others. It's because the half-wit swine in front doesn't like being overtaken, so he accelerates surreptitiously as you approach him. Once you've passed him he slows down again.
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Post by Paul Doherty on Jun 28, 2008 20:17:59 GMT
The same driver also has exceptionally good eyesight. So good, in fact, that he can see the imaginary lorries which always stop him moving into the leftmost lane on a motorway.
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Post by Bertie on Jun 28, 2008 20:51:49 GMT
Very funny chaps, but there is a serious point here, not imagined. I believe that there is a proper explanation in physics for this phenomenon.
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Post by Paul Doherty on Jun 28, 2008 21:22:39 GMT
Doppler?
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Post by Twoddle on Jun 28, 2008 21:34:10 GMT
That crossed my mind, but I can't see how the Doppler effect would explain the phenomenon that Bertie's described (which is one that I've noticed on many occasions).
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Post by Pete on Jun 29, 2008 1:54:43 GMT
OK, I'm guessing here but might it be to do with the fact that rear-view mirrors and wing mirrors are convex, so that they exaggerate the distance behind you?
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Post by Rajesh Valluri AKA Raj on Jun 29, 2008 2:18:48 GMT
OK, I'm guessing here but might it be to do with the fact that rear-view mirrors and wing mirrors are convex, so that they exaggerate the distance behind you? Bingo. When approaching the vehicle ahead of us, we are getting a normal unadulterated view and hence the rate of approach we see is the real rate of approach. Once we have passed the vehicle, the objects in the mirror are shown at a larger distance than they actually are and hence the rate of leaving is exaggerated.
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Post by Geoff on Jun 29, 2008 5:14:09 GMT
OK, I'm guessing here but might it be to do with the fact that rear-view mirrors and wing mirrors are convex, so that they exaggerate the distance behind you? But not all mirrors are convex. My side mirrors are convex but my rear view mirror is not. Has anyone observed the effect with plain mirrors? I, too, was thinking Doppler, but wouldn't be able to explain the effect observed by Bertie using that concept.
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Post by Verbivore on Jun 29, 2008 11:00:12 GMT
Perhaps because so many of us are verbivores*, not scientists or mathematicians? But I had a scientific upbringing, only switching to law in my late 20s. * Verbivore, forgive me but this is a perfect word for what we do here, methinks. And using the word generically is intended as a compliment.Nothing to forgive, Pete. I believe the term verbivore was coined by Richard Lederer in the title of his (1992?) book, Adventures of a Verbivore. I merely appropriated it (having first sought RL's permission – which request was not responded to, so by default not objected to). As I could find no verbivore trademark registered I figured I was on safe ground (though RL does own the domain verbivore.com; mine is ~.com. au). Once I went commercial, the manager of the university press I do a lot of my work for started referring to me as The VerbiWHORE. ;D ;D I am just one verbivore, not The Verbivore. There is plenty of room for others.
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Post by Dave M on Jun 29, 2008 17:06:06 GMT
Hi, Bertie I don't think I've noticed the effect you refer to, but I'll hazard this guess:
Imagine that you're travelling at 5mph faster than the car in front, and your approach mentally "starts" at some point half a mile back. At that distance, you will be reducing the gap at just under 17% per minute.
When you're quarter of a mile behind, you're closing the gap at 33% per minute - and when you're just 100 yards behind, you're closing that gap at 144% per minute!
Once you pass the car, you start to "undo" the pattern in exactly the same ratios, BUT ... you'll care about the passed car for just a few yards. Your memory of approaching the car is built up of a long period of varying rates, including the final approach and the less dramatic early stages, whereas your memory of leaving it is made up only of the dramatic "close" period.
[Technical points: 1. Your speed, realative to the other car, has stayed the same; it's the rate of closing/expanding the gap (proportional to the current gap) that changed. 2. I'm assuming you don't manage to be at a significant portion of the speed of light ... ]
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Post by Pete on Jun 29, 2008 18:09:36 GMT
Perhaps because so many of us are verbivores*, not scientists or mathematicians? But I had a scientific upbringing, only switching to law in my late 20s. * Verbivore, forgive me but this is a perfect word for what we do here, methinks. And using the word generically is intended as a compliment.Nothing to forgive, Pete. I believe the term verbivore was coined by Richard Lederer in the title of his (1992?) book, Adventures of a Verbivore. I merely appropriated it (having first sought RL's permission – which request was not responded to, so by default not objected to). As I could find no verbivore trademark registered I figured I was on safe ground (though RL does own the domain verbivore.com; mine is ~.com. au). Once I went commercial, the manager of the university press I do a lot of my work for started referring to me as The VerbiWHORE. ;D ;D I am just one verbivore, not The Verbivore. There is plenty of room for others. To us, you'll always be The Verbivore. The rest of us are mere verbivores.
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Post by Pete on Jun 29, 2008 18:11:23 GMT
OK, I'm guessing here but might it be to do with the fact that rear-view mirrors and wing mirrors are convex, so that they exaggerate the distance behind you? But not all mirrors are convex. My side mirrors are convex but my rear view mirror is not. Has anyone observed the effect with plain mirrors? I, too, was thinking Doppler, but wouldn't be able to explain the effect observed by Bertie using that concept. Geoff, I am not an expert on these things but I have never seen a car which didn't have a convex rear-view mirror, to maximise the field of view.
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noel
New Member
Posts: 14
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Post by noel on Jun 29, 2008 18:24:37 GMT
I'd go with Pete's explanation and cite as evidence that, when getting into the lane in front of the having-been-overtaken vehicle and looking in your rear-view mirror, you tend to feel you are less far in front of it than you had expected. I'll try it out tomorrow.
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Post by Twoddle on Jun 29, 2008 18:50:41 GMT
I'd go with Pete's explanation and cite as evidence that, when getting into the lane in front of the having-been-overtaken vehicle and looking in your rear-view mirror, you tend to feel you are less far in front of it than you had expected. I'll try it out tomorrow. Try not to take anyone's front wing off, Noel.
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Post by Tone on Jun 29, 2008 20:02:51 GMT
Vadim, >I think you mean the third differential, Tone?<
(Belately!)
Yes, technically, of course. But it depends upon your mental "starting point". I really meant it as "the next".
Back in the '50s, when this effect first really became "known" (i.e. relevant), most people knew that acceleration was the "rate of change of speed", but never really thought about the fact that acceleration was in fact "rate of change of distance (from a certain point)". So the term "second differential", meaning, as I said, "next" tended to be used to stress the point that it was something "further", in the scheme of things, than acceleration, that those same "most" people just hadn't thought about before. (If it had been spoken of, at that time, as the third, I feel that many people would have stumbled with a "what's the first?" attitude.)
(And it would seem that not so many people think about it even now.)
So I'm sorry, but the name just stuck in my mind and I have used it thusly ever since.
I haven't been able to track the origin of "jerk" any more accurately than "middle 20th.", but that would fit in with that which I've said above.
And, about the rear-view mirrors, my cars (RHD) have, by choice, a flat interior mirror, a flat right-side mirror, and a convex left-side mirror.
Tone
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