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Rain July 3, 2022

Posted by Tomboktu in Uncategorized.
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In third year physics when I was in college, we did a short course on methods and models. It was intended to cover common features across diverse subjects like acoustics, solid state, nuclear, or thermodynamics, without being about any of them per se. (I think it was also designed to reign in our ambitions for final year projects to something more realistic below a Nobel prize. Ah, youth!) I remember the lecturer told us about the problem of the decay of the beta particle which seemed to break the laws of physics because momentum seemed to be lost, until Pauli came along in 1930 and said, no, the laws are fine: there is another particle being emitted which we can’t see. (It was given the name neutrino and detected in the 1950s.)

For the course project, we were given a project closer to home: to devise a model to tell you how to stay as dry — well, as ‘least wet’ — as possible while moving through a rain shower. Specifically, your bus shelter is 100 meters from the door of the college, and it’s raining. Devise a model to show your best strategy for staying as dry as possible as you cross that 100 m.

The lecturer explained that we needed to think about different factors and decide which we would include in our model and which to exclude. For example, did we want to take account of any differences that wind might introduce? If that caused the rain to fall at an angle of alpha degrees instead of straight down, did it make a difference if you lean into it at alpha degrees so that only the top of your head gets wet, or would it be some multiple of alpha to take account of rain drops that fell below head height just be fore you arrived and which you want to hit the ground before you got to them? Or does staying upright just mean that the same amount of rain falls on you, but some if is now on the front of your shirt rather than on the top of your head? Or maybe before we explored that angle, would it be better to develop a first model assume the rain fell only vertically?

We were warned — emphatically — that we were unlikely to devise a good model and that the purpose of the project was to show what we did and where we got stuck. It was only by trying to devise a model and trying to find a way of identifying the right equations for the situation that we wold realise the vast amount of hidden work behind the successful models presented to us as ‘obvious’ descriptions in the various courses we were studying. We had two weeks to submit our reports.

One of the class of about 30 cracked the problem. Despite the hint that the angle of the rain and the consequential angle you adopted as you moved through the shower might be a red herring, most of us poked at that unsuccessfully. Peter spotted the bleeding obvious that the rest of us missed: forget angles, forget raindrops falling under the acceleration of gravity just in front of your shirt, forget questions about whether a cylinder is an adequate representation of a human; the key question is how much time you spend in the rain. And that means that the way to minimise the amount of water that falls on you is to maximise the speed you use to cover the 100 meters from the college door to the bus stop, technically: ‘run like hell’.

Comments»

1. polly - July 3, 2022

Stop me if I’m telling Cedar Lounge back a story I got from you originally but that reminds me of the one about Niels Bohr working out the height of a tower using a barometer.

http://felix.physics.sunysb.edu/~allen/Jokes/bohr.html

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WorldbyStorm - July 3, 2022

That’s pretty good!

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2. WorldbyStorm - July 3, 2022

That’s brilliant, Tomboktu.

Liked by 1 person

3. Michael Carley - July 3, 2022
4. alanmyler - July 5, 2022

I’m struggling to recall any bus stops within 100m of the doors of the college. Also impossible to get to any such bus stops in a straight line, if they did exist, which I’d have thought might have complicated the model. Anyway, being an engineer rather than a scientist I’d have gone for the empirical approach and run multiple experiments first, to see what approach actually works in keeping one less wet, and then fudged a model to match the data.

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Tomboktu - July 5, 2022

Indeed, Alan. But I did say it was a physics class, so everything was roughly right. The nearest bus stop would be opposite Kevin St garda station, circa 140 meters away. That’s 100 meters within a physicist’s margin of error, though the fact that the college building no longer exists wouldn’t be.

An astronomer, a physicist and a mathematician were traveling to a conference in Edinburgh on the train. Just after they crossed the border into Scotland, they see a black sheep.
‘Oh look’, says the astronomer, ‘In Scotland, all sheep are black’.
The physicist sighs ‘No. In Scotland, some sheep are black’.
The mathematician shakes her head: ‘No, no, not quite. In Scotland, there exists a field that contains a sheep at least one side of which is black.’

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alanmyler - July 5, 2022

Very good.

Out of interest, who was the lecturer, I’m curious about whether or not we might have had him too? For first year physics we had Des Hickey, I really liked his classes, he was witty. Second year we had a guy we used to call Leo Sayer, shortish guy with very curly hair, he had done his post grad in the US in the same university as Carl Sagan or somebody equally famous that I’ve since forgotten the name of. He was forever name dropping the guy.

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Tomboktu - July 5, 2022

It was Des Hickey.

Somebody said he was in the interesting position of being a staff rep and a student rep on governing bodies at the same time, as he was studying at NCAD while teaching in DIT.

Des was also sane. In final year, somebody organised a lunch-time session on getting through your finals. One lecturer have a 20-minute talk on the importance of exercise, giving up smoking, cutting down the drinking, changing to a healthy diet, and so on. Des was up next and said that the only one of those we should adopt in the run up to the finals was cutting out the drink because trying to give up smoking, changing your diet, or taking up exercise, never mind all three, in the few months before stressful finals would only add even more stress. But we could do those afterwards.

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5. yourcousin - July 5, 2022

Being a simple construction worker, I would’ve put on a rain suit and just gotten on with it. All that math is above me.

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