Things Not Generally Known was the title of a book by J. Timbs, published in 1856. (This page, previously titled "Science Facts that People Get Wrong").
That motto, from the Royal Society coat of arms is traditionally translated as "dont take anyone's word for it". There now follows some miscellaneous nonsense you hear or read about from time to time. BUt dont take my word for it either: I could be wrong.
Last updated 14 January 2004; corrections and additions 18 July 2006, 18 Aug2006
18 July 2006 Why I wrote this page... Hmmm. Reading this now I can see I must have been having a Bad Hair Day at the time :-( Some of these points are, it has to be said, rather fatuous. You certainly should not attach equal importance to everything you read here. There were a couple of topics that I wanted to discuss seriously, 1 and 9, and I wondered if I could think up enough other topics to make a 'list'. You'll see I was scraping the barrel with some of them!
But it is interesting, is it not, that societies count things differently from time to time? Why should we have four tastes and then five; nine colours in the rainbow and then seven (or six)? And on body senses, are there five, or six, or more?. New Scientist magazine ran an article a couple of years ago, which detailed 21 distinct body senses. And several myths and counter-myths about lightning rods are commonly heard, although I still manage to forget the 'real' answer myself, despite having it pointed out to me several times over the years.
OK... I'll shuffle these topics around into some sort of order, with the bar-room arguments and fatuous comments at the bottom.
Copyright © David Gibson 2004. Details
Friction has nothing - or at least very little - to do with it. Spacecraft (and supersonic aircraft) heat up because they are travelling through the atmosphere faster than the speed of sound. Supersonic travel imparts a retarding force quite separate to, and more severe than friction.
Put simply, the spacecraft cannot push the air out of the way fast enough (because the shock wave can only travel at the speed of sound) and so the air becomes compressed. The retarding force is caused by the compression of the air in the shock wave. And, as everyone who has pumped up a bicycle tyre knows, if you compress a gas it gets hot - the phenomenon is the adiabatic compression of the shock wave. And - no - bicycle pumps do not get hot because of friction - try pumping one when it is not connected to a tyre and it will not get hot.
If it were friction that generated re-entry heat then we would expect a blunt spacecraft to get hotter than a streamlined object, because it slows down faster (and therefore must give up its kinetic energy faster). In fact, it is the streamlined shape that gets hotter because the shock wave it generates is closer to the vehicle, heating it up more.
The idea that it is the air rubbing against the spacecraft that heats it up is nothing but a popular myth. (Think about it: if the heating really was frictional then spacecraft would simply have to have a non-stick coating!). The fact that research has concentrated on thermal insulation and control of the shape of the shock wave rather than on perfecting a non-stick coating should suggest that friction is not the cause of the heat. Of course, friction does play its part, but not as the major source of heat generation.
There is (or used to be) a standard A-level Physics experiment to measure gamma, which is the ratio of the specific heat capacity (s.h.c.) of a gas at constant pressure, to the s.h.c. at constant volume. This ratio is related to the speed of sound in the gas, and is tied in with a discussion about adiabatic and isothermal processes. For an ideal diatomic gas, gamma can be shown to be exactly 1.4. During an adiabatic process, the quantity TV^(gamma-1) is a constant, so if we instantly reduce the volume (V) of a sample of (ideal diatomic) gas to, say, a half then its absolute temperature (T) increases by almost 32%. If it starts at 20C (293K) it will therefore end up at 114C! Clearly this is enough to explain the rise in temperature of bicycle pump - its not the friction of the piston against the wall of the pump.
A related formula can be used to work out the temperature of a gas that is compressed when a fast-moving object compresses the gas in a shock-wave. We can define the stagnation temperature, Tt as the temperature of the gas when it is brought to rest relative to the projectile - this will be in some part of the shock wave, and is not as a result of the molecules touching the projrectile at all. If the gas temperature is T0 and the projectile is moving at M times the local speed of sound then the stagnation temperature can easily (if you understsand fluid dynamics, that is) be calculated to be Tt = T0[1 + ½(gamma-1)M^2]. So, if we assume that gamma is 1.4, T0 is - I dont know - 200K, then if the projectile is moving at Mach 10, the stagnation temperature is 4200K.
Note that the above formula did not include any 'coefficient of friction' - the temperature is not due to frictional effects, it is due to the compression of the gas. But, Nullius in Verba, the above formula makes all sorts of assumptions, including M>>1. Also, of course, the stagnation temperature is not the temperature of the projectile, it is the temperature at some point in the shock wave. The skill in designing re-entry heat shields is to shape them so that there is a large shock wave, so the hot regions of the gas are kept well away from the heatshield.
You would think that it would be easy to confirm what Ive said above by doing a quick search on the Internet. But you will find that many aeronautics sites - including some of NASA's own pages - refer to the "frictional heat of re-entry" and they attribute the re-entry braking forces to friction. Some of the mistakes are due to journalistic miscomprehension, but if you read others closely, you'll see that it is just sloppy writing - they mention friction, but then go on to talk about the shape of the shock wave. Only if you delve into some of the very technical pages - companies that have actually designed heat-shields, for example - do you find the heating correctly attributed to adiabatic compression.
You are slightly better off looking in text books. To pick one almost at random, Douglas, J, J Gasiorek & J Swaffield, 2001, Fluid Mechanics, Prentice Hall derives the above expression for supersonic flow; and my ancient Nelkon & Parker A-level Physics textbook gives a derivation of the 'bycycle pump' formula.
Just to pick three pages more-or-less at random, from a Google search...
This is not a correct description of why Foucault's Pendulum behaves as it does. The behaviour is due to the earth's rotation, and the earth is underneath the pendulum - of course - but the use of the phrase "rotates underneath" is a vague description. Whether it is incorrect is a moot point, but it is not "helpful" as an aid to understanding the pendulum. Consider this...
If you were to set up a pendulum at the north pole, and set it swinging, then it would continue to swing in the same plane (Newton's laws) whilst the earth "rotated underneath it". Foucault demonstrated this by suspending a pendulum from a drill chuck and rotating the chuck, observing that the plane in which the pendulum swung did not rotate. The earth rotates once a day so, at the north pole, we see the pendulum's plane of swing apparently rotating once a day. (No problems so far). Now consider the behaviour at the equator. Here the motion of the earth does not cause the pendulum to rotate. Clearly, at latitudes in between, the pendulum must take longer than a day to rotate. It can be shown, both experimentally and mathematically, that the period of rotation is one day, divided by the sine of the latitude. So, here in Leeds, at nearly 54 degrees, a pendulum would take nearly 30 hours to complete one revolution. At the latitude of the tropics of Cancer and Capricorn (23 deg, 27 min) the figure is about 60 hours.
So... to anyone who tries to tell you that the effect is caused by the earth "rotating underneath" the pendulum (the implication being that something moves round and the pendulum keeps still) - ask them this: What is it that is taking 30 hours to rotate?. The earth rotates once per day - you cannot argue with that! you might be told that you are looking at only a component of the rotation (similar to how we resolve bearings into a northing and an easting) but this is completely ludicrous. It doesnt matter how you look at it - the earth rotates once a day. If some 'components' of it didnt, it would fall apart!
So why is it that something appears to have a periodic effect that takes an arbitrary time longer than a day. Sitting in my room, what can I "feel", with my senses, that takes 30 hours to complete a cycle? ...or takes 47 minutes longer if I move to London? Clearly the explanation that "something" is rotating underneath the pendulum must be wrong!
Foucault's Pendulum is a beautiful experiment! It appears to reveal something uncanny about the world - some steady cyclic event that does not match the daily path of the sun and stars across the sky. What could it possibly be, and could I feel it any other way? Robert Crease was right to include this experiment in his recent book, The Prism and the Pendulum, where it is billed as one of the "ten most beautiful experiments in science" even if he did rather dodge the issue and fudge the explanation.
The explanation of what is really causing Foucault's Pendulum to rotate is far more subtle than the earth "rotating underneath", although it is caused ultimately, of course, by the earth's rotation (what else could it be?). To answer a question I raised above - no, you cannot detect this periodicity with your internal senses. Its an effect that is only experienced by moving objects. Since this is the Internet, I do not need to explain further, as you can probably find out for yourself. However - be warned - the physical effect that gives rise to this motion of the pendulum is another topic where people frequently get things wrong!
Why is it that 'popular knowledge' says that there are only five bodily senses? I can think of at least two that are not on the usual list - the sense of balance and the sense of body position, known to neurologists as proprioception. OK, so you could argue that proprioception is an 'internal' sense (i.e. it is merely the brain, keeping track of where it has put the limbs) but the sense of balance is detecting an external force - gravity - so surely it should be included with the traditional five? Discounting unproven 'senses' such as telepathy, how many senses do we really have? According to http://www.newscientist.com/channel/being-human/mg18524841.600.html it is 21.
Why is it that 'popular knowledge' says that there are four tastes? We all 'learn' at school about sweet, sour, salt and bitter tastes. But the fifth, known as umami, was identified as long ago as 1908. Ben Schott's latest book, Schott's Food and Drink Miscellany correctly includes it - but why has it been forgotten about for so long? And why are our school teachers perpetuating this and other myths?
An interesting problem: a rainbow consists of a continual gradation of colour across the spectrum, so why should we artificially divide it into seven colours - red, orange, yellow, green, blue, indigo, violet? The number of colours we recognise depends on our culture. For example, 'orange' did not exist as a colour in England a few hundred years ago (the hue was called 'golden'), and I would suggest that 'indigo' is falling out of favour now. An old book I have, Things Not Generally Known, by J. Timbs, published in 1856, lists six colours in a rainbow, but then subdivides three of them, to produce nine colours in total. For example, it subdivides purple into indigo and violet. The nine colours are, according to Timbs, red, scarlet, orange, yellow, pomona, green, blue, indigo and violet. But this is probably just as artificial as Newton's decision to divide the spectrum into seven colours, as described at en.wikipedia.org/wiki/Indigo (Thanks to a correspondent for that link).
Depends on what you mean by highest. The highest mountain in the world is not Everest, it is Chimborazo in Ecuador. Everest is the highest above sea level, true, but Chimborazo, at only 20,700 feet (6,310m) is the furthest from the centre of the earth by some 2150m. That makes it the highest mountain (assuming you want to define height that way). Search at google.com/search?q=chimborazo+centre+earth
18 Aug 2006: I had previously added a comment to the above, that "the peak of Chimborazo will be the place where you have the most potential energy", in order to try to explain what I meant by "highest". But Wyatt Johnson wrote to me pointing out that, on consideration of potential energy, Everest would be higher. He wrote, ... if you were to consider gravity as the only source of potential energy, than yes, furthest from the center of the Earth would yield the highest potential energy (assuming Earth's gravity field were longitudinally symmetric). But Earth's rotation is another source of potential energy. At the same radius, you'll have more potential energy nearer the poles than the equator. Objects will tend to roll (or in the case of liquids, flow) to the lowest potential energy possible. Our oceans don't form a sphere - they form an oblate spheroid. The sea level reflects an even potential across the planet. So since Everest is the highest peak relative to mean sea level, it also has the most potential energy relative to any other point on the surface.
On 18 July 2006, Joseph Cooper wrote... You should mention that Mauna Kea is actually the "tallest" mountain in addition to the two you mention. (See http://en.wikipedia.org/wiki/Mauna_Kea)
No. Heating water under pressure raises its boiling point so that the steam is at a temperature greater than 100C. It is the higher temperature that causes the food to cook quicker.
On 18 July 2006, Charles Cazabon wrote... Steam can easily be at temperatures greater than boiling. It's the liquid phase of the water that the pressure cooker raises the boiling point of, and therefore allows the liquid water to get hotter than the 100-degree maximum under normal pressure. I'm sure you already understand this, but the wording on your page is funky. [He means 'unclear' and, yes, it was what I meant to say :-) ]
* This was a typo. It orginally said "steam". I meant to say "heat", because that was the gist of the argument we had in the pub :-)
No. Well, ... OK, yes / but ... oh I dont know, I have got confused again now. This is not the sole purpose of a lightning conductor. It does provide a path for the lightning; but if that were its only function then any old bit of metal would do. However, the 'inventive step' in the design of a lightning conductor was supposedly to make it have a very sharp point at the tip. This gave it an additional function and allowed it to protect a greater area around it by creating an ion cloud - or, at least, that was what used to be thought, but ...
On 18 July 2006, Zachary Brock wrote... You incorrectly state that a sharp tipped lightning rod is preferable for grounding lightning strikes. You may be interested to know that research proves you quite wrong. [See here]
Also on 18 July 2006, Keith Steiger wrote... Wikipedia's entry for lightning rods pointed me to [this] which experimentally proved that blunt lightning rods work better.
On 27 July 2006, Robin Laing wrote... Actual studies have shown that blunt rods work better at directing lightning. See Researchers find that blunt lightning rods work best.
They do, but there is one very obvious exception without which life as we know it would cease to exist.
Im surprised this generated so much correspondence, as it was just a throwaway remark, really. I was referring to the fact that ice is less dense than liquid water.
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