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Mythbuster TV classics
More entertainment than genuine science
By Geoffrey Dean
Dr Dean is a former CSIRO scientist and organiser of the WA Skeptics Awards.
The TV Mythbuster information in this article is from the above book published by Wilkinson Publishing, Melbourne 2006. The Mythbusters fansite www.mythbustersfanclub.com is said to get 4-10 times the web-traffic of any other show on the (American) Discovery channel "simply because people are fascinated discussing this kind of stuff" (page 44).
An interesting question
Most people don't know that the TV Mythbusters programme is essentially
Aussie made. It was conceived in Sydney by its producer, Australian
Peter Rees, and its post-production work is carried out in Sydney. You
can see its Aussie origins in its irreverent approach to everything
(Americans tend to defer to authority), and by the many Aussies on its
production crew of 20-50 people (in the early years it was 100% Aussie).
Now here's an interesting question: Do the myths busted by entrants to the WA Skeptics Awards for Young Critical Writers compare with the myths busted by the TV Mythbusters? As shown below, the answer is No, absolutely not. But as explored later, there is also the question of science vs entertainment.
Ten typical myths addressed by Awards entrants
01. Are horoscopes true? No.
02. Do ouija boards work? No.
03. Do Feng Shui images work? No.
04. Is the I Ching accurate? No better than fake readings.
05. Is crystal therapy magic or myth? Myth.
06. Can graphology really reveal your personality? No.
07. Are blonde women dumber than brunettes? No.
08. Was the moon landing a hoax? No.
09. Does a pyramid preserve food and inhibit evaporation? No.
10. Can students detect message in songs played backwards? No.
Seventeen Mythbuster myths (from above book)
01. Can lightning be attracted by a metal tongue stud? No.
02. Can going up in an airplane cause breast implants to burst? No.
03. Can a live electrical appliance dropped in your bath kill you? Yes.
04. Can a quicksand pull you down and kill you? No.
05. Can jumping in a runaway elevator save your life? No.
06. In a car crash, could an unsecured tissue box kill you? No.
07. Do you stay dryer if you run rather than walk through rain? No.
08. Too much CO2 in your stomach, eg from fizzy drinks, can burst it. No.
09. Will a decorative tattoo explode in an MRI scan? No.
10. Does driving with windows down use less fuel than having the air
conditioning on? Yes but only below 80 kph.
11. Can a lightning strike travel down your house wiring and kill you if
on the phone or in the shower? Yes.
12. Can a domestic ceiling fan cut off your head? No.
13. Can a child be floated with helium-filled party balloons? Yes but
you'll need nearly 200 balloons per kg.
14. Will hair-cream explode if sparked in an oxygen-rich atmosphere? No
but it will burn.
15. When on a playground swing, can you swing through 360 degrees? No.
16. Can cola clean rust, destroy car paint, and dissolve a tooth? Not
appreciably, but it can polish chrome.
17. Can the microwave power in a tanning unit cook your insides? No.
The above myths are urban legends and are described by the book as "myths that might happen to you". Myths that are unlikely to happen to you, such as escaping from Alcatraz, are not included in the book even though they appeared in the TV programme.
The myths compared
The above examples indicate that the TV Mythbuster myths are usually
physical, whereas the myths busted by entrants to the WA Skeptics Awards
are usually (but not always) psychological. People in the former are
just another physical object. People in the latter are usually part of a
psychological process that any secondary student can safely investigate
at home or school without the disclaimer so necessary for the TV
Mythbusters:
Whatever happened to the science?
Alas, not much science escapes from the TV Mythbusters, even when all
you need is science rather than gee whiz spectaculars. For example a
quicksand is much denser than water, so it can only float you, not sink
you as the movies would have you believe. A tissue box travelling at
car crash speed does not have enough energy to kill you. A tanning unit
does not emit microwaves so it cannot cook your insides. But it doesn't
end there.
Sometimes the Mythbusters result can be confusing when science is ignored. For example the Mythbusters found that you stay slightly dryer in rain if you walk rather than run. But the book quotes an experiment by two meteorologists at the National Climatic Data Center in North Carolina who found the opposite: you get appreciably wetter if you walk (page 80). To the Mythbusters the difference was a complete puzzle. But it is readily explained by science, which also adds an understanding not found in the TV programme, as follows:
Raindrops are falling on my head
First we need to know how intense the rain is. The Mythbusters aimed for
5-7 cm per hour, which over each square metre of ground delivers 5-7 x
(100 x 100) / (60 x 60) = an average of 17 cubic cm of rain per second.
The above book does not tell us, but 5-7 cm per hour is heavy rain,
somewhere between "pouring" (about 4 cm per hour) and "torrential"
(about 8 cm per hour), which rates I measured some years ago by standing
in assorted heavy rains with a very large funnel, a stopwatch, and a
measuring cylinder. For comparison, the heaviest rain recorded in
Australia over 24 hours was nearly 5 cm per hour at Bellenden Ker,
Queensland, on 4 January 1979. Rain just heavy enough to raise visible
splashes on a bitumen road is about 0.5 cm per hour.
Interestingly, the rate in cm per hour of the heaviest rains ever observed is given quite closely by K x square root of the duration in hours, where the duration can be anything from minutes to months. For the UK the value of K is about 6, for the world it is about 33. If we insert the above value for Bellenden Ker and work backwards, the value of K for Australia is about 24. So on Bellenden Ker, for a duration of five minutes, we expect the heaviest rain ever observed to deliver 24 x sqr(5/60) = nearly 7 cm per hour.
Heavy rain (torrential in this picture) does not photograph well. Which is why fake hose-delivered rain in movies has to be much heavier than ordinary rain, as in Gene Kelly's famous dance sequence in the 1952 movie Singin' in the Rain. Otherwise it would hardly register.
Next we need to know how much rain there is in the air. According to the Mythbusters, the terminal velocity of raindrops is 6.7 metres per second or 24.1 kph. In fact the velocity depends on drop diameter, see graph below, and varies from 0.0001 meters per second for the finest mist up to 9 metres per second for the largest raindrops.
The above graph shows that the Mythbusters value of 6.7 metres per second corresponds to a drop diameter of about 2 mm. The maximum drop diameter that can be sustained in free fall without breaking up is about 5.8 mm. For drop diameters between 0.2 mm and 4 mm the terminal velocity is roughly D x (4 - D/2) metres per second, where D is the diameter in mm. Snow falls at a much lower rate, so even light winds can create snowdrifts.
The above graph shows that the Mythbusters value of 6.7 metres per second is realistic enough for the purpose of our calculation. So over each square metre of ground, one second of Mythbusters rain occupies a height of 6.7 metres and delivers 17 cubic cm of rain. So every cubic metre of air contains 17/6.7 = 2.5 cubic cm of rain.
Finally we need to know how big you are. Your area seen from above will be something like 0.1 square metre, and your frontal area will be something like 0.8 square metre, both somewhat more if you are large. We shall reasonably assume that you and your clothing will soak up any rain that hits you, and that the rain is not so misty that it floats away as you push through it. We are now ready to calculate.
Now for the singin' and dancin'
For every metre you walk regardless of speed, you will sweep out 0.8
cubic metre of space and thus receive sideways the rain that it
contains, namely 0.8 x 2.5 = 2.0 cubic cm of rain. And for every second
in the rain you will receive vertically from above another 0.1 x 17 =
1.7 cubic cm of rain. So the total amount of rain received will depend
on your exposure time and therefore on how fast you walk.
If you stroll at 1 metre per second (= 3.6 kph or 2.3 mph) you will for every metre travelled sweep out sideways 2.0 cubic cm of rain and receive vertically 1 secondsworth or 1.7 cubic cm of rain, total 3.7 cubic cm. If you dash at 4 metres per second (= 14.4 kph or 9.0 mph, a four-minute miler does 15 mph) you will for every metre travelled, other things being equal, sweep out sideways 2.0 cubic cm of rain and receive vertically 1/4 secondsworth of rain or 1.7/4 = 0.4 cubic cm, total 2.7 cubic cm. So (as the meteorologists found) you stay dryer if you run.
But other things are not equal. When you run, your arms and legs may flail, and you may even lean forward a bit, all of which increases your area as seen from above. You may also raise splashes in puddles. So (as the Mythbusters found in their multi-puddled hangar) you may stay dryer if you walk. It all depends on how you run and how big the puddles are. Interestingly, the above calculations suggest that wind and raindrop size have an effect only by making you run faster or slower, and indeed the Mythbusters found that blowing their synthetic rain with giant fans made almost no difference.
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In their book Why do buses come in threes? The hidden mathematics of everyday life, Rob Eastaway and Jeremy Wyndham reach the same conclusion. "To stay as dry as possible, you should run as fast as you can" (page 77). But not if the wind is directly behind you. Here you can minimise the rain hitting your back by going no faster than the horizontal speed of the rain. "If you are a person of typical build and the rain is coming from behind you at the speed of a gentle walking pace, you will be hit by less rain if you amble along than if you run full pelt" (pages 78-79). |
Entertainment 1, science 0
Of course calculations like the above are boring, boring, boring
compared to the thrill of creating rain in an aircraft hangar. But the
primary purpose of commercial TV (Australian SBS excluded) is not to
educate but to create the maximum number of viewers for commercials. Too
bad if this guarantees no-brainers between commercial breaks -- science
and critical thinking is not something that commercial networks can
afford to encourage. So if you want genuine brain food and freedom from
dire physical hazards, go for this website and the WA Skeptics Awards!
From www.undeceivingourselves.com (1900 words 5 graphics) Home Fast-Find Index