From www.undeceivingourselves.org
(1900 words 5 graphics)
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).
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.
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.
Sometimes the Mythbusters result can be confusing when science is
ignored. For example the Mythbusters found that you stay
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 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.
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.
From www.undeceivingourselves.org
(1900 words 5 graphics) |