Why does speed confuse so many people?
The Sunday Nation
Nairobi,
04 May 2014
Speed is a simple but confusing concept. A motorist was once stopped by the police for speeding and she retorted: “Officer, why are you accusing me of driving at 120km per hour yet I’ve only driven for 10km since I left my house?”!
In science, speed has the special distinction of being the only combined measurement that hasn’t been given a single name. Force, for example is the product of mass and acceleration but it is measured in newtons; preasure is the ratio of force to ares but it is expressed in pascals, power (energy per second) is stated in watts and so on. But speed is always stated in kilometres per hour, metres per second etc.
Navigators in the shipping and aviation industries state speed in “knots” where one knot is equal to one nautical mile per hour. A nautical mile is different from the ordinary (civil) mile. The former was devised to be approximately equal to one minute of arc (equal to a 60th of a degree) measured along any longitude on the Earth’s surface. However, knots are not used outside navigation.
Now the speed of sound in air is approximately 300 metres per second. In other words, if some one stands 300m away and shouts, you will have to wait one full second before you hear the voice.
Since there are 1,000m in one kilometre, this speed can be stated as 0.3km/s. But also, one hour has 60 minutes and one minute has 60 seconds, therefore one hour has 3,600s (60 x 60). Therefore, 0.3km/s is equal to 1,080km/h (0.3 x 3,600). This is the sort of manipulation that the motorist referred to above was not able to comprehend.
Interestingly, the speed of light also starts with 300… it is approximately 300,000km per second. By similar manipulation, this works out to about one billion kilometres per hour. That’s very fast indeed: so fast that if God poured a “bucketful” of heavenly water over the sun to put out its fire, we would have over eight minutes of sunshine before realising what has happened.
From the 150-million-kilometre distance, the sun’s light takes about 8 minutes and 20 seconds to reach Earth. We can now understand one of the challenges of space exploration: the great distances involved makes communications tricky.
Imagine a spacecraft is some 300 million kilometres away – somewhere halfway to Jupiter. A message sent from mission control on Earth will take about 16 minutes to reach the craft. Then the craft acknowledges the instruction and the reply takes another 16 minutes. The full cycle time is over 30 minutes!
But I find that there is one thing about speed that seems to confuse many people. This is the question of whether a vehicle with large wheels (say, a bus) travelling at 100km/h moves faster than one with smaller wheels (say, a car) also doing 100km/h. I have given the answer to that twice before, so I won’t repeat it!
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