Don’t get confused: a kilo is a kilo…is a kilo!

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

22 February 2009

One of the most popular questions that I get is whether a small car doing 80km/h moves as fast as a large bus/lorry travelling at 80km/h. I discussed the issue in detail a few yeas ago and the answer is that the two are moving at the same speed…80km/h!

The expectation that the bus will be faster because of its larger wheels has no foundation at all. 80km/h means that if the vehicle maintains that speed for one hour, it will cover a distance of 80km.

That question is similar to asking whether a kilogram of wool is as heavy as a kilogram of iron. Of course it is! The expectation that the wool might be lighter also has no foundation.

Perhaps it arises from the knowledge that a given volume of iron weighs a lot more than an equal volume of wool. This shows a mix-up of the meanings of volume and mass: they are not the same thing. The fact remains that one kilogram is one kilogram! It does not depend on the type of material being weighed.

That brings to mind a related question: how heavy is one litre? In this case, the answer depends on the kind of substance one is referring to. If it is pure water (at a temperature of four degrees Celsius and at sea level) then it is one kilogram, but not so for other materials.

A litre of mercury, for example, weighs about 14kg while that of air (at zero degrees celsius and at sea level) is only 1.3 milligrams!

The specification of the environmental conditions is necessary because the volume of a substance (especially gases) is affected by the temperature and pressure - which, in turn, depends on the altitude.

Still, Kenyans are peculiar: our tailors buy raw materials in metres (a metric quantity) yet they take measurements in inches (an imperial standard)? It probably has to do with the sizing of clothes. For example, I wear size 32 trousers and size 16 shirts. These numbers are the fitting measurements of my waist and neck, respectively, measured in inches.

For that reason, many times people will ask tailors a question like: “How long is one and a half metres?” Well, one and a half metres is one and a half metres long!

Of course what they are asking for is the equivalent of that length in inches. Thus the tailor look for a calculator to work out the answer…all the while with a measuring tape (that has both inches and metres) hanging around his neck! Why not simply read it out?

Tailors are not alone in this mixing of measurements. The dairy industry is worse: farmers sell milk to processors in kilograms (a mass) while the same is sold to consumers in litres (a volume). Perhaps we can now understand why the two quantities cause so much confusion.