One more reason why small potatoes are better than large ones

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

11 May 2008

Last week’s article ended on the question of whether it is better to buy vegetables by volume (debes, kasukus or counting per piece) or by mass (in kilos). The first method is preferred by kiosks in the estates while the second one is favoured by the supermarkets.

Before going into the details, it is important to note that there is a difference between mass and weight. As explained in a previous article (October 2005), the former is quantity of matter in a substance and it is measured by a scale balance. The latter is the gravitational force “felt” by the object and it is determined by a spring balance. Many people think that the two words mean the same thing because the action of measuring these quantities is called “weighing”.

While mass is constant, weight varies depending on the location where it is measured. For this reason, it is illegal to sell goods by weight! This explains why spring balances have a warning that says “Not to be used for trade”.

Back to the current problem; the easiest and direct way to find out whether, say, a kasuku of peas is cheaper than a kilo is to weigh them and then compare the price per kilo.

But kiosk traders can complicate matters: they might arrange a pile of four small potatoes and offer them at, say, Sh20. Then they will have a second set with three large ones still going for the same price. Now, which is the better deal?

I saw this kind of offer recently: By holding them in my hand, I estimated the large potatoes to be about 6 centimetres in diameter and the small ones about 4cm. From these measurements, one can calculate the volumes fairly accurately for comparison. (If you have forgotten, the formula was: “four-thirds-pi-r-cubed”)

It turns out that each large potato is about 38 centimetres cubed (cc). Therefore three of them are 114cc. The small ones are 33.5cc each and four make a total of 143cc. Clearly, by choosing the smaller potatoes, one gets 17.5 percent more food. They are the better deal.

Now, if the potatoes were sold by the kilo, their size would not matter. The buyer may choose many small ones or few large ones to get the same mass and pay the same price. For this reason, selling by mass is a fairer trading practice than using volume – even though it might not always be cheaper.

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A reader who prefers anonymity insists that I was mistaken to say that it is impossible to design a perpetual motion machine (i.e., one that runs without addition of energy from “outside”). He even sent me drawings of his intended contraption and after analysing them, I concluded that it CANNOT work.

This is my advice to anyone with such an idea. It is easy to design anything but there is only one way to prove that it can work: build it and demonstrate it to the world!