# Edison Rock

Shortly before his death, the famous American inventor Edison wished to distinguish the most sensible young man in his country, appointing him generous financial support for further education. From all parts of the republic young people were sent to him, one from each state, selected by the school authorities. These fifty young men were subjected to a written examination in Edison's house: they had to answer 60 questions of a special quiz, co-invented by the inventor and his co-workers. The judges were Edison himself, the "automobile king" Ford, the celebrated pilot Lindbergh and several prominent American teachers. One of the questions of the Edison quiz, which I want to offer to you, was the following:

Imagine that you found yourself on a tropical island of the Pacific without any guns. How would you move a 3 ton load there, for example a granite block of 100 feet long and 15 feet high?

The problem seems insoluble. What can you do with your bare hands with Edison's three-ton rock stone - a block of such impressive sizes?

Let us, however, delve deeper into the task and try to imagine this Edison rock clearly. We know its weight, its length, its width, but there is not a word about its thickness in the task. Why did Edison not talk about her? Is not there a clue?

We shall find out for ourselves what the thickness of this Edison rock must be. First of all, we determine its weight by weight. The rock is granite, and how much a cubic meter of granite weighs, we can learn from the directory. In the "weights table" of different materials, we find that the specific gravity of the granite is a round number of 3. This means that the cubic centimeter of the granite weighs 3 g or the cubic meter of granite weighs 3 tons. One follows from the other, because in a cubic meter a million cubic centimeters, and in one ton - one million grams. But if each cubic meter of Edison's rock weighs 3 tons, and the weight in a block is just 3 tons, then it's clear that its volume is only one cubic meter. With such a small volume, Edison's rock nevertheless stretched 100 feet in length and 15 feet in height. Obviously, it is very thin. Let us estimate the thickness of Edison's rock. The volume, as is known, is obtained by multiplying the length by the width and by the thickness. Therefore, by dividing the volume by the length and by the width, we will know the thickness. So let's proceed with the volume of our rock: divide 1 cubic meter first by 100 feet (ie at 30 m) then by 15 feet (ie, approximately 5 m), or even better - immediately at 30x5, t .e. on 150. What happens? Only 150 m, or about 7 millimeters.

This is the thickness of Edison's rock: only 7 mm! On the island rises, we see a thin granite wall, a kind of wonder of nature. To overturn such a wall is not at all difficult even with bare hands: to press against it more tightly or to tilt on it with a running start - and it will not stand.