Universal reference | The game of science
‘La gran ola de Kanagawa’ (1830-31) de Hokusai.Metropolitan Museum of Art
Our castaway in distress from last week, to get rid of the fire, has to resort, paradoxically, to more fire: if he starts a fire towards the center of the island, the wind will move the two fronts of flames in the same direction, so that between the two there will be a safe mobile strip, and when the first fire reaches the area devastated by the second, it will be extinguished due to lack of fuel.
It is one of those “Columbus egg” riddles that seem obvious when the answer is known, but are only solved by those who are capable of counterintuitive thinking, since, at first glance, it seems to add more fire to the Fire is aggravating the situation by causing an increase in the suffocating temperature that the hapless castaway has to endure.
And speaking of temperature, let’s go back to degrees Celsius. The well-known joke “Zero degrees, neither cold nor hot” can help us to realize (jokes are often revealing) that, contrary to what happens with other scales, the temperature scale does not refer to the body human.
The famous Protagoras judgment, “Man is the measure of all things”, finds its most literal expression in the historical measurements of length: span, foot, inch, cubit, breaststroke … And time is also measured “protagonist”, as the second approaches much in the span of a beat of our hearts. Following the same criteria, the positive degrees should correspond to what it is for us to feel hot and the negative degrees to the cold, as suggested by the aforementioned joke, and the 0, therefore, should be around 20 ºC. And, in fact, some early thermometers did not indicate degrees, but only “cold” or “heat”, and heat was determined by putting them in contact with human skin.
But thermometric scales (Fahrenheit, Celsius, Réamur) were established in the 18th century, at a time when scientists were looking for more stable and precise referents than human body magnitudes, and the freezing and boiling points of water were the most reliable. and accessible.
As is well known, water also served as a reference to define the kilogram as a unit of mass based on the meter: a kilogram is the mass of a cubic decimeter of water at 4 ºC and at sea level (actually the first gram, in 1795, as the mass of a cubic centimeter of water). Currently the much more precise definition of kilogram is based on Planck’s constant; but the classic “aquatic” definition is still an acceptable approximation.
Note that, thanks to water, a universal reference -as well as a solvent-, it is enough to define a unit of length to deduce almost all the others (with which, in a certain way, we return to Protagoras, since the human body contains more than one 60% water).
Even the unit of time, the second, can be deduced from the meter – and vice versa – in a simple way. In fact, according to the current definition (behind that of “the ten millionth part of the quadrant of the terrestrial meridian”), the meter is the distance traveled by light in 1/299792458 of a second. But there is a much more accessible way to deduce the second from the meter (or vice versa). Which?
And finally, two aquatic classics:
In a glass of water filled to the brim, an ice cube floats. What happens when the ice melts?
We have two glasses, one with water and one with wine. Take a tablespoon of water from the first glass, add it to the second and stir well. Then we take a tablespoon of wine (slightly watered down) from the second glass and pour it into the first. After the operation, will there be more water in the wine than wine in the water or vice versa?
Carlo Frabetti is a writer and mathematician, member of the New York Academy of Sciences. He has published more than 50 popular science works for adults, children and young people, including ‘Damn physics’, ‘Damn maths’ or ‘The great game’. He was a screenwriter for ‘La bola de cristal’.
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