Tech UPTechnologyAre you sure you know what energy is?

Are you sure you know what energy is?


The star concept of science developed by Newton was that of force. For a very simple reason: force represented the action of moving things, the cause of movement. It was his intellectual adversary, the German Leibniz, who drew attention to a certain quantity associated with movement and that, in a world without friction, should always remain unchanged. He called it the living force -vis viva- and we know it today under the name of kinetic energy .

On the other hand, since the time of the builders of water mills it has been known that, to make them work, the water has to fall from a certain height . Leibniz reasoned that it must also have an associated magnitude and called it vis mortuum or dead force, since an object at rest did not possess vis viva , but could potentially produce motion. Today we call it potential energy . If the kinetic energy is calculated as half the product of the mass times the square of the speed, the potential energy is also defined as a product, but between the weight of the body and the height above the surface of the Earth. (Are we intuiting that energy is just a number?)

The sum of these two types of energy defines the mechanical energy, and in the absence of friction this quantity must remain unchanged. Thus, the water that reaches a mill, thanks to the irregularities of the terrain, will have a certain potential energy. As it falls towards the wheel, it will lose it -every time it is at a lower height- but it will not disappear, rather it will be converted into kinetic energy -because it is acquiring greater speed- which in turn will be converted into work done by the wheel. Obviously, any body can have kinetic energy and potential energy – for example, an airplane – and both can change value, but what will always remain unchanged is their sum, the mechanical energy. This is the meaning of the famous phrase “energy is not even created…”.

This result, exclusive to mechanics -the discipline of physics that deals with movement and its causes-, was extended to all physical phenomena thanks to the work of Carnot, Mayer and, above all, Joule, which culminated many centuries of research on fire, heat and gases. And the change came. Until 1851 physics was the science of force; thereafter physics became the science of energy .

The problem is that it is such an abstract concept that even today, despite its widespread use, it continues to be misused and misunderstood . Even moral characteristics are attributed to it, or is it that bad energies do not exist? It is a process that comes from ancient times: take natural phenomena, deify the causes that generate them and give them human attributes. When some speak of energies (thus, in the plural) they seem to refer to a subtle and mysterious substance that fills the universe; others do not go as far and perceive it as something that swarms everywhere. They don’t really know what it is and they haven’t quite identified it with something material, but it’s there . Both thoughts show the existence of a curious mechanism of the human mind: objectification . Putting a name to something, be it energy or intelligence, implies that that something must have an independent objective existence; it cannot be just a concept.

The true meaning of energy

The one who has most clearly explained what energy is and its conservation principle is the admirable and peculiar physicist Richard Feynman : “Imagine a child who has blocks that are absolutely indestructible, that cannot be divided into parts. Each one is equal to the other. Suppose you have 28 blocks. His mother places him next to the 28 blocks at the beginning of the day. At the end of the day, out of curiosity, she carefully counts the blocks and discovers a phenomenal law – whatever her son does with the blocks, there are always 28 left!”

What is the analogy of this with the conservation of energy? The most notable thing to take away from this example is that there are no blocks .

The concept of energy only points out the existence of an intangible property of matter, something that allows us to predict the course of natural events. A stone hanging twenty meters high has ‘something’ that another placed on the ground does not, and that ‘something’ is revealed when it is released and falls. Saving the distances, it is similar to what feelings represent in the human being . They are not tangible like blood, spleen or neurotransmitters, but they are there. We cannot touch them or say “this substance is fear” or “love weighs ten kilos”. Obviously there is an underlying biochemical basis and very real processes, but sensations as such are not ‘things’.

It can be said louder but not clearer: energy as such does not exist , it is just a number.


Feynman, R. P. (1992) The character of physical law, Penguin

Silver, B. L. (1998) The ascent of science, Oxford University Press

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