Energy, so what is it? -- Really!
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Aristotle coined the word, from the Greek, meaning "at work"

Two millennia  later, along came steam engines.

Physics education research suggests that puzzle solving - active brainwork - is the best route to understanding abstract principles.  Links here lead to many puzzling points.
n the 19th century, science went to work on the problem of finding how to get the most work out of steam engines.

A new concept of "energy" evolved which was very different from Aristotle's idea. 

The new "energy" is more abstract, and the concept is more powerful. It's also more subtle. 

The words "work," "heat," and "temeperature" also acquired new meanings, concepts more subtle and more powerful than the older meanings

Today the words "energy," "work," "heat" and "temperature" are amost always used exclusively in their prescientific senses.  The more abstract, scientific meanings are rarely recognized.  
the broadly useful power concept
 Energy:   Something in food and fuel that keeps us and our engines going. Energy:  A very abstract something about which we know only how to calculate its value and that it's conserved.
 Free Energy:  Energy we get at no cost. Free Energy:  Energy that, statistically, can be transferred as work.  (Free from incoherence.)
Work: Physical or mental effort or activity directed toward the accomplishment of something, (American Heritage Dict.) Work: Energy transferred by mechanisms other than temperature difference.
Heat: A fluid-like entity that resides within matter and which raises the temperature of the matter;  A  feelling from some sensors in our skin; elevated temperature. Heat:  Energy transferred solely because of temperature difference. 
Temperature: A  feelling from some sensors in our skin; elevated temperature. Temperature: A parameter in a statistical distribution which determines the rate at which heat flows between regions which are at different temperatures.

Increasing abstractedness from 1685 to 1925:

One definition of energy is in terms of work being done on an object.  Work energy is the product of the force exerted on the object times the distance over which the force acts.  However, only the component of the force in the direction of the motion is used in the calculation, and if the force varies, the integral must be used:

the line integral of the dot product of two vectors, Force and incremental distance

In general, science concepts become more and more abstract as science develops.  Nevertheless, this definition is centuries old.  Since then, energy has undergone several refinements.

E = mc2

is Einstein's recognition that, in Einstein's words, "Energy and mass are merely different expressions of the same thing.  This does not say that mass can be converted to energy.  This is a logical equivalence and not a logical mutual exclusion, although mutual exclusion is the more common (mis)interpretation..  The equivalence states that if the value of one increases the value of the other increases; were it a mutual exclusion, one would indeed be "converted to" the other, that is, when one increases the other would decrease.  Einstein's  energy-mass equivalence is a significant increase in abstractness of the energy concept.

Quantum mechanics takes energy into further depths of abstraction.  Energy and time, taken together, form a unified-whole "conjugate pair" of variables.  The pair has additional meaning which takes the concept further from arbitrary human influence:  This is the Uncertainty Principle at work.  The Uncertainty Principle is often thought to suggest the opposite, that it bestows some control on the real world merely by arbitrary choices we make when we measure things. It does not.

As science has moved into the 21st century, the energy concept has become less and less important, being replaced by ever deeper abstractions.

Drop a line over the edge of the top of a high building and hook the handle of the briefcase sitting on the sidewalk below so you can pull the briefcase to the top of the building.  While you were raising the briefcase the force you were putting on it was in the same direction as the motion.  You put energy into the briefcase because you were working against gravity.  You get it back as kinetic (motion) energy if you drop it to the sidewalk.  Splat!

However, carry the brief case along the horizontal sidewalk and the force you are putting on the briefcase is perpendicular to the motion.  No work done on the briefcase!  You added no energy that you can get back by dropping it to the sidewalk.

Only the component of force in the direction of motion adds energy to, "does work on," the briefcase


Nevertheless, the older concepts are powerful tools
and are very useful to those who master them.
Each one represents another little piece of the world, and using it enables the user to improve his or her interactions with that world. The deeper abstractions that science keeps discovering are the discoveries we make by ever extending our abilities to recognize those patterns of patterns of patterns of...

As we thus continually refine our knowledge we are following Estling's prescription "to find out as much about reality as we possibly can in what time we have and not waste our precious diminishing time..."