Work has nothing to do with energy expenditure

G De Berredo-Peixoto

doi:10.1590/1806-9126-rbef-2020-0054

G De Berredo-Peixoto

Open Access

https://doi.org/10.1590/1806-9126-rbef-2020-0054

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Abstract

Abstract We show that, contrary to common sense, the author of mechanical work does not necessarily provide the required energy. By the most simple example one can ever conceive – a standing person starting to move, this point is made clear: the floor does not spend energy and it does work on the person (in relation to its own reference frame). We analyse two additional examples in order to ilustrate the subtle aspect of Newtonian theory: despite something is said about where the work is going to, namely the Work – Kinetic Energy Theorem, absolutely nothing is said about where the work comes from (we explain in our conclusions, making reference to Aristotelian philosophy, why it happens in that way). In all these examples, explicit calculations of work are done in both inertial and non-inertial frames.

Highlights

If you work hard all day long, you certainly will be very tired by the end of the day
This is so because you spent a great amount of energy, and work is commonly associated to energy
It is very usual to think in that way: since energy is necessary to do some work, the required energy for some work must be provided by the author of that work

Summary

Introduction

If you work hard all day long, you certainly will be very tired by the end of the day. The first of them (Section 2) is remarkable simple: a person just begins to walk Perhaps it is the simplest example one could ever imagine, and still it already illustrates that the entity that does the work is not necessarily the source of energy. In this case, we argue in a similar way, but, in addition, a non-trivial calculation of work in an accelerated frame is promptly done. We argue in a similar way, but, in addition, a non-trivial calculation of work in an accelerated frame is promptly done In this case, the Work - Kinetic Energy Theorem has to be modified

The simplest example: a person begins to walk

Second example: two people move away from each other without friction

Third example: a child begins to walk in a treadmill

Concluding remarks: the underlying philosophy