Before proceeding, we discuss what we mean by “work” and how this is different from many standard introductions to the topic. The easiest way to describe what is “work” is via an analogy to force:
Law I: Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.In this first law, Newton defines a particular kind of “event” for which there is a cause: an object moving at a constant speed, or an object at rest, is not an “event” - there is no happening to explain. (In this way, he departs from the idea that all objects tend to a state of rest.) A change in motion is an event, and force is the entity responsible for that event. Force, that is, is not the change in velocity — it is the entity responsible for that change. Force is a causal entity; a change in motion is the result. Calling a force a “push or a pull” is empirically true given the definition of force (and is even implied by calling force a thing that can be “impressed” on an object), but it is not itself the definition. And calling a force “an acceleration” mixes up the cause (F) with the effect (ma).
Similarly, work is a change in energy (a transfer or transformation of energy) caused by a force. (By way of comparison, heat is a change in energy caused by a temperature difference.) Work is the “event” and the cause for the event is a force applied over a distance. Like the student who mistakes ma for the force, rather than the effect of the force (and mathematically equivalent to the force), many texts mistake
for the work, rather than the cause of the work.Why does this matter?
If we the goal of physics to provide us with coherent, mechanistic explanations for events in the world (hammer and van zee), it is best that we know what kind of ‘events’ merit explanation and what kinds of entities are causal. Work is an event - not a cause. And in seeking the cause, we are—as with Newton’s First Law—led to forces as a causal entity.
... Herein lies the total genius of Energy Theater — while it took Newton some serious thinking to decide that an object in motion will stay in motion (and so was not “an event” worth explaining), Energy Theater makes the important 'events' visible - every transfer or transformation event is either an instance of work or heat. What the teachers have done is begin to parse work from heat, and relate work to F.d.
Are you troubled?
What might give you pause is to say: wait! - I thought work was a change in KE, not all transfers and transformations in energy. It just so happens that every energy transfer or transformation is “laundered” through the kinetic energy bank - so it just so happens that to keep track of enery transfers and transformations, you need only account for changes in KE. So a transfer or transformation of energy (caused by a force) is equivalent to accounting for changes in kinetic energy KE. This is the genius of the teachers’ 1st and 2nd laws (1. When forces transfer energy, they transfer kinetic energy. 2. Kinetic energy is present in all transfers and transformations (potential energy always transforms into or from kinetic energy). )
What puzzles me
I didn't know this prior to teaching it. Maybe this is the beauty of responsive teaching - you do more than you knew you could-- it is generative teaching.
