Energy Theater Force Diagrams and the Five Laws (from my EPSRI Congress Presentation)

The recent posts about energy theater diagrams have inspired me to finally post about my EPSRI congress presentation.

In E2 this year, they tried to answer the following challenge:

Give a causal or mechanistic description of energy transfers and transformations.
Why does energy transfer or transform? (Wealth analogy)
Must have something to do with forces…
What is relationship between energy & force?
(Physicist answer: W=F.d)

As an answer, they came up with the following five laws of forces and energy:

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).
   
3. (a) A force on an object in the direction of motion increases kinetic energy. (b) A force on an object opposite the direction of motion decreases kinetic energy. (c) A force on an object that is not in motion neither increases nor decreases kinetic energy. (d) Forces within objects transmit energy.
   
4. Transfers of energy are due to contact forces. Transformations of energy are due to non-contact forces.
5. Forces transfer or transform energy proportional to their magnitude.

To check these laws, they invented another kind of "Energy Theater Diagram" which they used to determine the relationship between forces and energy transfers and transformations in a particular scenario. Below are a couple frames from a movie of a hand pushing a ball under water, and a picture of a diagram on the front board picture representing the analysis of a single time step in this movie:




The idea is that you draw all the forces in the scenario, and each force must correspond to a transfer or transformation of energy, or you have to use the laws to explain why it doesn't. For example, Fbh, the force of the ball on the hand, corresponds to 2KEh -> 2KEb, or the transfer of 2 units of kinetic energy from the hand to the ball. This is a transfer of kinetic energy, consistent with laws 1 and 2, it is in the direction of motion and giving kinetic energy to the ball, consistent with law 3, it is a contact force transferring energy, consistent with law 4, and the number of energy units, 2, is determined in relationship to the number of energy units involved in other transfers and transformations by the relative magnitudes of the forces, consistent with law 5.

This was a very disciplined use of diagrams, and led to lots of questions (and answers) about the scenario that would not have come up otherwise. Leslie and I are still talking about exactly how to analyze this, but one idea would be to look at the questions it produces and how commitment to the laws and the representation leads to these questions.