We characterize Newtonian particles as being in some kind of steady balance with their environment. They are presumably interacting with countless photons, bouncing around a lot, and colliding with other particles. But despite much agitation, there is still a central tendency that might be called *realistic* motion, or perhaps *naturalistic* movement. Particles that depart too far from this balance may be called non-Newtonian, or even unphysical. To be more exact about this we define the kinetic and potential energy.

## Kinetic Energy

Consider a material particle P, described by its rest mass $m$ and momentum $p$. The **kinetic energy** of P is

$\begin{align} K \equiv \frac{\, p^{ 2}}{2m} \end{align}$

Since $m > 0$ for material particles, $K$ is never negative. And in an inertial frame, momentum is proportional to the wavenumber ${\kappa}$. So $K$ is proportional to $\kappa ^{2}$. Then recall that the wavenumber depends only on the coefficients of dynamic quarks. So the kinetic energy depends strongly on P's dynamic quark content. Sensory interpretation: Dynamic quarks and the wavenumber have already been interpreted as representations of somatic and visual sensations. So the kinetic energy depends mostly on these audio-visual sensations too.

## Potential Energy

Let us also include the mechanical energy $\, E$ in the description of P. The difference between $E$ and the kinetic energy defines another number $\mathcal{U}$ called the **potential energy**

$\mathcal{U} \equiv E - K$

And recall that if the motion of a material particle is not relativistic, then

$\begin{align} E \simeq m c^{2} \left( 1 + \frac{p^{2}}{2m^{2}c^{2}} \right) = m c^{2} \left( 1 + \frac{K}{mc^{2}} \right) = m c^{2} +K \end{align}$

So the potential energy may be approximated as $\mathcal{U} \simeq m c^{2}$. And for slowly moving Newtonian particles, the potential energy depends strongly on the mass. Then remember that for heavy particles, a sensory interpretation of the mass relates mainly to baryonic quarks and thermal sensations. And so for Newtonian particles, the potential energy is mostly associated with thermal sensations too.

## What is Dynamic Equilibrium?

We say that P is in **dynamic equilibrium** when $\mathcal{U} = K$ so that kinetic and potential energies are equal to each other. At equilibrium there is an equal sharing, or *equipartition*, of mechanical energy between kinetic and potential types. But the potential energy is defined above by $E$ less $K$. So for a particle in dynamic equilibrium

$E= \mathcal{U} + K =2K$

This account of dynamic equilibrium is succinct. And it theoretically links the equipartition of energy to our traditional understanding of momentum as the product of mass and velocity. But it is not very meaningful in practice. Difficulties arise from using a hypothetical condition of perfect isolation to set the zero-value for energy measurements. Recall that initially our discussion of energy adopted the reference sensation of *not* seeing the Sun to grasp the notion of having *no* internal energy. So in principle, we do not have a tangible laboratory reference standard for absolute-zero on the energy scale. And this is noticeable when energy measurements can be made to about a part in 10^{12}. Moreover, there are conflicts with theories of dispersion and gravitation which may deny the possibility of perfect isolation. Anyway, these difficulties are manageable because physical phenomena often occur within distinct energy regimes. So for example, nuclear energy can usually be ignored when doing benchtop chemistry. In the laboratory we measure energy changes, that are related to each other by $\Delta \mathcal{U} = \Delta E - \Delta K$. Then a null-value standard for calibrated energy-difference measurements can be selected for experimental convenience. Results are reported using $\mathcal{U}^{\prime}$, a slightly different version of the potential energy with a shifted-origin

$\mathcal{U}^{\prime} = \mathcal{U} + \sf{\text{an arbitrary constant}}$

Then $\Delta \mathcal{U}^{\prime} = \Delta \mathcal{U}$. These energy-differences are more susceptible of precise laboratory observation than absolute values. But $\mathcal{U}^{\prime} \! ≠ K$ and equipartition is inapt for shifted energies.

Sensory interpretation: As noted above, in an ideal reference frame the kinetic energy characterizes the sensory magnitude of visual stimuli, whereas the potential energy depends more on thermal perception. So there must be a balanced experience of both thermal and visual sensation for events to be objectified as particles in dynamic equilibrium. This requirement for eyes-open visual awareness means that, for example, a dream about flying while asleep cannot meet equilibrium conditions. And neither can watching cartoons on TV, because television only transmits audio-visual sensations, not thermal sensations. So dynamic equilibrium is more like experiencing ordinary circumstances in classrooms and laboratories on Earth. Unlike many movies, dreams and hallucinations.Related WikiMechanics articles.

Velocity | You can jump ahead to this article which shows how the equipartition of energy and dynamic equilibrium are related to a traditional understanding of momentum as the product of mass and velocity. |