Bidang, Iban people. Sarawak 20th century, 50 x 101 cm. Lintah motif. From the Teo Family collection, Kuching. Photograph by D Dunlop. |

Consider a composite atom P described by a repetitive chain of historically ordered events

$\Psi \left( \bar{r}, t \right) ^{\sf{P}} = \left( \sf{\Omega}_{1}, \sf{\Omega}_{2} \ldots \, \sf{\Omega}_{\it{i}} \, \ldots \, \sf{\Omega}_{\it{f}} \, \ldots \right)$

where each event $\sf{\Omega}$ is characterized by its momentum $\bar{p}$ and time of occurrence $t$. Let P interact with a particle called $\sf{X}$ between initial and final events so that there is some variation in P's motion described by

$\Delta \bar{p} = \bar{p}_{\it{f}} - \bar{p}_{\it{i}}$

$\Delta t = t_{\it{f}} - t_{\it{i}}$

According to the usual narrative of Newtonian mechanics, the particle $\sf{X}$ impresses a*force*like a push or a pull that causes the change in P's motion. Isaac Newton says that

"A change in motion is proportional to the motive force impressed and takes place along the straight line in which that force is impressed."^{1}

This relationship is called Newton's second law of motion. It can be mathematically expressed by defining an algebraic vector called the **force** as

$\begin{align} \overline{F} \equiv \frac{ \Delta \overline{p} }{ \Delta t } \end{align}$

If P is a Newtonian particle, then its momentum is related to its velocity by $\bar{p} = m \overline{\sf{v}}$ where $m$ is the mass. Also for many sorts of interactions the mass of a Newtonian particle can be considered constant, then $\Delta \bar{p} = m \, \Delta\overline{\sf{v}}$ and the force can be written in terms of the acceleration as

$\overline{F} = m \overline{a}$

Momentum is conserved. So all particles that have momentum can cause changes in the momentum of another particle if they are absorbed or emitted. Then X, the particle that is absorbed or emitted, is often called a force-carrying particle. Force-carrying particles that are material or charged are usually easy to detect in the immediate vicinity of an interaction. So the forces they impart are called **contact** forces. Phenomena like automobile collisions and gunshot wounds can be analysed using contact forces. X is much less conspicuous if it is ethereal and neutral. Such particles can be difficult to detect, and the forces that they carry may seem to come from far away. So they are often referred to as **exchange** particles to suggest a remote origin, and their effects are called **action-at-a-distance** forces. If X is imaginary then its force may seem to come from nowhere. Some of these force-carrying particles are difficult to characterize and distinguish as individuals, so it is often more convenient to group lots of them together and refer to them collectively as force fields.