Bead panel from a baby carrier, Bahau people. Borneo 20th century, 35 x 28 cm. From the Teo Family collection, Kuching. Photograph by D Dunlop. |

"To any action there is always an opposite and equal reaction; in other words, the actions of two bodies upon each other are always equal and always opposite in direction."^{1}

$\begin{align} \overline{F} ^{\mathbf{A}} \equiv \frac{ \Delta \bar{p} ^{\mathbf{A}} }{ \Delta t } = \frac{ - \bar{p}^{ \sf{X}} }{ \Delta t } \end{align}$and$\begin{align} \overline{F} ^{\mathbf{B}} \equiv \frac{ \Delta \bar{p} ^{\mathbf{B}} }{ \Delta t } = \frac{ \, \bar{p}^{ \sf{X}} }{ \Delta t } \end{align}$

so that

$\overline{F} ^{ \mathbf{A}} = - \, \overline{F} ^{ \mathbf{B}}$

The force of the cause is of equal size and in the opposite direction to the force of the effect. These quantities are measureable, they can be quantitatively evaluated in our laboratories. And for WikiMechanics, they are well defined in terms of sensation.