the body force per unit mass, and $\bq^\iota$ is an interaction term denoting the force per unit mass exerted upon $\iota$ by all other species present. The final term with the (reference) gradient denotes the contribution of the flux to the balance of momentum. In practise, the relative magnitude of the fluid mobility (and hence flux) is small, so the final term on the right hand side of Equation (\[linearmombalance\]) is negligible, resulting in a more classical form of the balance of momentum. Furthermore, in the absence of significant acceleration of the tissue during growth, the left hand-side can also be neglected, reducing (\[linearmombalance\]) to the quasi-static balance of linear momentum.

The balance of momentum of the entire tissue is obtained by summing Equation (\[linearmombalance\]) over $\iota = \mathrm{c,f}$. Additionally, recognising that the rate of change of momentum of the entire tissue is affected only by external agents and is independent of internal interactions, the following relation arises.

$$\sum\limits_{\iota =
  \mathrm{c}}^\mathrm{f}\left(\rho^\iota_0\bq^\iota+\Pi^\iota
\bV^\iota 
\right)= 0. \label{qrelation}$$

This is also consistent with Classical Mixture Theory [@TruesdellNoll:65]. See @growthpaper for further details on balance of linear momentum, and the formulation of balance of angular momentum. We only note here that the latter principle 