# Mass brick¶

This brick represents a weak term of the form

\[\int_{\Omega} \rho u\cdot v\ dx + \ldots\]

It mainly represents a mass term for transient problems but can also be used for other applications (it can be used on a boundary). Basically, this brick adds a mass matrix on the tangent linear system with respect to a certain variable.

The function which adds this brick to a model is:

```
ind_brick = getfem::add_mass_brick
(md, mim, varname, dataexpr_rho="", region = size_type(-1));
```

where `dataexpr_rho`

is an optional expression representing the density
\(\rho\). If it is omitted, the density is assumed to be equal to one.