$($d$)$ The Weber number, We$,$ is a non$-$dimensional quantity which can be defined

mathematically as:

$We=\frac{U^{2}D}{}$

where $$ is the fluid density, $$ is the fluid surface tension, $D$ is the length scale of

the body and $U$ is the flow velocity. The Weber number can be described as the

ratio of forces as follows:

$We=\frac{\text{I}\text{n}\text{e}\text{r}\text{t}\text{i}\text{a}\text{f}\text{o}\text{r}\text{c}\text{e}}{\text{S}\text{u}\text{r}\text{f}\text{a}\text{c}\text{e}\text{t}\text{e}\text{n}\text{s}\text{i}\text{o}\text{n}\text{f}\text{o}\text{r}\text{c}\text{e}}$

Define the Mach number, $Ma,$ both mathematically $($specifying what all symbols

mean$)$ and as a ratio of forces.

$[5$ marks$]$