The LaTeX package SIunits (which seems to be a descendant from the *units* package that I once used) addresses one of the most nagging issues in physics and engineering: getting the units well typeset. While the source is a bit cumbersome, it is quite readable:

$R=8.314$ \joule\per(\mole\usk\kelvin)

There are prefixes, so that “\mega\ohm” is ok. Many ready-to-use units are predefined, such as \kilogrammetrepersecond for your momentum, or\kilogrammetrepersecondnp if you prefer negative powers instead of division.

As a bonus, \celsius adds the little “degree” symbol that’s so easy to get wrong, and \kelvin does not, as it should be:

$0$\celsius=$273.15$ \kelvin.

Also, units maybe inside formulae or outside:

$9.81 \newton$ or$9.81$\newton

The space in the first equation is ignored, btw, so both produce the same output. This space, is moreover, one of the few things left to the author:

$9.81$ \newton or$9.81$\newton ???

It seems the first one is correct, but one may use this to get things always right:

\unit{120}{\kilo\meter\per\hour}

This later choice is a bit more cumbersome, but convenient: math mode is invoked, no need to use $$; also, no need to remember the extra \ at the end of the command: “a force of $1$ \newton\ is required \ldots”.