# What is pros of weight density?

I've been studying mechanical simulation with nastran/patran, nowadays.

I knew weight density is used in imperial unit system, instead of mass density.

I feel using weight density is tedious, because I have to put a conversion factor to change weight density into mass density, when I perform dynamic simulation, always.

I just had used SI unit in my whole life, so that I don't know which advantage there is in using weight density, compared to using mass density.

Could anyone explain why weight density is used in imperial unit, instead of mass density?

And then, what is the advantage of weight density, compared to mass density?

• I think the more common term for this quantity is specific weight. It is not an "imperial unit system" quantity, but an engineering quantity. It can be expressed as force per unit volume, which can be expressed in any unit system, including the metric system. You should read the wikipedia page on its uses: en.m.wikipedia.org/wiki/Specific_weight – Paul Aug 26 '16 at 4:28

Of course there is an analogous situation in SI units. If you want to include the weight of an object as a force in your model, the weight of a mass $m$ kilograms is $mg$ Newtons. But there are many engineering situations where the weight of an object is negligible compared with the other forces acting on it, and can be ignored.