What is your application? If you are using Nmms, which means you are living in a Force world. So if you have a cantilever beam with a load on the free end, the load would be in Newtons = kg x m/sec^2. You use mass when you define the density of the model, however, you can choose any units you like including Kg/m^3.
Tobalcane
"If you avoid failure, you also avoid success."
“Luck is where preparation meets opportunity”