If you make calculations that use something like the mass of an object and that mass is constant like a projectile, it greatly reduces the complexity.
If you make calculations that use something like the mass of an object but that mass changes over time like a rocket burning fuel youll need to adjust your calculations accordingly,
to be accurate.
If you go to Amazon.com or some other retail dealer youll likely find a current price for Windows 7 Home premium well below that of the cited $120. Also note that the Windows 7 family 3 pack was sporadically available for ~$150. Therefore the price wasn't exactly constant,...
Recall that I referred to being accurate not being outright wrong. They are very different things.