Mass is not weight! Mass is an invariant property of a rigid body. The weight of the same body varies depending on the gravitational system in which it sits. See Section 1.10 (p. 16) for a discussion of the use of proper mass units in various measuring systems. We will assume the mass of our parts to be constant in our calculations. For most earthbound machinery, this is a reasonable assumption. The rate at which an automobile or bulldozer loses mass due to fuel consumption, for example, is slow enough to be ignored when calculating dynamic forces over short time spans. However, this would not be a safe assumption for a vehicle such as the space shuttle, whose mass changes rapidly and drastically during liftoff.