Definition of mass in engineering mechanics?

Answer 1

Mass is the measurement of inertia, or resistance to change in motion. It is not to be confused with weight, a measurement of the force due to gravity exerted on an object.

For many practical purposes, mass and weight vary numerically by a constant, namely gravitational acceleration. Due to the common misconception of mass and weight being the same thing, units can often get confusing. For example, the English unit for mass is the slug, rather than the pound (a unit of weight). Some unit systems, such as the American Engineering System, take up the convention of making mass and weight numerically equal. In such a case, the units specify whether mass or weight is intended, so you would have pound mass (lbm) and pound weight (lbw). The SI unit for mass is the kilogram (kg), which, like the pound, is commonly and erroneously used as a unit of both mass and weight.