Note 4 — INTRODUCTION.
Reading edition
Vis inertiæ, or force of inertness, is the force by which a body, when at rest, resists being put into motion, or, when in motion, resists arrestation. The force, in this latter case, is called momentum, being directly as the weight multiplied by the velocity. Thus, two pounds, moving at the rate of one foot per second, exercise exactly the same momentum as one pound moving at the rate of two feet per second.
The force of a spring, or of explosive compounds, cannot be called momentum; neither velocity nor weight enter into its constitution; though, when transferred to a projectile, it produces momentum proportional to the force with which it acts, the weight moved, and velocity imparted.
Muscular force does not come within the definition of momentum, although it produces this property in a hammer, proportionably to its weight and the resulting velocity. Nor is the force of gravity momentum, though momentum be generated by it in falling bodies.