The change of motion is proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.

This sentence is Newton’s formulation of what later became known as the Second Law of Motion. It appears in the opening definitions and axioms (“Axiomata sive Leges Motus”) of his Latin treatise *Philosophiæ Naturalis Principia Mathematica* (1687), where he set out three laws to ground a new mathematical physics of terrestrial and celestial motion. Newton’s “motive force impressed” refers to an external force acting on a body, and “change of motion” uses “motion” in the Principia’s technical sense of quantity of motion (momentum). The law provided the conceptual and mathematical basis for analyzing impacts, projectiles, and planetary orbits within a single framework.

Newton is stating that an applied force produces a change in a body’s momentum proportional to the strength of that force, and that the change occurs along the line of action of the force. In modern notation this is closely aligned with the impulse–momentum relation (force producing a rate of change of momentum), and in the common special case of constant mass it underwrites the familiar form *F = ma*. The emphasis on direction (“right line”) is crucial: forces are vectors, and they determine not only how much a motion changes but also the direction in which it changes. This principle is foundational for classical mechanics because it links causes (forces) to measurable dynamical effects.

1) “The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.”
2) “Change in motion is proportional to the force impressed and takes place in the direction of the straight line in which the force is impressed.”