牛頓當時敘述是：“運動的變化成正比於施加的動力，並且（方向）依照力施加的直線方向”。（原文見內）

原文：”Mutationem motus proportionalem esse vi motrici impressae, et fieri secundum lineam rectam qua vis illa imprimitur.

英文翻譯：The mutation of motion is proportional to the motive force impressed; and to be done according to the straight line in which the force is impressed.