Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by forces impressed.
Did you ever know that after this statement, reported with minor differences by all Mechanics textbooks as Principle of Inertia, Newton added the following three examples to better explain his concept of inertial motion? In his words:
Projectiles persevere in their motions, except insofar as they are retarded by the resistance of the air and are impelled downward by the force of gravity. A spinning hoop, which has parts that by their cohesion continually draw one another back from rectilinear motion, does not cease to rotate, except insofar as it is retarded by the air. And larger bodies – planets and comets – preserve for a longer time their progressive and their circular motions, which take place in spaces having less resistance.
Apparently these examples, interpreted as casting doubts rather than light on the initial statement, were dropped by commentators and authors, including those engaged in a chauvinistic campaign against Galileo’s “circular inertia”. I.B. Cohen in the Guide to Newton’s Principia which precedes his new translation (above quote is taken from it) could not avoid the matter. He wrote :
On first encounter, these examples may seem confusing since each involves curved paths and yet the subject of the first law is uniform linear (or rectilinear) motion.
and, by mentioning the first two examples only, vaguely concludes that
… it is only the tangential or linear component that is inertial, not the curved motion.
Also E. Mach deemed convenient to skip the matter. Under this condition, did we really understand Newton’s First Law?
