Recent research has shown that serious misconceptions1frequently survive high school and university instruction in mechanics. It is interesting to inquire whether Newton himself encountered conceptual difficulties before he wrote the Principia: (a) Did he have serious difficulties? (b) If so, were they difficult to overcome? We shall present evidence from Newton's writings of affirmative answers to both questions. Newton's development of his system of mechanics was hampered by a persistent belief in ‘the force of a [moving] body’ from 1664 to 1685. His belief in centrifugal force was an additional restraining factor that remained intact until Hooke's intervention in 1679 and weakened only gradually over the next two years. Three additional years passed before the resulting successes weakened his commitment to impetus sufficiently to permit conceptualization of mass as an inert surrogate. This paper will compare Newton's pre‐Principia beliefs with those of contemporary students in the areas of impetus force and centrifugal force. We shall emphasize the retarding effect on Newton's development of inappropriate but deep‐rooted models at a qualitative level, and the extent to which his experience suggests the necessity for students to struggle conceptually in order to construct the models employed by physicists.
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