Changing the Order of Newton's Laws--Why & How the Third Law Should be First

Newton's laws are difficult both for teachers and students at all levels.^1-3 This is still the case despite a long history of critique of the laws as presented in the classroom. For example, more than 50 years ago Eisenbud⁴ and Weinstock⁵ proposed reformulations of the laws that put them on a sounder, more logically consistent base than is presented in many textbooks without resorting to "intuitional or anthropomorphic contrivances."⁵ In 1990, Arnold Arons⁶ wrote that "the Law of Inertia and the concept of force have, historically, been two of the most formidable stumbling blocks for students." One might imagine, therefore, that by 2012 remedial strategies would have resolved these difficulties, but there is little evidence that the problem has been satisfactorily addressed. Diagnostic tools such as the Force Concept Inventory^7,8 have cast light on areas of difficulty; remedial strategies have included historical approaches, computer simulations, analogical approaches, and many more.^9-12 Nevertheless, papers on the subject are still being published.^8,9,11-13 Textbook descriptions of the laws have also contributed to the problem: in general the topic is presented with little discussion and the third law in particular is often given cursory treatment. This paper addresses the introduction of the laws in the classroom, especially the order in which they are customarily presented, and discusses particular issues attached to the third law that constitute major impediments to understanding.^14-16 We have devised a six-hour workshop, for middle-school teachers and senior students, that has achieved some success in enhancing understanding of Newton's laws by adopting a different order of presentation of the laws from that traditionally given in physics texts. Our approach is deliberately intuitional and experiential, as we believe the ideas need to be felt or owned by students before introducing them to a more rigorous formalism.