Historical development of Newton’s laws of motion and suggestions for teaching content

Based on the above discussion, it is suggested that the teaching content of Newton’s Laws of Motion should provide modernized definitions while also abandoning the ancient ideas which appeared in Newton’s publications of the key tools/terms, such as force, inertia, and inertial frames of reference. For example, most popular textbooks ignore the operational definition of inertial frame via Newton’s 1st Law (Sawicki, 1996; Chang, 2006); i.e., by means of the criteria of when F=0, a=0 to determine the existence of inertial frame. Concurring with Larkin’s (1983) notion, Arons (1990) critiqued ‘most of these (popular textbooks) start with “force” as though it were a primitive, already fully understood… not requiring explicit operational definition’ (p.57). Based on the modern view, “force” should be defined by its cause (external agents only), ontological assumption (interaction), and limitations (inertial frames of reference).

In addition, statements similar to Newton’s original publication may become meaningless, or even misleading, such as the causal link between inertia and NFL. For example, ‘Newton’s first law is also known as the law of inertia. The word inertia is from the Latin word for sluggish or inactive. In modern terms, inertia is the property of matter that causes objects to resist changes in motion’ (Jones & Childers, 2001, p. 103). ‘The inertia of a body is its tendency to resist any change in its state of motion’ (Benson, 1996, p. 56). The statements provided by the two textbooks imply the ancient notion of innate resistive force regarding inertia, as well as attributing inertia as the cause of maintaining constant motion, which contradicts the modern view of Newton’s Laws (stated in M1–M2). Since NFL is valid only when external force is absent, Galili and Tseitlin (2003, p. 52) argued the logic flaw that ‘if there are no forces applied, why should the body persist? It would simply continue (originally emphasized) to move.’

The confusion regarding the meanings of “inertia” still exists, and has been addressed by several physicists (e.g., Niles, 1985). Many teaching resources still categorize demonstration examples of inertia as either “inertia of motion” or “inertia of rest” (e.g., Freier & Anderson, 1981), implying the two ancient notions, innate motive/resistive forces, embraced by Newton. Hudson (1988, p. 34) contended the inappropriate examples of demonstrating inertia in that ‘the result of any demonstration of inertia should be dependent on mass. Many of the demonstrations ‘explain’ as examples of inertia are in reality more related to some other effect, and mass plays a very small role in the results.’

For teaching content design, it is suggested that teachers adopt Halliday, Resnick and Walker’s (2005) decision to discard the terms “inertia” and “Law of inertia” in their popular textbook (the 7th edition). Coherently, Knight (2004, p.117) summarized in his textbook that ‘(t)he first law tells us that no ‘cause’ is needed for motion. Uniform motion is the natural state of an object (M4)’. Moreover, explicitly stating that ‘inertia is not the cause of moving objects continuing in motion’ (Hewitt, 1998, p.22) is favorable.

In sum, the teaching content of Newton’s Laws of Motion should 1) comprise comprehensive descriptions regarding the new tool of inertial frame of reference, 2) modernize the meanings of force and inertia, and 3) discard the causality between inertia and motion. In order to comprehend the knowledge of Newton’s Laws of Motion, discarding ancient ideas may be as challenging as becoming acquainted with new tools for many students.

Acknowledgements

The paper was supported by the National Science Council in Taiwan (NSC-100-2511-S-018 -033 -MY3).