Newton and Newton's 3rd Law

Here's a fascinating letter from The Physics Teacher (A very cool magazine, if I do say so myself), about Newton's own discovery of the idea of forces coming in equal and opposite pairs. I've bolded the most important part

NEWTON AND NEWTON’S THIRD LAW
Just as students have difficulty with Newton’s third law as discussed in the article by Lehavi and Galili, it is of interest to realize that Newton’s under- standing of the significance of the mutual interaction of bodies in their motion did not occur until very late leading up to his writing of the Principia. Cohen published an article on this, which also appeared as an appendix in a revised and updated version of his book.

Newton wrote an essay entitled “De motu” in which he described a calculation of the orbit of a planet in our solar system. He ran into the problem that his calculation did not fit the actual data sufficiently. He realized that assuming a stationary Sun was a potential source of error. He recalculated, but this time he allowed the Sun to move in response to the pull of the planet. The revised calculation yielded numbers within the error of measurements of the orbits of the planet available at the time. Apparently, the notion of the third law of motion was not an explicit and automatic part of Newton’s thinking until this point.

According to Cohen this work was completed shortly before Halley’s visit with Newton, which led to the writing and publication of the Principia. This is in distinct contrast to the impression one often gets from descriptions of Newton’s life in physics textbooks that he went home to avoid the plague dur- ing college and in two years worked out all of his contributions to physics. We should know better and do better in our textbooks because serious historical scholarship on Newton is now eas- ily accessible.

The final paragraph in the article by Lehavi and Galili illustrates the magnitude of the problem caused in physics education by the emphasis in texts on presenting as much of the canon of physics as possible. At best students develop a kind of technician’s view of physics, which is that the goal of learn- ing physics is to remember a list of calculations and facts. In doing so students miss the essence of physics, which is the construction of4 explanations of physical phenomena. Is it any wonder then that students yearning for such intellectual challenge choose other fields to go into?

Dewey Dykstra
Boise State University