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The Physics Teacher





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AIP Publishing


Newton’s laws1 have engendered much discussion over several centuries.2,3 Today, the internet is awash with a plethora of information on this topic. We find many references to Newton’s laws, often discussions of various types of misunderstandings and ways to explain them. Here we present an intriguing example that shows an assumption hidden in Newton’s third law that is often overlooked. As is well known, the first law defines an inertial frame of reference and the second law determines the acceleration of a particle in such a frame due to an external force. The third law describes forces exerted on each other in a two-particle system, and allows us to extend the second law to a system of particles. Students are often taught that the three laws are independent. Here we present an example that challenges this assumption. At first glance, it seems to show that, at least for a special case, the third law follows from the second law. However, a careful examination of the assumptions demonstrates that is not quite the case. Ultimately, the example does illustrate the significance of the concept of mass in linking Newton’s dynamical principles.


This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Asim Gangopadhyaya and James Harrington, "Can Newton’s Third Law Be “Derived” from the Second?" The Physics Teacher 55, 236 (2017) and may be found at

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Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License.

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