Comment on “Free-Body Diagrams and Bogus Work”
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Rod Cross
[1]
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[1] University of Sydney
University of Sydney
Australia
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- Localización: The Physics Teacher, ISSN 0031-921X, Vol. 62, Nº. 9, 2024, págs. 708-709
- Idioma: inglés
- Enlaces
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Resumen
In a recent article in this journal, Bassichis1 outlines a case for omitting thermodynamics in introductory mechanics courses in order to focus on Newton’s laws. He considers several examples, including a ball rolling without slipping down an inclined plane. Since the friction force is a static force, and there is no displacement of the point of application of the friction force, the claim is often made that a static friction force does no work. It is then easy to calculate the magnitude of the friction force and the velocity of the ball at the bottom of the incline. That is a standard textbook approach, but it ignores the fact that work is done by the friction torque to increase the rotational kinetic energy of the ball, and it ignores the fact that an equal and opposite amount of work is done by the friction force to reduce the translational kinetic energy of the ball. Consequently, it is the total work done by the friction force that is zero, as pointed out by Carnero et al.2 There is rarely any discussion of the work done by the friction force to change the rotational kinetic energy of the ball or to change the translational kinetic energy of the ball. The two separate effects are almost always mysteriously combined in the statement that no work is done by a static friction force. Indeed, some authors in the past claimed that the work done to change the translational kinetic energy is not real work. Instead, they labeled it is as fake or pseudowork or bogus work (as emphasized by Bassichis)
