Carl Mungan, United States Naval Academy
mungan@usna.edu

In the November 2014 issue of The Physics Teacher, Michael O’Shea argues that the maximum backpack weight a person can carry does not simply equal a percentage of one’s body weight. Rather, there is some intermediate hiker’s weight at which the backpack mass is maximized, because a person has to carry not just the pack but also their own upper body. The author’s photographs of mountain hiking reminded me fondly of my own month-long Outward Bound trek in central BC (a long time ago). Hewitt’s solution to Figuring Physics on page 564 of the December 2014 issue seems slightly wrong to me. He says the weight of a can of air and of an evacuated can would be equal. My issue is that should only be true if the evacuated can collapsed down to zero internal volume, which by inspection is not true for the partly collapsed can in his sketch. I invite some reader to actually try the experiment and see if they can measure a difference in weights. Page 34 of the January 2015 issue has a lovely example of the numerical simulation of the motion of a charge in a nonuniform magnetic field (namely that of an infinite straight wire) as an interesting counterpoint to the standard textbook example of helical motion in a uniform field. The authors do a nice job of deriving the two coupled differential equations. Although they cannot be solved analytically, they do have a simple dimensionless form which I find appealing.

I have previously seen granite sphere fountains, such as the one shown in Fig. 1 of the article by Snoeijer and van der Weele in the November 2014 issue of the American Journal of Physics, but I did not realize they are actually levitating a one-ton stone on a thin film of flowing water. The authors convincingly show that the levitation is due to lubrication (like a giant ball bearing) not buoyancy. In the December 2014 issue, Garfinkle and Rojo explain why meteors impact Earth with speeds that are always in the range 16 to 72 km/s. I have published a simplified analysis of the same issue. Finally, Kagan has a thorough review of the method of analyzing electrical circuits using nodal potentials (rather than the standard introductory physics method of branch currents) in the January 2015 issue. I have summarized his key example of a Wheatstone bridge of resistors for use in my classes.

Page 693 of the November 2014 issue of Physics Education presents a simple model of drafting of one race car closely following behind another. The idea is that if the front car totally cancels quadratic air drag on the second car (because they essentially form one longer car of the same cross-sectional area) then we expect the terminal speed to increase by the square root of 2, i.e., by as much as 40%. Also I was amused by the simple but clear explanation of why a superball thrown under and bouncing off the underside of a table (after hitting and returning to the floor) will retrace its path back to the thrower on pages 125 and 126 of the January 2015 issue. Turning to the European Journal of Physics, article 065012 in the November 2014 issue discusses anharmonicity in various oscillating systems: a disk bouncing between two bumpers on a horizontal air table, a marble rolling along a V-shaped track, an interrupted pendulum, a quartic rather than the usual parabolic potential, and the combination of linear damping with Hooke’s law. Article 015005 in the January 2015 issue analyzes the normal modes of a vertically hanging chain with a point mass at its lower end. The addition of the mass enables Neumann functions as solutions, rather than just the Bessel functions one gets in its absence. Both journals are accessible at IOP Science Journals website.

Page 2195 of the December 2014 issue of the Journal of Chemical Education discusses the thermodynamics of a rubber band, including a comparison of the theoretical equation of state with experimental results on the tension as a function of temperature and length. The references include a link to a YouTube video of a heat engine constructed by replacing the spokes with rubber bands and heating one region with a lamp. Additional theoretical and experimental details are available in the online Supplementary Info for the article.

The Indian Academy of Sciences publishes the journal Resonance of science education. Physics articles in the November 2014 issue include a review of nonlinear ocean waves on page 1047, and a brief derivation of Wien’s displacement law starting from classical action on page 1058.

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Disclaimer – The articles and opinion pieces found in this issue of the APS Forum on Education Newsletter are not peer refereed and represent solely the views of the authors and not necessarily the views of the APS.