November 2017 Issue, Volume 85, No. 11November 2017 issue of American Journal of Physics


Confining rigid balls by mimicking quadrupole ion trapping
The rotating saddle not only is an interesting system that is able to trap a ball near its saddle point, but can also intuitively illustrate the operating principles of quadrupole ion traps in modern physics. Unlike the conventional models based on the mass-point approximation, we study the stability of a ball in a rotating-saddle trap using rigid-body dynamics. The stabilization condition of the system is theoretically derived and subsequently verified by experiments. The results are compared with the previous mass-point model, giving large discrepancy as the curvature of the ball is comparable to that of the saddle. We also point out that the spin angular velocity of the ball is analogous to the cyclotron frequency of ions in an external magnetic field utilized in many prevailing ion-trapping schemes.


Guest Editorial

Guest Editorial: Truth, beauty, and supergravity by Stanley Deser. DOI: 10.1119/1.4994807


Entanglement isn't just for spin by Daniel V. Schroeder. DOI: 10.1119/1.5003808

Confining rigid balls by mimicking quadrupole ion trapping by Wenkai Fan, Li Du, Sihui Wang, and Huijun Zhou. DOI: 10.1119/1.5005927

Dielectric media considered as vacuum with sources by Hans Olaf Hågenvik, Kjell Bløtekjær, and Johannes Skaar. DOI: 10.1119/1.5003810

Variational approach to the soft-Coulomb potential in low-dimensional quantum systems by Federico Grasselli. DOI: 10.1119/1.4994809

Cosmic ray topography by Matthew Bressler, Lydia Goodwin, and Abaz Kryemadhi. DOI: 10.1119/1.4996874

An acoustic demonstration of an avoided crossing by William Newman, Alexandria Skinner, and Shawn A. Hilbert. DOI: 10.1119/1.4996871


Personalized instructor responses to guided student reflections: Analysis of two instructors' perspectives and practices by Daniel L. Reinholz, and Dimitri R. Dounas-Frazer. DOI: 10.1119/1.5002683


Comment on “Not all counterclockwise thermodynamic cycles are refrigerators” [Am. J. Phys. 84, 413–418 (2016)] by João P. S. Bizarro. DOI: 10.1119/1.5005928

Reply to “Comment on ‘Not all counterclockwise thermodynamic cycles are refrigerators’” [Am. J. Phys. 85, 861–863 (2017)] by R. H. Dickerson, and J. Mottmann. DOI: 10.1119/1.5005929

Three new roads to the Planck scale by Valerio Faraoni. DOI: 10.1119/1.4994804

Comment on “Magnetic field calculation for arbitrarily shaped planar wires” [Am. J. Phys. 68(3), 254–258 (2000)] by S. Gratkowski. DOI: 10.1119/1.5005528

A demonstration of decoherence for beginners by L. Lerner. DOI: 10.1119/1.5005526


A compact grism spectrometer for small optical telescopes by Dominic A. Ludovici, and Robert L. Mutel. DOI: 10.1119/1.5000801


John Stewart Bell and Twentieth-Century Physics: Vision and Integrity by Michael Nauenberg. DOI: 10.1119/1.4983117

Rays, Waves, and Scattering: Topics in Classical Mathematical Physics by James A. Lock. DOI: 10.1119/1.5005527

Drawing Physics: 2,600 Years of Discovery from Thales to Higgs by Dwight E. Neuenschwander. DOI: 10.1119/1.5002679


American Journal of Physics 85, 883 (2017);


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