AJP April 2021 issueApril 2021 Issue,

Volume 89, No. 4

Acoustic levitation and the acoustic radiation force

We investigate acoustic levitation in a vertical standing wave in an attempt to understand the basic physical mechanism responsible for this phenomenon. We find that a description in terms of a simple pressure force leads to the prediction of stable equilibria that occur slightly below the anti-nodes of the standing pressure wave. We then demonstrate that such a prediction is at odds with experimental data, which show that levitating particles come to rest slightly below the nodes of the standing pressure wave. Finally, we outline a theoretical approach based on fluid dynamics that correctly predicts the locations of the levitating particles, which leads to a simple qualitative description for this fascinating phenomenon.


In this issue: April 2021 by Beth Parks. DOI: 10.1119/10.0003615


The mobile phone as a free-rotation laboratory by Michael S. Wheatland, Tara Murphy, Daniel Naoumenko, Daan van Schijndel and Georgio Katsifis. DOI: 10.1119/10.0003380
Never thought of deliberately tossing your smartphone in the air? Well, you should. As this paper shows, by so doing, you can observe and measure a rotation instability. Indeed, for any object that has three different moments of inertia, the rotation about the intermediate axis is unstable: Any small perturbation in spin about the other two axes grows exponentially. This effect has interesting consequences for tennis rackets, wingnuts and skateboards. Appropriate for undergraduate mechanics lecture demonstration or labs. The link above will also take the reader to the article's video abstract.

Euler's rigid rotators, Jacobi elliptic functions, and the Dzhanibekov or tennis racket effect by Christian Peterson and William Schwalm. DOI: 10.1119/10.0003372
The torque-free rotational motion of an object with three different moments of inertia is studied using Jacobi elliptic functions. This analysis is complementary to the more traditional analysis based on the use of conservation laws. Suitable for an advanced undergraduate mechanics class. The link above will also take the reader to the article's video abstract.

The rolling elliptical cylinder by Johan Lindén, Kjell-Mikael Källman and Markus Lindberg. DOI: 10.1119/10.0002362
An elliptical cylinder rolling down an inclined plane without slipping is an easy-to-conceptualize mechanical system. However, the Lagrangian for this system is nontrivial and the resulting dynamics are interesting. This article, which is suitable for upper-level mechanics students, uses video analysis to track the motion of a rolling elliptical cylinder and compares the results with the numerical solutions of the equations of motion. The link above will also take the reader to the article's video abstract.

Evaluating wind turbine power coefficient—An undergraduate experiment by Edward W. K. Chan, Ken K. K. Tsung, Terence K. K. Chan and Nai-Ho Cheung. DOI: 10.1119/10.0003730
A laboratory experiment to introduce physics students to renewable energy technology: Undergraduates can explore the efficiency of wind turbines through experiments using an inexpensive apparatus. Suitable for introductory-level students, with extensions for those at higher levels.

Frequency-dependent capacitors using paper by Bart H. McGuyer. DOI: /10.1119/10.0002655
Parallel-plate capacitors can be made by sandwiching sheets of printer paper between sheets of aluminum foil. They are shown to have complex dielectric behavior consistent with the Cole-Cole model. Suitable for advanced labs, especially those interested in resourceful, low-cost experiments. Several variants on the experiment are suggested.

Plasma generation by household microwave oven for surface modification and other emerging applications by Benjamin K Barnes, Habilou Ouro-Koura, Justin Derickson, Samuel Lebarty, Jesudara Omidokun, Nathan Bane, Othman Suleiman, Eguono Omagamre, Mahdi J. Fotouhi, Ayobami Ogunmolasuyi, Arturo Dominguez, Larry Gonick and Kausik S. Das. DOI: 10.1119/10.0002706
A simple and low-cost plasma generating device can be constructed from a household microwave oven and a vacuum flask. This device can then be used in surface treatment of a substrate for several cutting-edge research applications. A large set of concept-based cartoon questions are also provided to engage student discussion at the undergraduate and even high-school levels.

Acoustic levitation and the acoustic radiation force by David P. Jackson and Ming-Hua Chang. DOI: 10.1119/10.0002764
Acoustic levitation of spheres is first explained incorrectly in terms of a ponderomotive force and then correctly using the Navier-Stokes equation, at a level appropriate for undergraduates studying fluid mechanics. The link above will also take the reader to the article's video abstract.

Tesla's fluidic diode and the electronic-hydraulic analogy by Quynh M. Nguyen, Dean Huang, Evan Zauderer, Genevieve Romanelli, Charlotte L. Meyer and Leif Ristroph. DOI: 10.1119/10.0003395
The irreversibility of flow through complicated channels, as in Tesla's proposed fluid rectifier, is used to introduce students to some fundamentals of fluid dynamics and to present them with design challenges that emphasize intuition. This is appropriate for sophomores, especially for projects.

A guide for incorporating e-teaching of physics in a post-COVID world by Daniel J. O'Brien. DOI: 10.1119/10.0002437
The global pandemic has required instructors and institutions to rely on remote learning far more than ever before. Despite the difficulties of this transition, there is now an opportunity to engage with new technologies and to raise awareness of systemic issues in physics education. This article presents a broad picture of the difficulties facing students during the pandemic and offers recommendations going forward. Concrete suggestions for the use of social media and smartphones in the classroom are also provided. The article is of interest to the entire physics educational community during these challenging times and also in a recovery that we hope is coming soon.

The Persico equation for minimum uncertainty states by Vincenzo Barone. DOI: 10.1119/10.0002719
In 1930, Enrico Persico presented a method of determining minimum uncertainty wavefunctions for any pair of non-commuting physical observables, but it was lost to history. This paper presents the historical context of the discovery and derives the result at a level for undergraduate students of quantum mechanics.

The bound-state solutions of the one-dimensional hydrogen atom by Rufus Boyack and Frank Marsiglio. DOI: 10.1119/10.0002639
Some problems are harder in one dimension than in three dimensions. By considering a “regularized” version of the one-dimensional hydrogen atom, the authors determine properties of the even-parity and odd-parity solutions. Appropriate for advanced undergraduate and graduate-level quantum mechanics.

A first encounter with the Hartree-Fock self-consistent-field method by Robin Santra and Michael Obermeyer. DOI:10.1119/10.0002644
The Hartree-Fock self-consistent-field method is a first-principles technique for computing eigenstates of multi-electron atoms and other atomic systems. While this method is powerful, it can seem abstract to students. This paper offers an introduction to the Hartree-Fock method at a level appropriate for upper-level undergraduate students.


How far can planes and birds fly? by Sanjoy Mahajan. DOI: 10.1119/10.0003729
Semi-spoiler alert: They have a similar geometries and comparable fuel fractions and energy densities. This extension of the November 2021 “Back of the envelope” on the energy used in flight presents a calculation that can be enjoyed by physicists at all levels.


Digital imaging of a random walk by computer simulation: Using a simple model to interpret the effects of finite spatio-temporal resolution by Swayamshree Patra, Swagata Dey, Krishanu Ray and Debashish Chowdhury. DOI: 10.1119/10.0002718
A simple computer simulation explores the effects of the spatial and temporal resolutions of digital imaging. The size of the pixels of the virtual camera and the time interval between successive image acquisitions fix the spatial and temporal resolutions and yield novel patterns in the trajectories. The simulation process may be interesting to students of computational physics, and understanding it may also avoid errors in the interpretation of experimental data.


Gravitational Few-Body Dynamics: A Numerical Approach by Patrick Hamill. DOI: American Journal of Physics 89, 443 (2021); https://doi.org/10.1119/10.0003728


American Journal of Physics 89, 444 (2021); https://doi.org/10.1119/10.0003736

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