April 2026
Volume 94, Issue No. 4
The common New England sight of a cranberry bog presents a rich tapestry of fluid dynamics and soft matter phenomena. Here, we present four connected problems exploring the behavior of cranberries in their stages of harvest: the buoyant rise of a cranberry in a flooded bog, the stable floating configuration of a cranberry on the surface, the aggregation and interaction between many floating cranberries collected with a boom, and the piling of cranberries onto a truck for transportation. We model these phenomena from first principles and develop simple computational simulations of their collective behaviors. Additionally, we describe tabletop experiments to accompany these problems, either as in-class demonstrations or lab activities. Throughout, we draw connections to broader physical principles in soft condensed matter and fluids, allowing the real-world example of the cranberry bog to serve as a bridge between the undergraduate curriculum and topics in soft matter research.
EDITORIAL
In this issue: April 2026 by John Essick; Harvey Gould; Jesse Kinder; Claire A. Marrache-Kikuchi; Beth Parks; Daniel Schumayer; Todd Springer; Jan Tobochnik; Keith Zengel. DOI: 10.1119/5.0332496
LETTERS TO THE EDITOR
Reflections on a comment by David Griffiths about quantum mechanics by Nick Tufillaro. DOI: 10.1119/5.0320795
Reply to Aleksandar Gjurchinovski's comment
SEASONAL ARTICLES
Higher multipoles of the cow by Benjamin V. Lehmann. DOI: 10.1119/5.0287411
Editor's Note: The spherical cow is a useful approximation familiar to physicists. But what does it leave out? How can you systematically improve on this approximation?
This light-hearted yet rigorous analysis explores controlled approximations in which the spherical cow emerges as the first term. First, a multipole expansion of the mass distribution of a cow reveals the dire shortcomings of the spherical cow approximation in computing gravitational radiation from a rotating cow. Next, gradient flow and harmonic maps are introduced in order to explore a multipole expansion of the shape of a cow and to resolve a paradox introduced by the spherical cow approximation: no force is required to tip a cow.
While the premise is playful, the physics is no joke. The author has provided a memorable vehicle for teaching multipoles, gradient flows, harmonic maps, and the consequences of oversimplification.
PAPERS
Mechanical forces induced by chemical potential: From thermodynamic principles to environmental applications by Jacco H. Snoeijer; Bruno Andreotti. DOI: 10.1119/5.0252905
Editor's Note: This paper brings two valuable updates to our thermodynamics courses: examples that illustrate the relevance of thermodynamics to understanding environmental processes, along with a new way to understand the chemical potential in terms of mechanical forces. Readers (and their students) will enjoy learning about topics ranging from cloud formation to osmotic energy production.
The physics of cranberry bogs by Caroline M. Barotta; Jack-William Barotta. DOI: 10.1119/5.0300074
Editor's Note: Do you teach your students about the physics of cranberries? Perhaps you should. They're cheap, aesthetic, and round. Born at the bottom of a bog, they buoy through drag forces to the surface and self-assemble via capillary attraction. When you stack them in a pile, they form a conical shape with an angle of repose determined by their shape and friction coefficients. Read this article if you're looking for fun new experiments you can adapt to any level of undergraduate physics teaching.
Spreading viscous fluids on a horizontal surface: Project-based learning in fluid mechanics by R. Bolaños-Jiménez; P. L. Luque-Escamilla. DOI: 10.1119/5.0269772
Editor's Note: This paper presents an undergraduate fluid mechanics lab investigating the spreading of a vertically falling viscous jet on a flat surface. The lab combines dimensional analysis as an entry point, hands-on quantitative experiments, and a simplified theoretical model that effectively captures the observed results.
The original Wigner's friend scenarios by Jay Lawrence. DOI: 10.1119/5.0303052
Editor's Note: Philosophical questions about the foundations of quantum mechanics continue to surface today. The Wigner's friend paradox imagines an observer inside a lab making a quantum measurement while an outside observer treats the entire lab as a quantum system. This setup leads the two observers to assign different quantum states to the same physical situation, challenging the idea of a single, objective outcome. The paradox succinctly highlights key interpretational tensions and invites examination of how the classical and contemporary theories attempt to resolve or soften these issues.
Accurate laser-based characterization of liquid refractive index for advanced undergraduate laboratories by Zhe Wang; Chantal Barwig; Gaurav Dave; Christine Selhuber-Unkel. DOI: 10.1119/5.0297246
Editor's Note: This paper presents an affordable instructional laboratory setup in which the refractive index of a liquid in a tilted cuvette is measured using laser beam displacement. Care is taken to precisely determine the centroid of the laser beam using a knife-edge measurement technique, and the angular symmetry of the problem is used to address a potential source of error. In addition, the authors model this system using a standard geometric optics approach as well as through a ray transfer matrix analysis. Results obtained from fitting these two models to the experimental data taken on water and 1-propanol are shown to be consistent with reference values, demonstrating the validity of the approach.
COMPUTATIONAL PHYSICS
Exploring the lattice Boltzmann method for two-dimensional turbulence simulation by Raquel Dapena-García; Vicente Pérez-Muñuzuri. DOI: 10.1119/5.0303644
Editor's Note: The authors describe the lattice Boltzmann method for simulating hydrodynamic problems and apply it to two-dimensional turbulence created by placing solid disks in the fluid. Their results show the scaling of the energy and the enstrophy (a measure of vorticity) as a function of length scale and discuss the different behavior of turbulence in two and three dimensions.
INSTRUCTIONAL LABORATORIES AND DEMONSTRATIONS
AC magnetic measurements with a self-oscillating LC circuit and its application to university education by Harshit Agarwal; Oleksandra Uralska; Jasmin Billingsley; Maxim Yamilov; Hyunsoo Kim. DOI: 10.1119/5.0305298
Editor's Note: This paper is a welcome tutorial on a technique for AC magnetometry, allowing the measurement of a sample's magnetic susceptibility with considerable sensitivity. The authors present a low-cost setup based on an LC self-oscillator circuit, which has an operating frequency that is rather sensitive to the susceptibility of a sample introduced within its inductor. After characterizing the performance of the circuit, data taken on a high-temperature superconducting sample are presented, demonstrating how the samples superconducting transition can be detected by this noncontact magnetic method, independent of the usual resistivity-based detection. This project will be of interest to instructors wishing to introduce a magnetism-related experiment in their laboratory curricula.
Swept-source and spectral-domain optical coherence tomography—Educational setups for the undergraduate lab by Keerthanan Ulaganathan; Kai Pieper; Max Althön; Antje Bergmann; Carsten Rockstuhl; Jens Küchenmeister. DOI: 10.1119/5.0300824
Editor's Note: This paper presents two methods for implementing frequency-domain optical coherence tomography (OCT), allowing undergraduate students to produce images of the internal structure of samples. Detailed descriptions of the underlying theory and required experimental setups are given for the swept-source and spectral-source OCT methods, along with depth-profile images obtained using these setups on layered samples. This work will be of interest to instructors wishing to add an optics-based characterization technique of contemporary interest in their advanced laboratory curricula.
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