**August 2021 Issue,**

**Volume 89, No. 8**

The force required to operate the plunger on a French press

The French press is a popular device for brewing coffee, comprising a cylindrical beaker—or “jug”—fitted with a lid and plunger with a fine wire mesh filter. The plunger is used to drive the solid coffee particles to the bottom of the jug, separating these grounds from hot liquid above. When using the French press in this way, a growing permeable pack of ground coffee is pushed through hot water by applying force to the plunger. We use a combination of kitchen-based and laboratory experiments to determine the force required to push on the plunger as a function of the speed of the plunger and the mass of coffee used. We calculate that for the recommended preparation method, the maximum force is 32 N to complete the pressing action in 50 s. We propose that home coffee preparation provides a fun, low-cost, and relatable learning opportunity for students and for those who are interested in coffee science.

#### EDITORIAL

In this issue: August 2021 by Beth Parks. DOI: 10.1119/5.0060663

Supporting student success by Beth Parks. DOI: 10.1119/5.0060646

#### PAPERS

Resource Letter RP-1: Race and physics by Katemari Rosa, Jennifer Blue, Simone Hyater-Adams, Geraldine L. Cochran and Chanda Prescod-Weinstein. DOI: 10.1119/10.0005155

This article provides resources to physicists who hope to better understand the experiences of people in groups that have been marginalized in science in order to make needed changes in physics and physics education.

The force required to operate the plunger on a French press by Fabian B. Wadsworth, Caron E. J. Vossen, Michael J. Heap, Alexandra Kushnir, Jamie I. Farquharson, Diana Schmid, Donald B. Dingwell, Lukas Belohlavek, Mila Huebsch, Lucille Carbillet and Jackie E. Kendrick. DOI: 10.1119/10.0004224

What can a cup of coffee teach you about fluid mechanics? This article helps answer this question by examining the force one has to exert on a French press plunger to make the water flow through the coffee grounds. Analyzing the laminar flow of water through this permeable medium, the authors relate the force to the initial mass of coffee, its density, the average grain size, and the velocity of the plunger that the operator imposes during brewing. They propose a simple model that can be used to illustrate undergraduate fluid mechanics lectures, not only in simple at-home experiments but also in more in-depth characterization in the lab, suitable for a student project.

On the consistency of the Lagrange multiplier method in classical mechanics by Nivaldo A. Lemos and Marco Moriconi. DOI: 10.1119/10.0004135

This work explores the relationship between the mathematical method of Lagrange multipliers and the physical forces that can be determined through its use. Readers interested in classical mechanics will find conditions under which the technique of Lagrange multipliers is physically consistent.

Black hole flyby by Sebastian J. Szybka. DOI: 10.1119/10.0004305

You want to launch a spacecraft to observe a nearby black hole. The craft will orbit the black hole and then eventually return to its initial position. How should you launch it? What is the shape of its trajectory, and what will onboard observers experience? These are among the fascinating questions investigated in this article which will be of interest to students in general relativity courses and to their instructors.

Thermal equilibrium of an ideal gas in a free-floating box by Scott Tremaine, Bence Kocsis and Abraham Loeb. DOI: 10.1119/10.0004955

One of the first systems studied by students in statistical mechanics is an ideal gas confined within a fixed rigid box. But what if the box is free to move in response to collisions with the enclosed gas molecules? The authors derive simple expressions for the mean square velocity and displacement of the box and then show how their results are relevant to a real-world phenomenon: the Brownian motion of a black hole surrounded by stars at the center of a galaxy.

Classical behavior of a quantum particle in a refringent medium by V. E. Kuzmichev and V. V. Kuzmichev. DOI: 10.1119/10.0003966

Quantum particles are known to recover a classical behavior in the ℏ→0 limit. However, there is another road to classicality: One can engineer the potential field in such a way that a quantum particle of mass m and energy E evolving in this medium has a classical momentum p=2mEn2⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯√. Schrödinger's equation then amounts to a wave-optics equation within a medium of effective index of refraction n, and quantum effects no longer govern the particle's motion. The position and energy dependences of n are derived in the 1D and 3D cases. This paper is appropriate for advanced undergraduate quantum mechanics, where the relationship between the quantum and the classical worlds is explored.

Coulomb scattering in the Born approximation and the use of generalized functions by Peter Collas. DOI: 10.1119/10.0005453

Scattering experiments are often used to study quantum systems at the atomic scale, and the Born approximation is used to simplify analysis of their results. The traditional derivation of the Born approximation uses a convergence factor, essentially treating the particles as interacting through a screened potential, but it applies even to electron-electron scattering where there are no surrounding electron clouds as there are in atoms. This paper shows two more elegant alternative methods to derive the Born approximation that do not require a convergence factor.

The floating point: Tales of the unexpected by David A. Faux and Janet Godolphin. DOI: 10.1119/10.0003915

Floating-point errors can pop up in interesting and unexpected places in computational physics, as demonstrated in the first paper. The Notes and Discussions paper that accompanies it shows how floating-point errors can affect timing devices in ways that have measurable impacts.

#### BACK OF THE ENVELOPE

Bending of starlight by gravity by Sanjoy Mahajan. DOI: 10.1119/5.0060579

The back-of-the-envelope column draws lessons from Wile E. Coyote to calculate the effects of general relativity.

#### NOTES AND DISCUSSIONS

The floating point: Rounding error in timing devices by David A. Faux and Janet Godolphin. DOI: /10.1119/10.0003919

Comment on “A magnetic velocity Verlet method” [Am. J. Phys. 88, 1075 (2020)] by Siu A. Chin. DOI: /10.1119/10.0005472

This Comment calls attention to the Boris solver for motion of charged particles in a magnetic field.

#### BOOK REVIEWS

Janus Point: A New Theory of Time by Milan M. Ćirković. DOI: 10.1119/10.0005309

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