# Browsing the Journals

**Carl Mungan, United States Naval Academy**

mungan@usna.edu

A comprehensive review of what undergraduate physics majors might need to know about tensors can be found on page 498 of the July 2013 issue of the *American Journal of Physics*. It includes discussion of dummy versus free indices, orthonormal bases, div/grad/curl/Laplacian, covariant versus contravariant components, basis transformations, fields, metrics, and Christoffel symbols. In the August issue, I appreciated the article by Paul Withers on page 565 about landing spacecraft on planets, where the drag force varies with altitude in the atmosphere. Descent with and without deployment of multiple parachutes is considered.

Eric Mazur’s group presents data showing that students learn more from lecture demonstrations if they commit themselves to a prediction of the outcome before they see the demonstration performed on page 020113 of *Physical Review Special Topics – Physics Education Research* (Volume 9, Issue 2).

In the September 2013 issue of *The Physics Teacher*, I enjoyed the new editor’s Rental Car problem in the box on page 329. (Note however that the graph should be drawn a bit more clearly: Dallas is at *x*=0, the parabola is symmetric about its vertex at *x*=250 km, and Sterlington is at *x*=500 km.) Page 394 of the October issue presents measurements showing that a transverse pulse travels along a hanging cable at a constant acceleration of *g*/2. I have thought about the same setup and discuss a modified wave equation (Wave Pulse on a Hanging Rope—C.E. Mungan, Spring 2005) that leads to this value of the acceleration.

One can create impressive sparks by bringing a grounded ball near the top dome of a Van de Graaff generator. An Italian trio of educators show that one can photograph such a spark through a diffraction grating to determine the atomic nitrogen and oxygen ions that compose the spark. Read about it on page 426 of the July 2013 issue of *Physics Education*. I also liked the experiment described in the next article on page 429. Two paddles are mounted on an arm attached to a rotary motion sensor to measure the density of air. An accurate value is found by dropping the prefactor of *C*/2 in the expression for quadratic air resistance, where *C* is the drag coefficient of the paddles. Although that neglect seems surprising, apparently it implies that *C* is on the order of 2 for their shape of paddles, at least if one includes the “added mass” effect of an object that is constantly scooping up new air as it rotates. (Recall that the standard formula for quadratic drag assumes an object falling at constant velocity, rather than one that oscillates or rotates.) Brown and Zürcher suggest on page 1095 of the September issue of the *European Journal of Physics* that the hysteresis in the length of duct tape when weights are hung from it could be a good way to introduce biology students to the stress-strain behavior of tendons. I also liked the idea of modeling real-world data, such as Usain Bolt’s measured performance in the 100-m dash, by considering both linear and quadratic air drag and the effect of wind on page 1227. Both journals can be accessed at the *IOPscience* Journals web page.

As a non-chemist, I found helpful the article on page 1003 of the August 2013 issue of the *Journal of Chemical Education*. It describes how can one construct pyramids out of plastic Lego blocks to describe the arrangement of elements in the periodic table.

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.