The Physics Educator

In high school and undergraduate physics, magnetic forces are often introduced as not being capable of doing work. In this paper, we explore the truth of this statement, analyzing the work done in various physical scenarios, including the bending of a wire in an external magnetic field, as well as the interaction between magnetic dipoles. In particular, the formation of surface charges and thorough analysis of the Hall force are elaborated. This paper addresses certain misconceptions regarding magnetic forces and work, and is expected to be of interest to students and educators alike.

We describe a simple experimental apparatus which allows the quantitative study of the classical analog of a quantum eraser. The apparatus consists of a laser diode, a thin wire, two polarizers providing the which-way information, and a third polarizer which erases that information. The experimental results are analyzed with the aid of a digital camera and the Tracker application, and are compared with a novel classical computation of the interference patterns, which is presented as well.

A simple hand helicopter can be made by attaching rotor blades to one end of a vertical stick. The helicopter moves upwards when the stick is spun above a threshold angular velocity and released — a mechanism of common toys found in toy shops. This study uses computational fluid dynamics to predict the airflow around a hand helicopter from which the lift and drag acting on the hand helicopter’s blades can be obtained. Newton’s Second Law is then applied to find the hand helicopter’s equations of motion. These are solved to predict the helicopter’s trajectory and maximum height. The angle of attack, length of blades and thickness of blades are varied and their effects on the lift coefficient, drag coefficient, lift-off and maximum height are investigated. The findings offer insight into the parameters which will allow a hand helicopter to achieve its optimal maximum height. The conditions required for the hand helicopter to fly stably are also investigated and explained using the Tennis Racket Theorem.

We give a detailed analysis of the interaction between overlapping charged balls, depending on the distance between them and their sizes. If one of the balls is entirely inside another ball, the interaction force is strictly proportional to the distance between the balls’ centers. This force has a maximum when some partial balls overlap depending on the ratio of their radii. For example, if the balls are the same, then the maximum corresponds to the distance between the centers equal to the balls’ radius. At small relative displacements of oppositely charged balls, they should perform harmonic oscillations around their common center of mass. The paper is addressed to undergraduates who deal with modern electrostatics.

In this letter, we give a simple and analytical solution to the twin paradox. It relies just on Lorentz transformations and does not involve accelerated frames or any kind of signals exchanged between the twins. We expect that as a result we can put a dead end to this century-old problem.

This short contribution reports the development of a test for assessing middle school students’ physics proficiency via multiple-choice single-select items in German language. The test assesses students’ content and procedural knowledge across various content areas that are typical of physics education at the middle-school level and is based on adapted items developed within the Third International Mathematics and Science Study (TIMSS). We report the study design we used to develop this test, as well as the results and selected parameters regarding the test’s psychometric quality.

In this brief article, I present the surprising, symmetry-breaking behavior of a homogeneous log of maple wood. The log is of square cross-section, and is placed to float in a tank of water. The straight symmetric floating state turns out to be unstable, and it immediately lolls over to an unexpected angle. This could explain why, when I was a young kid, my carefully made boats often did not float upright.