Welcome to the American Journal of Physics (AJP). AJP publishes papers that meet the needs and intellectual interests of college and university physics teachers and students. This Journal was established in 1933 under the title the American Physics Teacher, which covers Volumes 1 through 7. The name was changed to the American Journal of Physics in 1940.
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We discuss the design of a simple experiment that reproduces the operation of the Michelson stellar interferometer. The emission of stellar sources has been simulated using light emerging from circular endfaces of stepindex polymer optical fibers and from diffuse reflections of laser beams. Interference fringes have been acquired using a digital camera, coupled to a telescope obscured by a double aperture lid. The experiment is analogous to the classical determination of stellar sizes by Michelson and can be used during the day. Using this experimental setup, we can determine the size of extended sources, located at a distance of about 75 m from our telescope, with errors less than 25%.

A rattleback is a canoeshaped body that, when spun on a smooth surface, rotates stably in one direction only; when spun in the reverse direction it oscillates violently (i.e., it “rattles”) and reverses its direction of spin. This behavior can be traced to the misalignment of the principal axes of the body with respect to the symmetry axis of its bottom surface. Although analyses of the phenomenon exist in the literature, there is not a clear, direct presentation of the basic mechanism responsible for the reversal of direction. The goal of this paper is to present, as clearly as possible, a treatment of the phenomenon by focusing on the geometry of the usual rattleback. Two initial conditions are considered: rotation about the vertical axis with no oscillation, and oscillation about a horizontal axis with no rotation. For the first initial state, oscillatory motion about the two horizontal axes is analyzed using a combination of linearization and reasonable assumptions. The reversal is then analyzed using energy considerations. The analysis for the second initial state is more direct and elementary. In combination, these analyses explain the transitions from rotation to oscillation to rotation in the opposite direction. The nonreversal for the rotation opposite initial rotation is also accounted for. We also comment on how the rattleback might be modified allowing it to reverse in both directions and thus to repeatedly reverse its direction of rotation.

Unlike passive Brownian particles, active Brownian particles, also known as microswimmers, propel themselves with directed motion and thus drive themselves out of equilibrium. Understanding their motion can provide insight into outofequilibrium phenomena associated with biological examples such as bacteria, as well as with artificial microswimmers. We discuss how to mathematically model their motion using a set of stochastic differential equations and how to numerically simulate it using the corresponding set of finite difference equations both in homogenous and complex environments. In particular, we show how active Brownian particles do not follow the MaxwellBoltzmann distribution—a clear signature of their outofequilibrium nature—and how, unlike passive Brownian particles, microswimmers can be funneled, trapped, and sorted.

We have constructed a lowcost Kerr microscope for use in our upperdivision solidstate laboratory course by retrofitting a polarizing microscope. It was tested by imaging the magnetic domains on the surface of the polished ferromagnetic samples NdFeB and FeSi. The instrument serves as a learning platform for students who use it to study essential aspects of magnetic domains, as observed using the magnetooptic Kerr effect. By applying a controlled external magnetic field to a sample, magnetic domains can be observed and manipulated in real time with the aid of a digital camera. We offer technical guidance for the development of such a microscope and outline learning objectives for undergraduates in a formal lab curriculum.

There are few simple examples of the formal equivalence of wave mechanics and matrix mechanics. The momentum matrix for a particle in an infinite square well is easy to calculate and rarely discussed in textbooks. We square this matrix to construct the energy levels and use the energy theorem of Fourier analysis to establish the wavematrix connection. The key ingredients of the equivalence proofs of Schrödinger and von Neumann, such as the d/dx rule and the RieszFischer theorem, find simple expression within the particleinabox framework.