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We present the requirements, design, and evaluation of the cryogenic continuously rotating half-wave plate (CHWP) for the Simons Observatory (SO). SO is a cosmic microwave background polarization experiment at Parque Astronómico de Atacama in northern Chile that covers a wide range of angular scales using both small (⌀0.42 m) and large (⌀6 m) aperture telescopes. In particular, the small aperture telescopes (SATs) focus on large angular scales for primordial B-mode polarization. To this end, the SATs employ a CHWP to modulate the polarization of the incident light at 8 Hz, suppressing atmospheric 1/f noise and mitigating systematic uncertainties that would otherwise arise due to the differential response of detectors sensitive to orthogonal polarizations. The CHWP consists of a 505 mm diameter achromatic sapphire HWP and a cryogenic rotation mechanism, both of which are cooled down to ∼50 K to reduce detector thermal loading. Under normal operation, the HWP is suspended by a superconducting magnetic bearing and rotates with a constant 2 Hz frequency, controlled by an electromagnetic synchronous motor. We find that the number of superconductors and the number of magnets that make up the superconducting magnetic bearing are important design parameters, especially for the rotation mechanism's vibration performance. The rotation angle is detected through an angular encoder with a noise level of 0.07 μrad s. During a cooldown process, the rotor is held in place by a grip-and-release mechanism that serves as both an alignment device and a thermal path. In this paper, we provide an overview of the SO SAT CHWP: its requirements, hardware design, and laboratory performance.

The drag effect on a falling ball caused by air is a conventional subject in the most well-known textbooks of classical mechanics and fluid dynamics. Further, there are some papers that employ video analysis to track objects movements in the air making it possible to obtain position data as a function of time and its graphs. However, none of them addresses the case of a ball thrown downwards, maybe because this situation is neglected by textbooks or probably due to the experimental difficulty to perform this kind of launch manually. In this paper, both the fall and the vertical launch down of five different Styrofoam balls are filmed with an ordinary smartphone camera and analysed by using the free software Tracker. The position and velocities graphs depicted clearly the air drag effect on the ball's movements. The entire raw data are compared with a theoretical model and therefore terminal and initial velocities can be extracted as adjusted parameters from simple mathematical fits. The Reynolds number and the drag coefficient calculated for each ball agree with those found in the literature.

In general, undergraduate experimental physics laboratories do not usually have experiments designed to address rolling friction and to measure the value of the rolling friction coefficient. This work explores an experiment, which has the potential to arouse students' curiosity about rolling friction by addressing a counterintuitive aspect of the behavior of a sphere that rolls up, stops and then rolls down on an inclined track. In fact, due to the difference in the net static friction in the upward and downward movements caused by the rolling friction term, the sphere's acceleration when rolling upward is higher than when rolling downward. This difference, which had been predicted theoretically, was easily demonstrated graphically by video analysis, using both rubber and mouse balls. For two steel balls, this difference was more subtle, but it was still possible to detect it numerically. The experimental setup employed typical laboratory equipment and the free video analysis software "Tracker" to collect position and velocity data. The rolling friction coefficient was calculated with good precision by means of an angular coefficient of a fitted first-degree function involving appropriate variables.

Currently, the number of smartphones with an embedded gyroscope sensor has been increasing due games whose performance relies on 3D augmented reality. In general, teaching papers on the gyroscope sensor address very simple spatial configuration, where the fixed rotation axis coincides to the z-axis of the smartphone. This work presents five experimental setups with different spatial orientations of a smartphone on a turntable. The total angular velocity vector of the turntable is constant, but its projections on the three fixed Cartesian axes of the smartphone varies for the five spatial orientations investigated. For the sake of consistency, the magnitude of the vector sum of the three angular velocities components yielded by the gyroscope sensor was checked by video analysis using the free software, Tracker. The internal consistency between the results provided by this video analysis technique and data from the gyroscope assures us this sensor is reliable for teaching proposals. The main goals of this paper are (1) encourage teachers to effectively incorporate the gyroscope sensor of smartphones in to their classrooms, especially in high schools and undergraduate physics and engineering courses; (2) spread in the academic syllabus the Tait-Bryan convention, a very intuitive way to perform 3D rotations and; (3) explain how to interpret data of the three components of angular velocity obtained by the gyroscope sensor in different spatial orientations.

Nowadays, in almost any country in the world, it is difficult to find a young person who has never played with, seen, or heard of the most famous toy in 2017: the fidget spinner. According to Wikipedia: "The fidget spinner is a toy that consists of a ball-bearing in the center of a multi-lobed flat structure made from metal or plastic designed to spin along its axis with little effort." Articles on educational proposals related to fidget spinners have been presented recently. Naturally, an issue arises from this polemic context: can fidget spinners be used to illustrate or explain physics concepts such as moment of inertia, torque, and angular momentum? This paper presents a simpler, cheaper, and easier experimental setup to obtain moments of inertia of the fidget spinner by using the free software Tracker for video analysis. A consistency test with an aluminum cylindrical shell ensures the reliability of the experimental setup.

This work presents a low-cost experiment to investigate heat conduction inside potato cubes, using thermistors as a thermometric instrument and Arduino as a data acquisition platform. In the theoretical framework, a heuristic argument was proposed to switch the 3D heat equation to an equivalent and simpler 1D equation, which is valid only along points on the cube's symmetry axis. In the experimental domain, an empirical and realistic time-dependent temperature function was used as boundary conditions instead of assuming simple boundary conditions, i.e., the temperature of the cube surface in contact with the air is kept constant. An approximate solution of the one-dimensional heat equation with nonhomogeneous and time-dependent empirical and actual boundary conditions was provided. Adjusting the experimental data to the theoretical solution predicted by the heat equation provides the thermal diffusivity of the potato. The thermal diffusivity obtained agrees with the reference values found in the literature. [ABSTRACT FROM AUTHOR]

This work presents a low-cost and straightforward experiment to visualise Fourier synthesis using a coiled metallic spring whose weight is not negligible, oscillating in a vertical plane and fixed at both ends. A massive coiled spring still obeys the standard linear differential wave equation, but a new term accounting for the effect of gravity must be introduced in the equation. Hence, if the spring is plucked vertically at its centre so that its initial configuration is a triangle, the trapezoidal shape of the resulting oscillation, typical of a spring of negligible mass, is clearly distorted by the effect of gravity, when the amplitude of the oscillation moves away from its maximum value. The hypothesis that this distortion comes from some nonlinear effect produced by the larger pluck-length ratio used in the experiment was discarded when evaluating the magnitude of a mathematical criterion to estimate the nonlinear impact of oscillations with large amplitudes. As the spring’s tension is small (∼4 N, on average) and the frequencies of transverse waves are low (close to 1 Hz), oscillations could be filmed with a basic and cheap camera (60 fps), which makes it possible to carry out the experiment without the need to purchase equipment. In addition, as the frequencies involved are low, the oscillations could be reproduced and visualised in slow motion. The free software Tracker was used to collect the position data of the massive coiled spring at each instant and compare them with the related solution of the wave equation obtained by Fourier synthesis of the first six terms. [ABSTRACT FROM AUTHOR]

Author(s): The FANTOM Consortium and the RIKEN Genome Exploration Research Group Phase I & II Team*; FANTOM Consortium:; Y. Okazaki [1, 2]; M. Furuno [1]; T. Kasukawa [1, 3]; J. [...]

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Aquatic models used for both freshwater and marine systems frequently need to account for submerged aquatic vegetation (SAV) due to its influence on flow and water quality. Despite its importance, parameterizations are generally adopted that simplify feedbacks from SAV, such as canopy properties (e.g., considering the deflected vegetation height) and the bulk friction coefficient. This study reports the development of a fine‐scale non‐hydrostatic model that demonstrates the two‐way effects of SAV motion interaction with the flow. An object‐oriented approach is applied to capture the multiphase phenomena, whereby a leaf‐scale SAV model based on a discrete element method is combined with a flow dynamics model to resolve stresses from currents and waves. The model is verified through application to a laboratory‐scale seagrass bed. A force balance analysis revealed that leaf elasticity and buoyancy are the most significant components influencing the horizontal and vertical momentum equations, respectively. The sensitivity of canopy‐scale bulk friction coefficients to water depth, current speeds, and vegetation density of seagrass was explored. Deeper water was also shown to lead to a smaller decrease in vegetation height. The model approach can contribute to improving assessment of processes influencing water quality, sediment stabilization, carbon sequestration, and SAV restoration, thereby supporting an understanding of how waterways and coasts will respond to changes brought about by development and a changing climate. Plain Language Summary: Aquatic system models that capture aquatic vegetation are increasingly important to help us understand processes controlling carbon budgets (e.g., "blue carbon") and for planning restoration efforts (e.g., Adams et al., 2016, https://doi.org/10.1002/lno.10319). Current models all rely on "static" approaches to account for vegetation via bulk parameterizations, though we know vegetation motion is important (e.g., Abdolahpour et al., 2018, https://doi.org/10.1002/lno.11008). Our study is the first to model the feedback between blade‐scale vegetation motion and hydrodynamics to show it is crucial in shaping bottom mixing processes with implications for carbon and nutrient deposition. Key Points: A model is presented to capture the changing position of Submerged Aquatic Vegetation blades and validated using laboratory experimentsForcing terms that control seagrass movement within a meadow are evaluatedEstimates of deflected vegetation height and bulk meadow friction coefficients demonstrate the importance of simulating SAV motion [ABSTRACT FROM AUTHOR]