We study the motion of a rotating tripod sliding over a smooth surface. The tripod is important and unique in that it is the only case where all aspects of the motion can be solved exactly. From the three constraint equations, we calculate the three normal forces and friction forces, which we then use to calculate the trajectory and other physical quantities of interest. We report novel results: the normal force has an interesting form when the sliding speed and the speed of rotation are almost equal; the tripod can execute serpentine trajectories; trajectories can have large lateral deflections; the translational kinetic energy can decrease and increase during a full rotation; the tripod can tip at a very late stage of its motion. The tripod trajectories are compared to trajectories of rotating, sliding cylinders that have continuous contact rings. We discuss significant implications of our results. PACS No.: 45.40.–f

The Galactic magnetic field is an integral constituent of the interstellar medium (ISM), and knowledge of its structure is crucial to understanding Galactic dynamics. The Rotation Measures (RM) of extragalactic (EG) sources have been the basis of comprehensive Galactic magnetic field models. Polarised extended emission (XE) is also seen along lines of sight through the Galactic disk, and also displays the effects of Faraday rotation. Our aim is to investigate and understand the relationship between EG and XE RMs near the Galactic plane, and to determine how the XE RMs, a hitherto unused resource, can be used as a probe of the large-scale Galactic magnetic field. We used polarisation data from the Canadian Galactic Plane Survey (CGPS), observed near 1420 MHz with the Dominion Radio Astrophysical Observatory (DRAO) Synthesis Telescope. We calculated RMs from a linear fit to the polarisation angles as a function of wavelength squared in four frequency channels, for both the EG sources and the XE. Across the CGPS area, 55 • < < 193 • , −3 • < b < 5 • , the RMs of the XE closely track the RMs of the EG sources, with XE RMs about half the value of EG-source RMs. The exceptions are places where large local HII complexes heavily depolarise more distant emission. We conclude that there is valuable information in the XE RM dataset. The factor of 2 between the two types of RM values is close to that expected from a Burn slab model of the ISM. This result indicates that, at least in the outer Galaxy, the EG and XE sources are likely probing similar depths, and that the Faraday rotating medium and the synchrotron emitting medium have similar variation with galactocentric distance.

Supernova remnants (SNRs) are significant contributors of matter and energy to the interstellar medium. Understanding the impact and the mechanism of this contribution requires knowledge of the physical size, energy, and expansion rate of individual SNRs, which can only come if reliable distances can be obtained. We aim to determine the distance to the SNR DA 530 (G93.3+6.9), an object of low surface brightness. To achieve this, we used the Dominion Radio Astrophysical Observatory Synthesis Telescope and the National Radio Astronomy Observatory Very Large Array to observe the absorption by intervening H i of the polarized emission from DA 530. Significant absorption was detected at velocities −28 and −67 km s−1 (relative to the local standard of rest), corresponding to distances of 4.4 and 8.3 kpc, respectively. Based on the radio and X-ray characteristics of DA 530, we conclude that the minimum distance is 4.4 − 0.2 + 0.4 kpc. At this minimum distance, the diameter of the SNR is 34 − 1 + 4 pc, and the elevation above the Galactic plane is 537 − 32 + 40 pc. The −67 km s−1 absorption likely occurs in gas whose velocity is not determined by Galactic rotation. We present a new data processing method for combining Stokes Q and U observations of the emission from an SNR into a single H i absorption spectrum, which avoids the difficulties of the noise-bias subtraction required for the calculation of polarized intensity. The polarized absorption technique can be applied to determine the distances to many more SNRs.

Conceptual inventory surveys are routinely used in education research to identify student learning needs and assess instructional practices. Students might not fully engage with these instruments because of the low stakes attached to them. This paper explores tests that can be used to estimate the percentage of students in a population who might not have taken such surveys seriously. These three seriousness tests are the pattern recognition test, the easy questions test, and the uncommon answers test. These three tests are applied to sets of students who were assessed either by the Force Concept Inventory, the Conceptual Survey of Electricity and Magnetism, or the Brief Electricity and Magnetism Assessment. The results of our investigation are compared to computer simulated populations of random answers.

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