Dynamics of a bouncing dimer

We investigate the dynamics of a dimer bouncing on a vertically oscillated plate. The dimer, composed of two spheres rigidly connected by a light rod, exhibits several modes depending on initial and driving conditions. The first excited mode has a novel horizontal drift in which one end of the dimer stays on the plate during most of the cycle, while the other end bounces in phase with the plate. The speed and direction of the drift depend on the aspect ratio of the dimer. We employ event-driven simulations based on a detailed treatment of frictional interactions between the dimer and the plate in order to elucidate the nature of the transport mechanism in the drift mode.Comment: 4 pages, 5 figures, Movies: http://physics.clarku.edu/~akudrolli/dime

The anomalous behavior of coefficient of normal restitution in the oblique impact

The coefficient of normal restitution in an oblique impact is theoretically studied. Using a two-dimensional lattice models for an elastic disk and an elastic wall, we demonstrate that the coefficient of normal restitution can exceed one and has a peak against the incident angle in our simulation. Finally, we explain these phenomena based upon the phenomenological theory of elasticity.Comment: 4 pages, 4 figures, to be appeared in PR

Simple model of bouncing ball dynamics. Displacement of the limiter assumed as a cubic function of time

Nonlinear dynamics of a bouncing ball moving vertically in a gravitational field and colliding with a moving limiter is considered and the Poincare map, describing evolution from an impact to the next impact, is described. Displacement of the limiter is assumed as periodic, cubic function of time. Due to simplicity of this function analytical computations are possible. Several dynamical modes, such as fixed points, 2 - cycles and chaotic bands are studied analytically and numerically. It is shown that chaotic bands are created from fixed points after first period doubling in a corner-type bifurcation. Equation for the time of the next impact is solved exactly for the case of two subsequent impacts occurring in the same period of limiter's motion making analysis of chattering possible.Comment: 8 pages, 1 figure, presented at the DSTA 2011 conference, Lodz, Polan

Simple model of bouncing ball dynamics: displacement of the table assumed as quadratic function of time

Nonlinear dynamics of a bouncing ball moving in gravitational field and colliding with a moving limiter is considered. Displacement of the limiter is a quadratic function of time. Several dynamical modes, such as fixed points, 2 - cycles and chaotic bands are studied analytically and numerically. It is shown that chaotic bands appear due to homoclinic structures created from unstable 2 - cycles in a corner-type bifurcation.Comment: 11 pages, 6 figure

Dynamic buckling and fragmentation in brittle rods

We present experiments on the dynamic buckling and fragmentation of slender rods axially impacted by a projectile. By combining the results of Saint-Venant and elastic beam theory, we derive a preferred wavelength lambda for the buckling instability, and experimentally verify the resulting scaling law for a range of materials including teflon, dry pasta, glass, and steel. For brittle materials, buckling leads to the fragmentation of the rod. Measured fragment length distributions show two clear peaks near lambda/2 and lambda/4. The non-monotonic nature of the distributions reflect the influence of the deterministic buckling process on the more random fragmentation processes.Comment: 4 pages, 5 figures, submitted to Physical Review Letter

Impact testing to determine the mechanical properties of articular cartilage in isolation and on bone

The original publication is available at www.springerlink.comNon peer reviewedPostprin

Simulation for the oblique impact of a lattice system

The oblique collision between an elastic disk and an elastic wall is numerically studied. We investigate the dependency of the tangential coefficient of restitution on the incident angle of impact. From the results of simulation, our model reproduces experimental results and can be explained by a phenomenological theory of the oblique impact.Comment: 30 pages, 9 figures, submitted to J. Phys. Soc. Japa

Teacher evaluation in Portugal: persisting challenges and perceived effects

This paper reports on findings from a broader piece of research aimed at investigating the ways in which head teachers experience a new teacher evaluation policy in Portugal, particularly in regard to the challenges and perceived effects of the policy on school and on teacher development. Data were collected through a questionnaire with open and closed-ended questions. In total, 134 head teachers participated in the study. Findings suggest a number of tensions and problems, most of which are related to the key features of the model of teacher evaluation itself. Others issues pertain to the lack of recognition of the appraisers and to the emergence of tensions amongst staff leading to the deterioration of the school climate. Persisting challenges to policy implementation, perceived effects on teachers and schools as well as the dilemmas of head teachers are analysed. The article concludes with the discussion of ways forward.Financial Support by CIEC (Research Centre on Child Studies, IE, UMinho; FCT R&D unit 317, Portugal) by the Strategic Project UID/CED/00317/2013, with financial support of National Funds through the FCT (Foundation for Science and Technology) and co-financed by European Regional Development Funds (FEDER) through the COMPETE 2020 - Competitiveness and Internationalization Operational Program (POCI) with the reference POCI-01-0145-FEDER-00756

Speaking with things: Encoded researchers, social data, and other posthuman concoctions

We apply our heuristics for ‘interviewing’ nonhuman research participants (Adams and Thompson 2011) to the digital things of qualitative research itself: recording devices, data analysis software, and other sociomaterial concoctions recruited at different stages of contemporary research projects. We suggest that these ‘inorganic organized’ entities participate as co-researchers that inevitably extend but also disrupt research practice and knowledge construction, introducing new tensions and contradictions. Counterpointing phenomenology and Actor Network Theory, we usher some of the hidden and coded materialities of research practice into view, and glimpse unexpected realities co-enacted. Such immersive entanglements raise ethical questions about the posthumanist fluencies now demanded in social science research practice and we outline several considerations