Coupling between static friction force and torque for a tripod

If a body is resting on a flat surface, the maximal static friction force before motion sets in is reduced if an external torque is also applied. The coupling between the static friction force and static friction torque is nontrivial as our studies for a tripod lying on horizontal flat surface show. In this article we report on a series of experiments we performed on a tripod and compare these with analytical and numerical solutions. It turns out that the coupling between force and torque reveals information about the microscopic properties at the onset to sliding.Comment: 7 pages, 4 figures, revte

Coupling between static friction force and torque

We show that the static friction force which must be overcome to render a sticking contact sliding is reduced if an external torque is also exerted. As a test system we study a planar disk lying on horizontal flat surface. We perform experiments and compare with analytical results to find that the coupling between static friction force and torque is nontrivial: It is not determined by the Coulomb friction laws alone, instead it depends on the microscopic details of friction. Hence, we conclude that the macroscopic experiment presented here reveals details about the microscopic processes lying behind friction.Comment: 6 pages, 4 figures, revte

Multi-channel Bethe-Salpeter equation

A general form of multi-channel Bethe-Salpeter equation is considered. In contradistinction to the hitherto applied approaches, our coupled system of equations leads to the simultaneous solutions for all relativistic four-point Green functions (elastic and inelastic) appearing in a given theory. A set of relations which may be helpful in approximate treatments is given. An example of extracting useful information from the equations is discussed: we consider the most general trilinear coupling of N different scalar fields and obtain - in the ladder approximation - closed expressions for the Regge trajectories and their couplings to different channels in the vicinity of l = -1. Sum rules and an example containing non-obvious symmetry are discussed.Comment: 16 pages. Extended version published in JHEP. Uses JHEP.cls (included

Wopper, Version 1.1: A Monte Carlo Event Generator for Four Fermion Production at LEP-II and Beyond

We report on the status of the Monte Carlo event generator WOPPER. Version 1.1 of WOPPER describes four fermion production at LEP-II and beyond with leading logarithmic radiative corrections in the double W pole approximation. These approximations are appropriate for almost all practical purposes, but the inclusion of these finite width effects and radiative corrections is nevertheless indispensable for LEP-II physics.Comment: 6 pages LaTeX (Elsevier proceedings style) with 7 PostScript figures, (Contribution to the Teupitz Workshop 1994), IKDA 94/

Barkhausen noise in the Random Field Ising Magnet Nd2_2Fe14_{14}B

With sintered needles aligned and a magnetic field applied transverse to its easy axis, the rare-earth ferromagnet Nd$_2$Fe$_{14}$B becomes a room-temperature realization of the Random Field Ising Model. The transverse field tunes the pinning potential of the magnetic domains in a continuous fashion. We study the magnetic domain reversal and avalanche dynamics between liquid helium and room temperatures at a series of transverse fields using a Barkhausen noise technique. The avalanche size and energy distributions follow power-law behavior with a cutoff dependent on the pinning strength dialed in by the transverse field, consistent with theoretical predictions for Barkhausen avalanches in disordered materials. A scaling analysis reveals two regimes of behavior: one at low temperature and high transverse field, where the dynamics are governed by the randomness, and the second at high temperature and low transverse field where thermal fluctuations dominate the dynamics.Comment: 16 pages, 7 figures. Under review at Phys. Rev.

The Irreducible String and an Infinity of Additional Constants of Motion in a Deposition-Evaporation Model on a Line

We study a model of stochastic deposition-evaporation with recombination, of three species of dimers on a line. This model is a generalization of the model recently introduced by Barma {\it et. al.} (1993 {\it Phys. Rev. Lett.} {\bf 70} 1033) to $q\ge 3$ states per site. It has an infinite number of constants of motion, in addition to the infinity of conservation laws of the original model which are encoded as the conservation of the irreducible string. We determine the number of dynamically disconnected sectors and their sizes in this model exactly. Using the additional symmetry we construct a class of exact eigenvectors of the stochastic matrix. The autocorrelation function decays with different powers of $t$ in different sectors. We find that the spatial correlation function has an algebraic decay with exponent 3/2, in the sector corresponding to the initial state in which all sites are in the same state. The dynamical exponent is nontrivial in this sector, and we estimate it numerically by exact diagonalization of the stochastic matrix for small sizes. We find that in this case $z=2.39\pm0.05$.Comment: Some minor errors in the first version has been correcte

Extended Molecular Gas in the Nearby Starburst Galaxy Maffei 2

We present a 9'x9' fully-sampled map of the CO J=1-0 emission in the nearby starburst galaxy Maffei 2 obtained at the Five College Radio Astronomy Observatory. The map reveals previously known strong CO emission in the central starburst region as well as an extended asymmetric distribution with bright CO lines at the ends of the bar and in a feature at the north-east edge of the molecular disk. This northern feature, proposed previously to be an interacting companion galaxy, could be a dwarf irregular galaxy, although the CO data are also consistent with the feature being simply an extension of one of the spiral arms. We estimate the total molecular gas mass of Maffei 2 to be (1.4-1.7)x10^9 Mo or ~3-4% of its dynamical mass. Adopting the recently determined lower value for the CO-to-H2 conversion factor in the central region, our data lead to the surprising result that the largest concentrations of molecular gas in Maffei 2 lie at the bar ends and in the putative dwarf companion rather than in the central starburst. A gravitational stability analysis reveals that the extended disk of Maffei 2 lies above the critical density for star formation; however, whether the central region is also gravitationally unstable depends both on the details of the rotation curve and the precise value of the CO-to-H2 conversion factor in this region.Comment: accepted to ApJ (Sept 10 2004 issue

Hopping Conduction and Bacteria: Transport in Disordered Reaction-Diffusion Systems

We report some basic results regarding transport in disordered reaction-diffusion systems with birth (A->2A), death (A->0), and binary competition (2A->A) processes. We consider a model in which the growth process is only allowed to take place in certain areas--"oases"--while the rest of space--the "desert"--is hostile to growth. In the limit of low oasis density, transport is mediated through rare "hopping" events, necessitating the inclusion of discreteness effects in the model. By first considering transport between two oases, we are able to derive an approximate expression for the average time taken for a population to traverse a disordered medium.Comment: 4 pages, 2 figure