Center of mass

Summary

In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point at any given time where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may be applied to cause a linear acceleration without an angular acceleration. Calculations in mechanics are often simplified when formulated with respect to the center of mass. It is a hypothetical point where the entire mass of an object may be assumed to be concentrated to visualise its motion. In other words, the center of mass is the particle equivalent of a given object for application of Newton's laws of motion. In the case of a single rigid body, the center of mass is fixed in relation to the body, and if the body has uniform density, it will be located at the centroid. The center of mass may be located outside the physical body, as is sometimes the case for hollow or open-shaped objects, such as a horseshoe. In the cas

Official source

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A review in snow saltation dynamics and its implications for the surface mass balance

Daniela Brito Melo, Francesco Comola, Franziska Gerber, Michael Lehning, Varun Sharma, Armin Sigmund

Snow covered regions are frequently subjected to strong winds. This leads to the erosion of the snow surface and the occurrence of drifting and blowing snow events. To correctly predict and model these phenomena is of the utmost importance to assess snowpack stability and avalanche formation, as well as airborne snow sublimation and the resultant surface mass balance. Nonetheless, snow transport is frequently neglected or misrepresented in regional and mesoscale models. One of the main challenges is the accurate representation of snow transport close to the ground, where snow particles are transported by a process called saltation. This shallow layer comprises most of the horizontal mass flux and sets the lower boundary condition to model snow suspension clouds. A detailed study of snow saltation dynamics has been conducted using a Large-Eddy-Simulation flow solver coupled with a Lagrangian model for particle trajectories. The effect of particle size distribution and interparticle cohesion on particle speed, mass flux and surface friction velocity has systematically been investigated. The results show that snow cohesion and grain size heterogeneity can significantly increase saltation mass flux, specially at high friction velocities. Moreover, the agreement between simulation results and the saltation models typically used in large scale atmospheric models is highly dependent on the assumed bed characteristics. These findings can support the development of comprehensive saltation models that specifically take into account snow bed properties. These new saltation models can be implemented in mesoscale atmospheric models and have the potential to significantly improve surface mass balance predictions if grain-scale snow surface properties are available. This is possible in the newly developed CRYOWRF model which expands WRF’s surface modeling suite by including the advanced complexity, grain-scale snow model, SNOWPACK, along with a blowing snow scheme.

2021

A combined theoretical and experimental analysis of the resonant Rayleigh scattering by excitons in a disordered GaAs quantum well with monolayer islands is reported. Experimentally, the coherent resonant Rayleigh scattering is deduced by means of both statistical speckle analysis and passively stabilized spectral interferometry. Theoretically, the effect of disorder is modelled by solving the time-dependent Schrodinger equation for the exciton center-of-mass motion. The disorder potential used in the model includes the detailed monolayer structure of both heterointerfaces, the short-range disorder due to segregation, and a correlation between the upper and lower interfaces. The simulations reproduce all features observed both in frequency- and time-resolved measurements. The comparison between theory and experiment bears strong evidence that the monolayer fluctuations in the two heterointerfaces are highly correlated and have similar length scales.

V C H PUBLISHERS, SUITE 909, 220 E 23RD ST, NEW YORK, NY 10010 USA2004

Spatially resolved photoluminescence spectra of thin GaAs quantum wells are measured by near-field spectroscopy, and two-energy autocorrelation functions of the spectra are derived. We demonstrate distinctly different autocorrelation functions for a 3-nm-thick quantum well grown on a (311)A surface and for quantum wells grown on (100) substrates. The autocorrelation spectra of the (311)A GaAs quantum well are quantitatively described by a statistical disorder model with a single correlation length of 17 nm for the exciton center-of-mass motion. A shoulder in the autocorrelation spectrum at energies between 1 and 3 meV is identified as a signature of excitonic level repulsion in the disorder potential. In contrast, the characteristic feature in the autocorrelation spectra of the (100) quantum wells is an additional positive correlation peak at energies between 3 and 4 meV. This peak reflects the energy correlations between ground and excited exciton states in individual monolayer islands with a narrow size distribution. The results indicate that the disorder potential in these high-quality (100) GaAs quantum wells contains both contributions from monolayer islands and nanoroughness on a length scale of the exciton Bohr radius.

2004

A review in snow saltation dynamics and its implications for the surface mass balance

Daniela Brito Melo, Francesco Comola, Franziska Gerber, Michael Lehning, Varun Sharma, Armin Sigmund

2021