Browsing by Author "Newman, William I."
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Item The method of variation of constants and multiple time scales in orbital mechanics(2002-11) Newman, William I.; Efroimsky, MichaelThe method of variation of constants is an important tool used to solve systems of ordinary differential equations, and was invented by Euler and Lagrange to solve a problem in orbital mechanics. This methodology assumes that certain ``constants'' associated with a homogeneous problem will vary in time in response to an external force. It also introduces one or more constraint equations motivated by the nature of the time-dependent driver. We show that these constraints can be generalized, in analogy to gauge theories in physics, and that different constraints can offer conceptual advances and methodological benefits to the solution of the underlying problem. Examples are given from linear ordinary differential equation theory and from orbital mechanics. However, a slow driving force in the presence of multiple time scales contained in the underlying (homogeneous) problem nevertheless requires special care, and this has strong implications to the analytic and numerical solutions of problems ranging from celestial mechanics to molecular dynamics.Item Micro- and macro-scopic models of rock fracture(2002-03) Turcotte, Donald L.; Newman, William I.; Shcherbakov, RobertThe anelastic deformation of solids is often treated using continuum damage mechanics. An alternative approach to the brittle failure of a solid is provided by the discrete fiber-bundle model. Here we show that the continuum damage model can give exactly the same solution for material failure as the fiber-bundle model. We compare both models with laboratory experiments on the time dependent failure of chipboard and fiberglass. The power-law scaling obtained in both models and in the experiments is consistent with the power-law seismic activation observed prior to some earthquakes.