Energy and reference frames

Classical thermodynamics ignores microscopic properties such as the behavior of individual atoms and molecules. Nevertheless, a consideration of the classical mechanics of particles will help us to understand the sources of the potential and kinetic energy of a thermodynamic system. 

In classical mechanics, the energy of a collection of interacting point particles is the sum of the kinetic energy of each particle (where m is the particle s mass and v is 

The positions and velocities of particles must be measured in a specified system of Cartesian coordinates called a reference frame. Since the kinetic energy of a particle is a function of velocity, the kinetic energy depends on the choice of the reference frame. A particularly important kind of reference frame is an inertial frame, one in which Newton s laws of motion are obeyed (see Sec. G. 1 in Appendix G). 

A reference frame whose axes are fixed relative to the earth s surface is what this book will call a lab frame. A lab frame for all practical purposes is inertial (Sec. G.IO on page 503). It is in this kind of stationary frame that the laws of thermodynamics have been found by experiment to be valid. 

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