On Tue, 15 Jan 2002 19:44:46 John Delacour wrote: >If we consider our body, the bridge, as a single vertical "ray" or >line of particles rather than a three-dimensional mass of particles, >it will be easier to explain. The particles of an elastic body have >a certain relationship to each other in space when they are in >equilibrium. If a homogeneous elastic body is heated, for example, >the body, as we all know, will expand and the particles be more >distant from each other. The bond, i.e. the atomic force, between >them becomes weaker. At a certain temperature, the bond will become >so weak that the body liquefies, and at a higher temperature this >fluid will vaporize. In the cases we are considering, we are forcing >the particles of the body to take up different positions in relation >to each other. We are opposing a force or pressure to the forces >that holds them in equilibrium. So long as we don't overdo it, much >of the energy we have expended in stretching the wire or compressing >the bridge, or whatever, will remain as strain energy in the body, >and this will be converted to kinetic energy, when our force is >removed, to restore the body to its original shape. All this is to >emphasize that any elastic body, by definition, is composed of >particles that can to a degree (that being the elastic limit of the >material) be moved closer together or further apart by an imposed >force. > >To return to our bridge consisting of a vertical "ray" of particles >and let me return to an analogy I used earlier -- that of a >transparent tube containing a stack of little cylindrical magnets >disposed North<>South<>S<>N<>N<>S<>S<>N etc. Let the magnets >represent particles of an elastic medium and the repulsive force >between them the atomic forces between them. This ray of magnets is, >of course, compressible. If we close the bottom of the tube so that >the bottom magnet can't move, and press down on the top magnet, we >oppose our force to the magnetic force and the magnets are pressed >closer together. If we move the top magnet a certain distance and >maintain the pressure, the magnets will (very fast but >_not_instantaneously_) take up different positions in respect to each >other. Each magnet, or particle will have been displaced a different >amount and the ray will be in a state of compression. > >I'll break off there and continue the analogy once you have accepted >that this is a clear description of a demonstrable experiment. If >there are any problems with it, I'll try to make myself clearer in >response to your queries. So far, of course, I have attempted only >to draw a picture of a one-dimensional bridge, which will take us >quite a long way and is easy to visualize. > >JD > John, I don't have a problem with any of this. Please proceed. Phil F
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