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Particles are sets of entities -- sometimes fixed (atoms in a crystal) or sometimes moving (galaxies in a universe)
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They are characterized by force Fij on particle i due to particle j
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Forces are characterized by their range r: Fij(xi,xj) is zero if distance |xi-xj| greater than r
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Examples:
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The universe
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A globular star cluster
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The atoms in a crystal vibrating under interatomic forces
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Molecules in a protein rotating and flexing under interatomic force
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Laws of Motion are typically ordinary differential equations
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Ordinary means differentiate wrt one variable -- typically time
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