We present a logic for representing Newtonian-physics models of real world situations. The logic is a nonmonotonic temporal logic, with real valued fluents, where the time axis is the real numbers. We introduce three new concepts in this logic, which make axiom writing easier. The first concept is local interval operators which are used for quantifying propositions over time intervals. The second concept is a distinction between two different types of discontinuities, namely left and right discontinuities, which makes it possible to axiomatize momentary "chain reactions" in a convenient way. The third and most important new concept is explained discontinuities which implement our intuition that a discontinuity should have a cause. We discuss the rationale behind these concepts and define the semantics for the logic. We also discuss how to write explanation axioms. 1 Introduction There are many approaches to the problem of how to represent knowledge and reason about the physical world..
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