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Definition:
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The principle that mass in a system is neither created nor destroyed, expressed by the relation the rate of change of mass in an element of the system equals the rate at which mass enters the element minus the rate at which mass leaves the element. The definition rests on the convention that all flows are positive in the positive coordinate direction. With the commonest alternative convention, that inward flows are positive and outward flows are negative, the definition would be read with 'plus' replacing 'minus'.
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