Conservation Of Momentum

A macroscopic momentum balance for a flow system must include all equivalent forms of momentum. In addition to the rate of momentum con-vected into and out of the system by the entering and leaving streams, the sum of all the forces that act on the system (the system being defined as a specified volume of fluid) must be included. This follows from Newton's second law, which provides an equivalence between force and the rate of momentum. The resulting macroscopic conservation of momentum thus becomes

Note that because momentum is a vector, this equation represents three component equations, one for each direction in three-dimensional space. If there is only one entering and one leaving stream, then mi = mo = m. If the system is also at steady state, the momentum balance becomes

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