Bernoulli's equation.

The fundamental principles of conservation of energy apply in fluid dynamics. The expression of conservation of energy for an ideal fluid takes the form of Bernoulli's Law, which tells you how energy interconverts within the flow. One way or another, Bernoulli's Law finds its way onto a sizable portion of MCAT exams.

The energy of a fluid in motion can take various forms. An indication of the energy within a portion of the flow can be seen in the height of the flow-line, the speed of the flow, and the pressure within the moving fluid. Within some volume elements of the flow, the energy manifests as flow speed. Somewhere else, the energy may manifest as pressure. And at another position, the energy manifests as height.

As one form of energy decreases, one or both of the others must be increasing. Where the flow speed is high in a level flow, the pressure must be lower than another portion of the flow at that same level where the flow speed is low. Lower flow speed at the same level means higher pressure in an ideal fluid.
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