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Flow in a streamtube



However for the difference

Ekin(t+Δt) - Ekin(t)


we do not have to consider that part of the streamtube where nothing has changed, but only the small volumes of difference at the extremes of the amount of fluid under consideration, ΔV1 and ΔV2. Consequently

Ekin(t+Δt) - Ekin(t) = Ekin(ΔV2) - Ekin(ΔV1)






streamtube