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Flow in a streamtube
However for the difference
E
kin
(t+Δt) - E
kin
(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,
ΔV
1
and
ΔV
2
. Consequently
E
kin
(t+Δt) - E
kin
(t) = E
kin
(ΔV
2
) - E
kin
(ΔV
1
)
streamtube