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In the above picture we select a certain amount of fluid
which passes through the tube.
At a certain instant of time t
that amount of fluid occupies
the space C of the tube.
On the lefthand side that amount of fluid is pushed
by the upstream fluid with a force
F1.
On the righthand side that amount of fluid pushes
the downstream fluid with a force
F2.
In the picture below we show the same amount of fluid but at a later instant of time t+Δt when it occupies a different piece of space C' of the tube. |
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| Notice that the volume of the space C of the tube is equal to the volume of the space C' of the tube. |