- #1
chingel
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Homework Statement
I tried to reach the Bernoulli principle this way:
Two pipes are connected, one has a cross-sectional area of [itex]S_{1}[/itex] and speed of [itex]v_{1}[/itex]; [itex]S_{2}[/itex] and [itex]v_{2}[/itex] for the other. The pipes are horizontal, the connecting wall between them at the crossing from one pipe to the other is vertical, water flows from the first pipe to the second, the first one is larger.
The Attempt at a Solution
In any second, a mass of fluid [itex]m=S_{2}v_{2}\rho=S_{1}v_{1}\rho[/itex] has gone from the larger pipe to the smaller and its velocity has changed by [itex]dv=v_{2}-v_{1}[/itex]. Therefore the change in momentum is [itex]F=dmv=S_{2}v_{2}\rho(v_{2}-v_{1})[/itex]. This force comes from the differences in pressure between the two regions and since the size of the connection between them is [itex]S_{2}[/itex], because that is the area of the smaller pipe, then the difference in force at the connection is [itex]dF=S_{2}(P_{1}-P_{2})[/itex]. Equating the two forces and doing some simplifying, I get that [itex]\frac{P_{1}}{\rho}+v_{1}v_{2}=\frac{P_{2}}{\rho}+v_{2}^{2}[/itex], which is clearly the wrong answer.
In Wikipedia, there is the derivation using conservation of energy:
http://en.wikipedia.org/wiki/Bernoulli's_principle#Derivations_of_Bernoulli_equation
However, I would like to know where did I go wrong and why did it give the wrong answer.