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Question
In the steady flow, what is the factor that  decreased the pressure and increased the velocity when the fluid flows from larger cross sectional to smaller one although there is no external force

Answer
Hello Mohamed Asem,

when the cross section of a steady flow is reduced, then the velocity must increase to provide for the same volume to pass a point in a given time unit. This is just conservation of mass flow.

As a consequence from the increase in velocity, the pressure decreases according to Bernoulli's principle, which is essentially just a statement that the sum of kinetic and potential energy of the flow stay constant (conservation of energy).

See here for a more detailed discussion:
http://en.wikipedia.org/wiki/Bernoullis_principle

I hope I could help you a bit.

Peace,
 Kevin

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Kevin Johnson

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I will try to answer any question in physics, but I do not provide homework solutions or project ideas. I have some teaching experience at university level but I am also skilled at explaining things in simple terms. My specialty areas are granular materials, computational physics, particle physics, quantum physics, econophysics and general physics. Questions in GERMAN are welcome, too.

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I have graduated in physics, specialized in theoretical particle physics and quantum field theory, worked in the area of econophysics and am currently working on my PhD in granular materials and computational physics. I have some tutoring and teaching experience at University level.

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German Diploma in Physics (equivalent to M.Sc)

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