Bernoulli’s equation please help , straightforward calculation  ?

I know how to solve this and I understand that the velocity is gonna be equal at both A and B is the terms including them in the equation will be canceled out, but while solving this question I realized that I don’t really understand what the terms mean. We did P1-P2=1/2p(v2^2-v1^2) 

Then I used the pressure difference to calculate h. What does P1 represent though ? And 1/2pv1^2 ? 

I know how to derive the equation but I still don’t get it in terms of pressure. 

Also, Why isn’t the pressure at points A and B equal ? Isn’t pressure =pgh? They’re at the same level so where did the pressure difference come from ? 

Could someone please answer me clearly, I just want a better understanding of pressure of fluids in general. 

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2 Answers

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  • 1 month ago

    When ever you are dealing with a fluid flowing in a pipe there will be friction losses that reduce the pressure as the fluid flow in the pipe. Most of the friction loss is from the interaction of the fluid with the surface of the pipe and some comes from the turbulence of the fluid. If the pressure at A and B were the same there would be no flow. 

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  • Vaman
    Lv 7
    1 month ago

    D seem to be correct answer. I assume the cross section at A is 16 cm^2

    The first condition

    area*v = constant. This gives the velocity at B= 25*2=16*v

    v=25*2/16=3,15.

    Pressure change is

    p+1/2 density *area*v^2= constant.

    p1+1/2*1*25*4= p2+ 1/2 *1*16 *3.15^2

    p1-p2=29.38. height difference = 29.38/13.6=2.16

    D is the correct answer.

    Check the method.

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