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Fluid mechanics,interesting problem

  1. Apr 14, 2012 #1

    pcm

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    1. The problem statement, all variables and given/known data
    An open-topped vertical tube of diameter D is filled with water up to a height h.The narrow bottom-end of the tube ,of diameter d, is closed by stop .
    when the stop is removed ,the water starts flowing out through the bottom orifice with approximate speed v=√(2gh). However ,this speed is reached by the liquid only after a certain time t.obtain an estimate of the order of magnitude of t.Ignore viscous effects.


    2. Relevant equations
    equation of continuity(mass conservation), A.v=constant
    bernoulli's equation, P+0.5ρv2+ρgh =constant
    the total energy,potential +kinetic ,of water is conserved.

    3. The attempt at a solution
    i assumed that i) water level falls with "constant" acceleration a.
    then in a small time dt, i found the mass of water that came out of the hole,and decrease in water level.this gives me the loss of potential energy.i know velocity at the end of that small interval,i used it to find kinetic energy.using energy conservation ,i got a=g.
    i don't know how to proceed now. also i am not sure about my first assumption.and once the liquid starts falling out at v=√(2gh),will this acceleration magically disappear? does the fact that a=g,implies normal force from the bottom surface is 0 during the acceleration period?
     
  2. jcsd
  3. Apr 14, 2012 #2

    BruceW

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    Homework Helper

    I'm not sure about your assumption either. But I'm not certain what the answer is meant to be. Are you allowed to look up information for this question? I know what my guess would be for the answer, but it would require me to look up one fact online.

    Anyway, I'd start by thinking about the equation v=√(2gh) where does this equation come from? and keeping this in mind, why do you think it would require a short amount of time before it started to be true?
     
  4. Apr 15, 2012 #3

    pcm

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    no, i am not allowed to use other information.i am required only to give a rough estimate, taking reasonable values for the parameters,if required.numerical prefactor need not be accurate.

    i get v=√2gh ,by applying bernoulli's theorem at 1)a point just inside the water from top.
    2)apoint just outside hole.
    P1=P2=atmospheric pressure
    v1can be neglected at first,in comparison to v2.
    therefore, after cancelling pressures and rearranging,i get, v2=√2gh.
    now ,D2v1=d2v2,
    this gives v1=(d/D)2√2gh.
    it would require a short amount of time before it started to be true,because velocity cannot change instantaneously.

    this is how i got a=g:-
    (a is the acceleration of water in the tube,acceleration of water coming out would be a(D/d)2)
    mass of water that came out in time dt after opening the stop=0.5a(dt)2*0.25πD2water
    height descended =0.5a(dt)2
    velocity at time dt=a(dt).
    now using energy conservation ,
    mass*height descended*g=0.5*mass*(velocity)2.
    from there i get a=g.
     
  5. Apr 15, 2012 #4

    BruceW

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    I think you get a=g because the assumption you're effectively making is that the water is in free-fall. This is good, it gives us the parameter g. You need to get a timescale. And you have the parameter g and the height h. So from these two parameters, what powers should they be raised to, to give a parameter of time?

    Also, the reason I was asking about the equation v=√2gh was because this works by pressure being transmitted through the water, forcing water to come out at that speed. So my guess was that you could use the parameter of the speed of a pressure wave in water. But, you said that you can't look up this value, so I think it's best to ignore this idea.
     
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