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Drawing up a liquid by forward motion of a tanker

  1. Mar 11, 2015 #1
    1. The problem statement, all variables and given/known data
    We are in a planet where the gravitational acceleration is 1 ms-2.
    Tankers fly over the surface of a liquid ammonia lake (at 239K) at a height of 40m. A special, insulated, frictionless tube is lowered into the ammonia 1m below the surface.
    We want to draw up ammonia through the tube into an insulated tank by the forward motion of the tanker without pumping.
    The tank is initially full of hydrogen at 5 bar and 239K. The hydrogen is displaced through a vent at the top of the tanker by the incoming ammonia. The maximum depth of ammonia in the tank is 10m (the bottom of the tank forms the base).
    What is the minimum speed the tanker must fly over the lake to fill its tank?

    2. Relevant equations
    1/2 mv^2=mgh???? F=ma???

    3. The attempt at a solution
    the total height the ammonia has to travel to reach the tank is 41m. Something to do with the height and gravitational acceleration? And the maximum height of ammonia? And the pressure of the hydrogen in the tank initially????
     
  2. jcsd
  3. Mar 11, 2015 #2

    BvU

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    Not much to go by, eh ?
    And what would the three question mark equation contribute to solving this problem ?

    Not much of an attempt at solution either: 40 + 1 = 41 m and that's all.
    What v does that give when substituted in the four question mark equation ?

    But the problem statement gives me the idea 40 + 1 + 10 m might be more appropriate !

    The story made me think of Pitot tubes
    Google led me to Bernoulli equation. But if that isn't in the preceding sections of your textbook or class notes, then perhaps all you have is Newton ?

    I also wondered at the 1 m/s2. But even more at the 5 Bar of hydrogen that has to be expelled. Does that mean that even at such low g the atmospheric pressure is 5 Bar ? Without further info I'd be inclined to assume that.
     
  4. Mar 11, 2015 #3

    mfb

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    Energy conservation is a good approach. Moving a drop of ammonia from the tanker to the surface of the lake should not release energy.
     
  5. Mar 11, 2015 #4
    i know theres something to do with bernoulli's equation/energy balances
    I'm just stuck on the fact that theres a forward motion involved. (force in x direction to push liquid in y direction)
    According to the other parts of the question I guess I am safe to assume the ambient pressure is 1 bar.
     
  6. Mar 11, 2015 #5

    BvU

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    What other parts of the question(s) ?
    And why the whole 5 bar story if just opening the vent can get rid of 80% of the hydrogen ? Or do they want the NH3 to stay at 5 Bar ?
     
  7. Mar 11, 2015 #6
    The other parts are irrelevant to this question so i just extracted the required information. And yes, they want the NH3 to stay at 5 bar.
    If i can differentiate between the excess information and the required information i probably wouldn't be here. Sadly.
    im not given density or atmospheric pressure.
     
  8. Mar 12, 2015 #7

    haruspex

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    Did you mean, raising a drop from the lower end of the pipe to the surface should not require energy? If so, I agree.

    Annnoyyying, I assume the lower end of the pipe points forwards, so acts as a scoop. You quoted a useful energy equation. Note that the m's cancel, turning it into a SUVAT equation.
     
  9. Mar 12, 2015 #8
    Yes the lower end of the pipe points forwards to act as a scoop. I originally suspected that particular energy equation as it does not require pressure or density terms like in the energy/Bernoulli's equation.

    Do you mind expanding a bit on this? My mechanics side of my brain has never been well developed.
     
  10. Mar 12, 2015 #9

    BvU

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    Means you add 40 m to 10 m and get the 1 m for granted :smile: haru was nice to me by not saying I totally overlooked that,
     
  11. Mar 12, 2015 #10
    So I just need to use 1/2 mv^2 = mgh, no bernoulli's? using that I get v = 10ms-1
     
  12. Mar 12, 2015 #11

    BvU

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    And that's from 50 m. The 5 Bar is out of the picture ?

    Actually, I have no idea, but: can planes fly at 36 km/h in a 1 m/s2 environment ?
     
  13. Mar 12, 2015 #12

    mfb

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    Same thing?

    They can. Birds can even do it on earth.

    Where did the hydrogen pressure go now?
     
  14. Mar 12, 2015 #13

    haruspex

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    No, the tanker is 40m above the lake surface; the lower end of the pipe is 1m below the lake surface. And moving it from tanker to lake would be opposite to the way it is flowing.
     
  15. Mar 12, 2015 #14

    mfb

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    In equilibrium, nothing flows, and "the energy difference is zero" is equivalent to both statements.
     
  16. Mar 12, 2015 #15

    haruspex

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    Then I find your original comment most confusing:
    That seems to say there's no PE change.
     
  17. Mar 12, 2015 #16

    mfb

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    Sure there is a change in potential energy. We still have the velocity => kinetic energy to take into account.
     
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