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A Nice Problem

  1. Nov 2, 2009 #1
    Here is a problem from a physics contest of two years ago. I've translated it from Persian but it might have some problems because of my lack of good English.

    Assume a water pipe is in the middle room and the water is going through it. How can we find out the direction of water without cutting the pipe?​

    Could anyone help me to solve it?

    P.S. Is "water is going through it" correct? What do you call it in English?
    Last edited: Nov 2, 2009
  2. jcsd
  3. Nov 2, 2009 #2


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    What can we use? Doppler ultrasound device?
  4. Nov 2, 2009 #3
    I think I understand.

    You can measure the flow of water from a distance by using the Doppler effect and spectroscopy. If you direct one laser perpendicular through the pipe, and another at, say, 60 degree angle from perpendicular, you can compare the absorption of the laser and calculate the velocity. This is because when there is a net flow of the fluid, the position of the spectral line is displaced- comparison of the two lines will give you the velocity.
  5. Nov 2, 2009 #4
    Is the pipe transparent?

    If not, we can measure the pipe diameters at two distant points. Due to pressure drop the downstream diameter will be smaller. The diameter difference will be small but still much larger than the Plank length.:wink:
    Last edited: Nov 2, 2009
  6. Nov 2, 2009 #5


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    Build a fire underneath the middle of the pipe and monitor the temperature at the two ends. The water flow should add a slight preferential direction to the heat flow.
  7. Nov 2, 2009 #6
    bang the pipe in the middle and measure the sound at either end, there should be a doppler effect due to movement of the water.
    the problem with this is that some (most?) of the sound will be transmitted through the pipe and not the water, but there should still be a different in the sound at either end if the equipment was sensitive enough

    I wonder if you could improve the technique by instead of banging, you could perhaps generate a specific type of sound that had harmonics with the water and not the pipe, although I appreciate that sound transmital has possibly nothing to do with harmonics, and perhaps more the density of the medium?....
  8. Nov 2, 2009 #7
    I would never have though that i will get such nice answers!
    thank y'all!!
  9. Nov 3, 2009 #8


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    I might be reconfirming my status as something of a moron, but I would just follow the pipe to see where the hell it goes. You need only trace one way, because it will end as either a source or a drain, and you will then know the direction of flow.
    The math stuff was pretty cool, but since I never finished high-school I tend to take a more 'hands-on' approach to things.
  10. Nov 3, 2009 #9
    Hi there,

    I might see the problem way to simple, but normally water flows downward. Therefore, you just need to measure the height of the pipe at both ends of the room.

  11. Nov 3, 2009 #10


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    To get water to flow uphill all we would need to do is raise the resevoir of the water's source higher than any segment of the pipeline. Then we can orient the pipe in any direction and still have water pressure. It is about the height difference. Then again, we could also use artificial means of generating water pressure via a pump.
  12. Nov 3, 2009 #11


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    Hey there, Fatra.
    Your answer is quite logical but for one thing. I don't know anything about plumbing in the Middle East, but here in North America our water systems are pressurized. Water flows up, down, and sideways at equal speed. In a gravity system, your response will be correct.

    edit: Well B2W, you beat me to it. And your mention of an elevated reservoir was something that I overlooked. (Kind of weird, since the house that I lived in during the early 60's had a water tower pump-fed from a well.)
  13. Nov 4, 2009 #12
    just thought of another, you may be able to freeze the pipe in the middle thus creating a blockage. Then it is a simple matter of identifying which side is hollow and which is full of water (a few taps should be enough to confirm it).

    If you could not freeze it enough to create a complete blockage, the flow will probably be effected enough to measure the difference somehow anyway
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