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Reversed water sprinkler

  1. Aug 9, 2009 #1
    this is quoted from feynman's "surely youre joking, mr. feynman"


    "i once did an experiment in the cyclotron lab at princeton that had some startling results. there was a problem in a hydrodynamics book that was being discussed by all the physics students. the problem was this: you have an S-shaped lawn sprinkler- and S-shaped pipe on a pivot- and the water squirts out at right angles to the axis and makes it spin in a certain direction.everyone knows which way it goes around; it backs away from the outgoing water. now the question is this: if you had a lake, or a swimming pool-a big supply of water- and you put the sprinkler completely under water, and sucked the water in, instead of squirting it out, which way would it turn?"


    so, which way would it spin?
     
  2. jcsd
  3. Aug 9, 2009 #2

    russ_watters

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    The other way, but not as fast.
     
  4. Aug 9, 2009 #3

    rcgldr

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    The problem with the underwater sucking spinkler is that due to viscosity, the suction stream induces a larger stream of water that generates an opposing force onto the sprinkler parts, so the sprinkler will barely move, or perhaps move the "wrong" way. In addition, the flow inside the sprinker pipes is curved and this generates a force and torque in the "wrong" direction as well.
     
  5. Aug 9, 2009 #4
    Think about it like a pinwheel - wind going one way spins it that way, wind going the other spins it the opposite. It would indeed spin in the opposite direction, but the viscosity of the water would keep it extremely slow.
     
  6. Aug 10, 2009 #5
    that makes sense.thanks!
     
  7. Aug 10, 2009 #6

    rcgldr

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  8. Aug 10, 2009 #7

    mgb_phys

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    I had this in an interview once and the 'correct' answer was that it spins the same way.
    The logic being that in normal mode the thrust comes from the reaction of the water against the closed end of the tube opposite the nozzle (just like in the simple explanation of a rocket engine) .
    In the suck case - the water would rush in and hit the wall opposite the nozzle in the same way (although flowing in the opposite direction) there is still a change in momentum in the same rotation direction.
     
  9. Aug 10, 2009 #8

    russ_watters

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    The water has a momentum toward the nozzle and as a result of conservation of momentum, the nozzle has a momentum toward the water: the net angular momentum of both systems is zero. I think the answer given to you is wrong.

    In any case, I don't like such intentionally argumentative and misleading questions.
     
  10. Aug 10, 2009 #9

    mgb_phys

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    I was picturing it (the suck case) as equivalent to an empty nozzle having high pressure water blown into the open end - that would seem to make the nozzle move with the flowing water, like drinking from a firehose.


    I was on the receiving end of this one - I guessed it was the same direction largely on the basis that the only reason they would ask this sort of question is that the answer was counter intuitive!
     
  11. Aug 10, 2009 #10
    or perhaps i asked the question because i didn't know the answer and genuinely wanted to...
     
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