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Collision of liquid with floor

  1. Oct 12, 2013 #1
    Hi friends,
    I have an issue in solving a Collision of liquid with floor.
    Please Help me in solving this.
    Thank you all in advance.

    The problem is as:

    https://fbcdn-sphotos-d-a.akamaihd.net/hphotos-ak-ash4/q71/s720x720/1379505_1432382913655481_500299018_n.jpg


    Attempt:

    https://fbcdn-sphotos-a-a.akamaihd.net/hphotos-ak-prn2/q77/s720x720/1383432_1432380890322350_1121703847_n.jpg

    Friends according to question Option (A) is correct. Please try to help me in this.
    I will appreciate the help.
     
  2. jcsd
  3. Oct 13, 2013 #2

    Simon Bridge

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    When the tap is first turned on, there would be some reaction but while the water is gushing, there is no change in momentum.
     
  4. Oct 13, 2013 #3

    haruspex

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    To answer the question, you have to presume that the hose reduces in diameter greatly in the section of pipe the gardener controls. Thus, it effectively has no momentum before reaching that point. All the momentum the water leaving the pipe has therefore comes from a force supplied by the gardener.
    (A dropped firehose thrashes around in a dangerous manner.)
     
  5. Oct 13, 2013 #4
    $$F=\frac{d(mv)}{dt}$$
    $$F=v.\frac{d(m)}{dt}$$
    $$F=v.\frac{d(A.v.ρ)}{dt}$$
    $$F=v.A.ρ.\frac{d(v)}{dt}$$
    But once the water is constantly flowing ##\frac{d(v)}{dt}=0##.

    Like Simon Bridge said,

     
  6. Oct 13, 2013 #5

    haruspex

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    Looking at the diagram, there may be another interpretation. If we take that right-angled turn in the hose as accurate, the gardener must supply the force to turn the flow of water through that angle.
     
  7. Oct 13, 2013 #6
    But looking at the scale of diagram, no living man could support the tank!!!!:tongue:
    The tank must be supported by some other structure. So the force you are talking about must be balance by the rigid tank itself, not the man.
    The diagram shows, I think, that the tank is rigid till the (white-black) pipe junction and black portion which could be assumed as flexible is straight, so no force should be acting on the man.
     
  8. Oct 13, 2013 #7

    haruspex

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    Ha! Just realised I misread the OP as saying zero was the wrong answer, so I've been scratching around for other interpretations unnecessarily. Sorry for the noise.
     
  9. Oct 13, 2013 #8
    :tongue:
     
  10. Oct 13, 2013 #9

    Simon Bridge

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    The temptation is probably to think of the hose as being like a rocket... it's throwing mass out one way so the hose must move back the other way.
     
  11. Oct 14, 2013 #10
    Yes, but its just a temptation!!!
     
  12. Oct 14, 2013 #11
    Anyone who has handled a fire hose knows that this is in fact the case.

    Or just think about it - if you mount the whole system in the diagram on wheels would you not expect it to accelerate to the left? Where do you think the force that causes this acceleration acts?
     
  13. Oct 14, 2013 #12
    Oh I've just noticed the answer is supposed to be zero too - in that case the question is wrong. I notice that the English is not correct (waters the plants by a pipe), perhaps there is something else that has been lost in translation. Or perhaps the hose is in fact rigid so absorbs all the nozzle reaction itself.
     
  14. Oct 14, 2013 #13
    the tank should be connected to a rigid support other than the man, and seeing the scale of the diagram it clearly is the case. In reality, many other complications arise thats why force is non-zero. only the black portion is the hose, which is straight so there is no momentum change and hence no force is exerted.

    P.S. : The assignment carries a logo of MIET, located in India. English isn't their primary language. Nor is mine.:tongue:
     
  15. Oct 14, 2013 #14

    Simon Bridge

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    ... and they tend to be a little glib in their descriptions.
    See the other threads from this member off the same test/assignment :)
     
  16. Oct 14, 2013 #15
    yeah, they haven't cared to mentioned crucial information. Its up to the reader, to make assumptions!!!
     
  17. Oct 14, 2013 #16

    Simon Bridge

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    OTOH: we don't know the preparation - sometimes the school does "example questions" in class and the whole thing becomes a memory exercise... or the missing info is effectively provided in class.
     
  18. Oct 14, 2013 #17
    Well yes, the only way the force at the nozzle can be zero is if there is no change in velocity at the nozzle so the whole pipe must be 1mm internal diameter (to be fair this is what it says, but against that is (i) the loose grammar means that you can't rely on any statement; (ii) whoever heard of watering a garden with a 1 mm bore pipe; (iii) I can't be bothered to calculate the pressure drop in 1 m of 1 mm pipe at 12+ m/s flow, but I bet the header tank would have to be a LOT taller than shown in the picture to sustain this).

    Trick questions like this are not IME a good way to reinforce learning, but if you are going to set a trick question you'd better make damn sure it is right, otherwise the joke is on the person setting the question.
     
  19. Oct 14, 2013 #18
    It could be but nonetheless question is loosely constructed as MrAnchovy already pointed out!!

    This is regular here, even most of the regular course books include these kind of questions!!!
     
  20. Oct 14, 2013 #19

    Simon Bridge

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    Of course. I have said nothing to contradict that - I'm just not arrogant enough to armchair quarterback what happens in a classroom on the other side of the World.

    It is quite something to wrestle with when you get course material like this - however it was constructed.
     
  21. Oct 14, 2013 #20
    Yes, it is. But it is so common with Physics that "most of us" associate it with difficulty of Physics. Its just that the variety of questions that can be create related to a particular topic is so vast, that the one with even a little doubt in basic concepts will face huge problem solving these kind of questions.
     
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