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Lowest frequency of a water with drinking straw

  1. Jul 20, 2014 #1
    1. The problem statement, all variables and given/known data

    the ans given here is 'using the opened pipe method'. but i solve it using closed pipe method. which i have sketched using the pencil on the right of the diagram. why cant i solve it using 'closed pipe method' , i just couldnt understand.

    2. Relevant equations



    3. The attempt at a solution
     

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  2. jcsd
  3. Jul 20, 2014 #2

    CWatters

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    A straw is open both ends. It would only be closed one end when you put it into the drink. That is not stated in the problem. How do you know how far the straw is inserted into the drink? If that information was provided you could work it out by treating one end open and the other closed.
     
  4. Jul 20, 2014 #3

    CWatters

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    PS Do you understand why you get a node at a closed end and an antinode at an open end?
     
  5. Jul 20, 2014 #4
    The air will vibrate at open end so it is antinode. The air wouldnt vibrate at closed end so it is node.
     
  6. Jul 20, 2014 #5

    BruceW

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    This is a good point. It is hard to tell if the straw is closed at one end, or open at both ends. This website http://www.phys.unsw.edu.au/jw/flutes.v.clarinets.html might help a bit. But anyway, to find out I guess you could physically try to get a sound wave to go through a straw. If your mouth is fully closed around the straw, then it is closed at one end. But if you can only make the vibrations when you don't have a tight seal around the straw, then it is open at both ends.
     
  7. Jul 20, 2014 #6

    CWatters

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    Problem says "blows at the end". I guess not 100% clear but I would take that to mean blow across the end from a distance like you would a bottle.
     
  8. Jul 20, 2014 #7

    CWatters

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    Perhaps worth looking at the foot note in that link Bruce posted..

    If you seal your lips around the outside a straw the cross section at that point isn't going to be reduced compared to the diameter of the straw. On the contrary the mouth is much larger. eg it's more like a Flute than a Clarinet. I reckon it would be more open than closed.
     
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