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Why does breathing out have more effect on surroundings?

  1. Dec 19, 2015 #1
    Theres a candle in front of me while ive been working. It got me thinking; why does it take so little to blow the candle out, but if i inhale near the candle almost nothing happens. I guess more generally than this, why does suction have such a small impact on its surroundings compared to blowing?
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  3. Dec 19, 2015 #2


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    At most you can create a vacuum in your mouth when inhaling (and I doubt that very much). Thus: max inhale pressure differential: 105Pa. When blowing, you can create a much higher pressure differential.

    It is the same with suction pumps and pressure pumps: You can only suck water up to ≈10m (vacuum in the pump, 105Pa outside), but there is only a mechanical and practical limit to the height a pressure pump can push water.
  4. Dec 19, 2015 #3


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    When air is blown from a higher pressure source, the flow is directional (assuming that a dispersal type nozzle is not used). When air is drawn into a lower pressure zone, the flow is not directional, but instead is drawn inwardws from all directions, so the net flow in any specific direction is much less.
    Last edited: Dec 20, 2015
  5. Dec 19, 2015 #4
    I also think that, for biological reasons, breathing out is a stronger reaction than breathing in. From what I remember, expelling carbon dioxide is more critical than taking in oxygen.
  6. Dec 19, 2015 #5


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    That's a good explanation! To repeat/paraphrase: Blowing out creates momentum in the volume of air expelled, the directionality and coherence increases its effect. Breathing in only creates coherent directionality in the air going down your windpipe. I'll check out some of the other posts by Rcgldr. Also it was a good question by Sharker---hadn't ever occurred to me to ask.
  7. Dec 19, 2015 #6


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    naaaa, disagree, am sitting here reading this thread and my inhaling and exhaling are very even in strength

  8. Dec 19, 2015 #7
    have to agree with davenn, surely you breathe the same amount in as out, and you pretty much control the force. But the momentum explanation makes sense, thanks.
  9. Dec 19, 2015 #8
    there is a simple test using a U tube manometer filled with water (standard school demo) Blow into one end of the manometer....note the height difference(pressure)
    Suck on the manometer, note the height difference (pressure).....let me know which is greater !!!
    (the manometer needs to be about 2m tall)
  10. Dec 19, 2015 #9
    Breathing in creates just as much momentum, but outside of one's body cavities it is diffused over a larger volume so the velocity of the air is generally less.
  11. Dec 19, 2015 #10
    the fundanental thing is that you can 'blow harder than you can suck'..... this is so easy to check with a pressure gauge...
  12. Dec 19, 2015 #11


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    You might be able to blow out harder than you can suck in, but that's not why you can blow out a candle but you can't 'suck' out a candle. The reason is that the outgoing air has momentum, just as explain in posts 3, 5, and 9.
  13. Dec 19, 2015 #12


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    As others said, even you blow at the same rate as you suck in, the blowing will still be more effective in putting out the flame.

    Put it another way: When you suck in air through a barely opened mouth, you feel a strong stream of air hitting your tongue that could put out a candle. So the issue not our limited suction strength.
  14. Dec 19, 2015 #13
    I think that it is agreed that you can blow harder than you can suck....in pressure terms
  15. Dec 19, 2015 #14


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    No one said otherwise. It just doesn't have much to do with why you can blow out a candle but can't put one out by inhaling.

    One other thing to think about for those in the thread, is that when you blow out a candle you're pursing your lips together and forcing the air through a small orifice, which greatly increases the velocity of the air compared to inhaling. Trying to blow out a candle by exhaling through a wide open mouth isn't nearly as effective.
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