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Why is blowing stronger than sucking?

  1. Jun 22, 2013 #1
    If you blow on something with your mouth, you can blow it pretty hard. Like if you had a paperball on a table, you could blow it pretty far. But if you tried to suck in air to bring it towards you, it would be incredibly difficult, and you'd have to be extremely close to it.
    The same seems to apply to mechanical devices. We can make devices that blow extremely hard. Leaf blowers can blow leaves a long way, but there aren't any device that can suck leaves in from that far away. To suck anything up, you have to be pretty close to the object. To blow it, you don't have to be close at all.

    Why is sucking and blowing so different?
     
  2. jcsd
  3. Jun 22, 2013 #2

    rcgldr

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    Blowing is directional, the momentum of the blown air tends to continue to travel in the same direction although it still disperses. Sucking isn't directional, reducing pressure will draw air inwards from all directions not blocked by some object(s), so the velocity versus distance from the input port is less.

    On the other hand, take the case of a bus traveling down a highway. At the front of the bus instead of the air getting trapped and forming a large stagnation zone in front of the bus, the air tends to be diverted around the bus, with only a relatively small stagnation zone at the front of the bus. At the back of the bus, the air's momentum prevents it from following the trailing edge corners to fill in what would otherwise be a void left in the wake of the bus, so there's a relatively large low pressure area aft of the bus, accelerating air forwards, and most of the drag on a bus is due to the low pressure area aft of the bus. In this case sucking is stronger than blowing.
     
  4. Jun 22, 2013 #3
    Maybe the question is better answered by referring to human anatomy.Blowing involves breathing out and sucking involves breathing on. I just tried it and found that I could breathe out quickly and comfortably but it seemed that when I breathed in I couldn't do it as quickly and the experience was very slightly uncomfortable.
     
  5. Jun 22, 2013 #4
    Besides what others have said:

    Sucking means to decrease the pressure below atmospheric pressure and use the difference to move something. The maximum pressure difference you can create is equal to 1 atm (if you create a good vacuum).

    When blowing there is no such limit. You can blow air with many atmospheres of pressure.
    I mean by using a blower or a compressed gas cylinder.
     
  6. Jun 23, 2013 #5

    CWatters

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    The way air, or any fluid, flows around an object (or through a nozzle) depends on the shape of the object. For example most aircraft can't fly backwards because the whole design of the plane is optimised for airflow in one direction.

    Similarly the air flow through a nozzle (like your mouth) is very different when the air flow is in the "wrong" direction.
     
  7. Jun 23, 2013 #6

    sophiecentaur

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    There's a good physiological reason why humans find it easier to 'blow' than to 'suck' large volumes of air (in and out of the lungs). The diaphragm evolved with just enough muscle to draw sufficient air in (suck) under maximum heavy breathing conditions. If you try to breathe through a tube when you are under water, you can't do it with your diaphragm under more than a few tens of cm under the water. I have tried it using a swimming pool ladder and a tube and it is unbelievable just how shallow the water depth needs to be, in order to breathe. Long snorkels just don't work at all. All the other abdominal / thoracic muscles can contribute to creating excess pressure; the diaphragm is not needed - so we can 'blow' harder (but still much less than a metre head of water).

    Otoh, if you are talking in terms of sucking and blowing small volumes, using your sealed mouth alone (tongue, cheeks, etc.) then I reckon you can produce much bigger pressure differences, either way, but you can produce a better negative pressure within the mouth than excess pressure. I think this is more to deal with the quality of 'seal' you can produce around a tube on suck than on blow.
     
  8. Jun 23, 2013 #7
    Wow, all this discourse and nobody answered the question?
    The reason sucking is weaker than blowing has to do with the diaphragm. When we inhale, the diaphragm relaxes. There is only so much the diaphragm can relax. No matter how strong your diaphragm is at pushing out air, its ability to suck in air remains about the same. Pushing out air requires the flexing of the diaphragm, and some people have some pretty strong diaphragms, in which case they can blow with a great deal of strength.
     
  9. Jun 23, 2013 #8

    sophiecentaur

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    Not in my biology book, Dan. It is dome shaped (inverted) over the stomach and, when the muscles tighten, it goes flatter (reducing its area by muscle tension) and increases the volume of the lungs. The ribs prevent the chest from collapsing when this happens. When you force air out, you use ribs and stomach muscles - put your hand on your tum and experiment.
    Also, didya know that women tend to use their ribs for breathing more than men because the diaphragm is less use when you're (they're) pregnant and the abdominal cavity is crammed full of baby.

    See here
     
  10. Jun 23, 2013 #9
    Whoops
     
  11. Jun 23, 2013 #10

    DrDu

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    I remember a story by Feyman (From "Surely you are joking" or the like). He had an S shaped tube with two nozzles on the ends and an inlet in the center. He measured the rotational momentum when water was pressed out at different pressures. Then he had the idea to place it into a large water bottle and raise the external pressure instead to see whether sucking would have the same effect. He never mentioned whether the two are theoretically equivalent or not but only that the water bottle exploded and messed up a whole archive of films of particle collision tracks.
     
  12. Jun 24, 2013 #11

    sophiecentaur

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    Having seen some of the rubbish demo equipment that is available in schools (plastic bag and bell jar), I can appreciate that it's easy to get the wrong message. It's essential that the plastic diaphragm is 'concave' to be a good model.
     
  13. Jun 24, 2013 #12

    russ_watters

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    Since the OP linked this issue to be the same for the human body as for mechanical devices, the more relevant answer has nothing to do with physiology. Rcgldr's simple answer is correct: blown air is coherent while sucked-air is not.
     
    Last edited: Jun 24, 2013
  14. Jun 24, 2013 #13

    sophiecentaur

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    I'm not sure of what is the meaning of "coherent air". The reason air flows is a pressure difference, however it's generated. If you include a nozzle of some sort then that could affect the appreciation of the situation but how does the air 'know' which side is doing the action to make it flow? Who was sucking and who was blowing?
    There are 'obvious cases', I suppose, but do they actually indicate anything, other than 'usage' of the language?
     
  15. Jun 24, 2013 #14

    russ_watters

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    Try examining a bare table fan. You can feel a coherent airstream in front of it from across the room, but a foot behind it, you feel nothing. That's the effect the OP is asking about.

    And the answer to why the air behaves completely different is that there are two completely different processes making it move.
     
  16. Jun 24, 2013 #15

    sophiecentaur

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    I know what you mean, of course. The problem is the usual one - what we 'call' things. And, boy, how many threads just boil down to that? The word "suck' is very vague so, perhaps it would be best just to discourage its use rather than to try to nail it down with a half cocked definition.
    Air can be made to flow by interacting with a solid object. In that case, you could say that there is localised high pressure and velocity, giving more directed flow than when air is caused to move by a broad front of low pressure difference, which could be what happens with 'sucking'. It has a sort of parallel with Near-Field -Far -Field considerations with radio antennas.
     
  17. Mar 1, 2015 #16

    sophiecentaur

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    Biologically speaking, it's down to evolution. Our diaphragms developed to be only strong enough to get enough air for respiration. As with all of our body 'design', it's just enough to do the job and we never expect any pressure difference more than when we are immersed in water with our diaphragm at a few tens of cm under water, say. Otoh, we need to be able to cough with much more force, in order to shift obstructions and mucus. When we do that, we use muscles all over our torso.
     
  18. Mar 1, 2015 #17

    russ_watters

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    This thread is 2 years old and was resurrected by a troll (not you, sophiecentaur), so I'm locking it.
     
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