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By blowing at pencil, a piece of paper, or another object up to fifty centimeters away, I can cause it to move away from me significantly. But I can't move an object toward myself by inhaling sharply from that distance, even if it is extremely light. Why is that?

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4 Answers 4

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I think the reason is that when you are blowing on an object, you are making lots of air particles collide with it perpendicularly in one direction thus transferring a lot of momentum to the object. When you are sucking air in, the only force that's acting on the object is by the air particles that rush in to fill up the gap that you just created. These particles come in from all kinds of different directions failing to transfer momentum in a consistent way.

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I think the reason is twofold.

First, when you are inhaling the air goes from an unbounded domain, to a bounded domain, for exhaling this is the other way around. So for exhaling the air gets accelerated in the hole between the two domains (your mouth), because of continuity (contraction). When you inhale, there is no such requirement, there is only deceleration.

Second (also mentioned by Vinayak), is the direction. When you exhale, you can consider the flow of air as a turbulent jet. It is known for turbulent jets that the spreading angle is about $20^\circ$ (this has to do with turbulent diffusion of momentum) which is generally true for all fluids. This is the reason that after this $50cm$ you cannot blow anything away: the jet is wider, and thus weaker/lower velocity.

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That's what I was going to say, for the same reason the flow around a fan is not symmetric fore and aft. Going into the fan it is simply being drawn from all directions, but coming out of the fan it has momentum in one direction, so forms a jet. –  Mike Dunlavey May 7 '12 at 22:41
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Physiology probably makes up for another good part of the effect: The lung is more easily compressed than expanded because muscles primarily work by contraction (and the skeleton´s muscles are more or less wrapped around the chest).

Also, compressing an enclosed volume (e.g. a paperbag) is easier than it´s expansion because the volume´s surface is under tension when compressed (which stabilizes the process) but becomes unstable when expanded (you have to pull in a lot of directions, while compression can be applied at one point).

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I may be mistaken, but I think the mechanism of inhalation is not from the skeletal muscles around the rib cage, but from the diaphragm. The diaphragm pulls downward, expanding the thoracic cavity and creating a partial vacuum; the pressure differential between the inside and the outside forces air into the lungs. –  MJD May 7 '12 at 13:11
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@MarkDominus You also expand the rib cage (have to so that you can contract it again later...). Indeed some martial arts instructors have students practice trying to separate the two mechanisms which can be useful when you're in a lock or hold that impedes just one of them. –  dmckee May 7 '12 at 13:20
    
@MarkDominus there are different kinds of breathing, but diaphragmatic is one of them, and I have a feeling the diaphragm is always taking part - but I´ll be happy to correct if anyone has knowledge on this one. –  Yves Klett May 7 '12 at 13:21
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You can also answer this by just considering the pressure difference between your mouth and the outside-- without even thinking about fluid dynamics!

The absolute max limit of pressure difference you can ever get by sucking-in is simply atmospheric pressure. In other words, the best vacuum generator in the world will, at best, give zero psi abs pressure whilst the outside is ~15 psi.

The absolute max pressure difference you can get when blowing air can be quite high-- several atmospheres (consider inflating a balloon, it must have a higher pressure than atmosphere to even be a recognizable balloon).

This applies even if you're considering the case of blowing or sucking through a straw.

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I am not sure if you can inflate at several atmospheres. A ballon does not need so much for inflation anyway - you just have the impression that your head is gonna blow away at some 1/10 atmospheres already. Perhaps some savvy anesthetist can come forward to clarify! –  Yves Klett May 7 '12 at 13:48
    
OK, maybe not several, but definitely MORE than one atmosphere. That is already bigger than the max achieveable pressure difference limit from sucking, minus one atmosphere. –  Angelo May 7 '12 at 13:53
    
According to [this](sensorwiki.org/doku.php/sensors/air_pressure) the maximal pressure difference under standard conditions is not much more than 3.3 psi or around .23 bar. You may get higher pressures using the closed mouth cavity but as far as the lungs go, that seems to be it... –  Yves Klett May 7 '12 at 18:02
    
@YvesKlett, ok so... 3.3psi above 1 atm. –  Angelo May 7 '12 at 18:31
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