# Why is blowing so different than sucking? [duplicate]

Why is it so easy to blow out a candle from a significant distance, but nearly impossible to suck enough air to do the same?

Even without focusing the airflow through a nozzle or something, this affect seems to be present. For example, it's easy to feel the air coming out of a box fan, but very hard to feel the air going into one.

• This may have less to do with fluid dynamics and more to do with how the muscles surrounding your lungs are arranged. – probably_someone Feb 18 '19 at 19:22
• @probably_someone If your hypothesis is correct, you also have to explain why the same thing happens with a fan. – alephzero Feb 18 '19 at 21:43
• Possible duplicate of Why does blowing on a candle put it out but sucking doesn't? – Aaron Stevens Feb 19 '19 at 5:03
• Answers below seem to hide an obvious symmetry: Regardless of whether you "blow" or "suck", you have a situation where air is being forced through an orifice. The air enters the orifice from every direction on one side, but then the pressure difference accelerates it in a particular direction as it passes through. – Solomon Slow Feb 19 '19 at 14:35

Assume a constant density of the air $$\rho$$.

Consider an imaginary tube of air in the region of the candle, of area $$A_{\rm open}$$ and speed $$v_{\rm open}$$, being reduced to an area $$A_{\rm lips}$$ and speed $$v_{\rm lips}$$ at the lips.
Using conservation of mass $$A_{\rm open}\, v_{\rm open}\, \rho =A_{\rm lips} \,v_{\rm lips}\, \rho$$.

This is a greatly simplified analysis of what happens when you are sucking.
If $$A_{\rm open}\gg A_{\rm lips}$$ then the candle is in a region where the speed of the air $$v_{\rm open}$$ is not moving very fast and so is not blown out.

When you blow the air through your lips the air is directed towards the candle through the imaginary tube of air which only increases in area by a relatively small amount as blowing the air through your lips gives the air momentum in a direction of the candle.
In this case the speed of the air is fast enough to blow out the candle.

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When sucking the air which eventually enters the lips comes from a multitude of directions whereas when blowing the air exiting the lips is moving in approximately one direction.

Having the nozzle of a vacuum cleaner a few centimetres above a surface is not a very efficient way of removing dirt from a surface.
To remove the dirt efficiently the nozzle needs to be placed close to the surface to ensure that the air passing over the dirt is fast moving.

You could suck out the candle if you have your lips around the candle!

A coherent stream of air can be said to be comprised of air particles with a similar velocity magnitude and direction. For both sucking and blowing, there is a relatively coherent stream of air in your mouth.

Sucking is pulling air almost equally from all directions, into your mouth, where it becomes coherent. See illustration below. Blowing is introducing a coherent stream of air from your mouth into the air outside your mouth. In the case of blowing, the stream of coherent air that you have created will persist for some distance), at least until it reaches a nearby candle (the coherency is lost with distance due to boundary influences - see the small arrows in the blowing diagram).

Your lungs and mouth are designed to generate overpressure, not underpressure. You can barely suck the liquid out of a straw, while you can expel air with way more pressure.