# Why does blowing on a candle put it out but sucking doesn't?

Alternatively, why does the force created by blowing out air feel so much stronger than the force created by sucking in air?

Ok, so forget the human factor involved in blowing out candles. Consider a vacuum cleaner with a suction end and a blower end. Anyone who has tried it out notices the blower end creates a much stronger force than the suction end, despite the discharge being (more or less) equal at both ends.

Why does this happen?

• Fundamentally, the same ideas underlie a related inquiry: Why doesn't a bus blow due to internal pressure? – user10851 Sep 2 '13 at 8:08
• – Ilmari Karonen Sep 2 '13 at 13:23
• I read in a bio of Richard Feynman, about a fun question, that has a similar answer: what happens if you invert the flow in one of those rotating water sprayers? In what direction does it turn? – Émile Jetzer Sep 5 '13 at 18:40
• @ÉmileJetzer, the bio is titled "Surely you're joking, Mr. Feynman". The problem is called the Feynman Sprinkler problem. en.wikipedia.org/wiki/Feynman_sprinkler – Pranav Hosangadi Sep 6 '13 at 1:46

In a blower, the air is directed along the axis of the blower as it exits, creating a high-pressure narrow cone. Exit pressure can also be multiple times of atmospheric pressure.

At a sucker entry, the low-pressure zone is fed by a much wider angle of atmospheric air at atmospheric pressure. Additionally, the underpressure can at most be 1x atmospheric pressure. therefore the inflow has an upper limit to its velocity.

Watch water going down a drain. It has a rotational symmetry and goes into a vortex, whose boundary conditions perpendicular to the flow cover an area much larger than the hole of the drain.

Watch water coming out of a hose. The boundary conditions defining the vortex are to start with the area of the hose perpendicular to the motion.

It is the same with the air, blowing out the mouth is the bound of the vortex thus the energy per unit area carried by the flow is much larger than by inhaling. In inhaling the vortex boundaries are variable but certainly much larger than the hole of the mouth, thus the energy carried by the induced flow of air per unit area is much smaller.

It is the impact of air that blows out the candle, and the area of the flame is small so that a directional blow from the mouth can turn it off. In inhaling the area covered by the flow cannot be controlled and the energy per unit area impacting the flame is much smaller.

• Fantastic!: I was groping for something like this but really had no idea how to condense my squabbling thoughts into an answer. – Selene Routley Sep 4 '13 at 1:35