So I have a tube of wrapping paper. It's at least a couple of feet long and maybe two inches wide. If I press one end of the tube firmly against my mouth and blow, I barely feel warm air come out the other end. If instead I hold the tube about half an inch to an inch away from my mouth and blow into it, I feel a good flow of air come out of the other end.
My question is why is there a difference? In both cases I would expect that since air is going in one end, it would come out the other. When I press the tube against my mouth and blow, but very little air comes out the other end, where does the rest of it go?
PS: I don't know what tags to give this question and I can't think of a good question title. I'm new to the physics sub.
Best Answer
Very interesting observation. I think what you are observing when your mouth is away from the tube is MORE than just the air you exhale - because the air in the vicinity of your breath is being "dragged along" by something called "entrainment". This is the principle behind an ingenious fan, called the Dyson Air Multiplier:
A detailed explanation of how it works can be found here but it should be pretty obvious from the picture: a little bit of high speed air is being forced through holes in the rim of the "fanless" device (actually there is a fan in the base where the dark blue arrow points - it is responsible for creating the high speed jet of air) and this fast air drags a lot of slower air with it.
It's cute, and it does work. And that's what you are doing with your mouth.
The reason for the entrainment is Bernoulli's principle: when air goes faster, it has a lower pressure so it "sucks" on the air around it. When the slower molecules collide with the faster ones, they end up sharing the momentum. Instead of one fast one, you have two medium-speed ones now.