This is hard to explain without pictures, but if you’ve played with these little party favors, you’ll know what I’m talking about. The toy consists of a little plastic horn tube with a reed in it that goes “honk” attached to a curled-up paper tube that unrolls when you blow through the thing.
As you blow, the paper tube unrolls until it’s completely extended and straight, in line with the horn part. But then, if you keep blowing, the paper tube bends at the base where it’s attached to the plastic and winds up at roughly a right angle to the plastic part. You blow it straight out, and then it bends down and hits you in the chest.
The unrolling seems intuitive, but why the bend? It certainly seems to bend more forcefully than would be explained by gravity alone-- and anyway, it bends harder the harder you blow, so I don’t think it’s just falling.
Is this a mystery on the order of “why does the shower curtain blow in at the bottom,” or am I just missing something obvious?
It’s because the far end where the air is coming out retains a bit of curl; the tube never gets completely straight, meaning the escaping airflow isn’t along the line of the tube. This creates a downward force on the tube, causing it to bend at the root if you blow hard enough.
To test this, you could hold the horn upside-down or sideways when you blow it, and see if it still deflects. If the hypothesis is true, then the horn should also deflect when turned sideways, and possibly even when upside-down.
As an additional test you can close off the opening that allows air to escape and simply inflate the thing. Then you can cut an opening in another place, like the opposite side of the curl, which would ostensibly cause it to bend the opposite direction.
