Blowing between Cans
"Just put your lips together and blow"
Here is a challenge, using two empty soda cans and two straws on a flat table top can you arrange the items one way so that when you blow between the cans they move apart and another way so that when you blow between the cans they come together?
Material
To Do and Notice
Try to arrange the soda cans and straws so that when you blow between the cans they move apart.
Then rearrange them so that when you blow between the cans they move together.
Place the cans parallel to each other about an inch apart, 2.5 cm, on the table surface so that they can roll easily.
Blow between them and they will move apart.
Place two soda straws on the table parallel to each other. Closer together than the height of a can. Place the cans parallel to each other and perpendicular to the soda straws so that the cans will roll on the straws. Place the cans an inch, 2.5 cm, apart.
Blow between them and they will move together.
What's Going On?
In the top figure when you blow down between the cans the air comes to rest when it hits the table, trapped by the cans. At the word "Blow" the air has nearly atmospheric pressure and high speed, at the table surface it stops. Comparing the pressures at these two places using Bernoulli's equation we find that the air slows down between the cans so that the pressure between the cans increases. The high pressure air between the cans forces them apart.
In the lower figure.
The air can escape to the sides beneath the cans. The air starts at the word "Blow" at high speed and nearly atmospheric pressure. It constricts as it flows between the cans. Conservation of mass flow means that the air must speed up to pass between the cans. When air speeds up the pressure drops. Thus atmospheric pressure outside the cans is higher than the pressure between the cans and the cans are pushed together.
Scientific Explorations with Paul Doherty 

6 February 2004 