Bernoulli's Principle
The 13 March entry included the question: What do opening a new plastic trash bag and riding in an airplane have in common? No one has ventured an answer but in case anyone's interested I'll post mine, namely, "Both activities are heavily dependent on Bernoulli's principle."
The simple statement of Bernoulli's principle which dates back to the nineteenth century is: " The pressure in the stream of a fluid is reduced as the speed of the flow is increased." The 'speed of flow' is most often that of the fluid relative to the surface of a solid. So it applies, say, to the speed with which air flows past an airplane wing as the plane moves forward. Aircraft wings are shaped so that the flow is faster across the top than the bottom. The pressure difference produced provides the 'lift' that keeps the plane aloft.
There are many practical application of this behavior of fluid flow and airplane lift is probably the one most often cited by physics teachers. I've never seen a written reference to the principle's application to the opening of a plastic trash bag but it certainly does apply. When you 'snap' a new plastic bag the air rushing by the outer surfaces experiences a pressure reduction and the pressure of the air trapped between the interior surfaces forces them apart. Of course this works only if the bag is open so that air can flow in to fill the increased interior volume.
So, the process is not a matter of 'scooping in' air to fill the bag.
I remember once when, during a strong wind, a large section of face brick on a wall of one of the apartment buildings of the complex in Ypsilanti fell off. People were asking, "How did wind get inside to blow the wall out?" But of course it was normal air pressure inside the wall that forced the wall out when the outside pressure was reduced ala Bernoulli.