Ditto to this.

I'm not sure of my most egregious example, there have been so many. One that comes to mind was in Yosemite, on the well-worn, relatively easy path to the big falls at the end of the main valley. There are rock steps which, from the spray, can be slippery, and they're uneven. Still, it's an easy walk.

Unless, that is, you're wearing little, cocktail-party, spiky shoes with 3" heels and no heel strap. And we must have seen a half-dozen women struggling along with such footwear. Too stupid for words, really. (But, hey, they sure looked fashionable, at least they would have in the right venue.) HPH

 

[QUOTE=Matt_in_VA,Dec 18 2006, 10:22 PM] With the exception of perhaps one.

 

^^ Sort of. A simpler explanation lies in the conservation of mass.

For a fluid that is incompressible (or very hard to compress, such as water), mass conservation means that the mass flow in a pipe has to be constant -- in essence, the same number of molecules have to pass by each cross section in a given time interval, no matter the size of the cross section. Therefore, in smaller cross sections the flow has to speed up. (And mass conservation is simply the principle that, in the absence of nuclear reactions that convert mass to energy, a system's mass -- the number of molecules -- must be constant, or at least possible to keep full track of. In other words, matter doesn't just magically appear and disappear.)

Pressure comes in when you apply Bernoulli's principle to this -- in such flows, pressure decreases as the flow speeds up. That means that the pressure upstream of the smaller cross section is higher than the pressure within the smaller cross section. (However, if the small cross section then expands to the original size, the pressure downstream is also higher -- meaning the flow in the small place is pushed on from both sides. This is why mass conservation is a simpler explanation.) HPH

Traffic is definitely not an incompressible fluid, though. The spaces between cars vary, and drivers' reaction times are also all over the map. HPH

 

Back OT: The TV news is reporting that the search for the two other guys missing on Mt. Hood has been called off, for safety reasons. HPH

 

Yeah, that was yesterday. A new storm blew through Wed evening. Realistically most of us felt that this had been a body recovery exercise for many days now, but you have to treat it as though you might find them alive because there's still a remote chance of it happening.