pizzazz wrote:5 is a lot?
Yes.From KA's Q&A. This week: The bottom of reality unseeable?
A more mathematical example is boundary layer flow over a flat plate. Basically, the flow profile looks kind of like the right side of the letter U. At the flat plate, the no-slip condition applies, and the velocity is 0. As you get farther above the plate, the velocity increases, but it only increases as high as the free-stream velocity.
Formally, if you construct this system non-dimensionally (so instead of height in centimeters or whatever, we just normalize it to "units"), then the boundary layer flow only approaches free-stream flow in the limit to infinity.
Practically, however, we know this not to be the case, and we observe a point when the boundary layer flow is equal to the free stream flow within, say, 99.5%. The difference in velocity at that point can be considered to be negligible (we couldn't even measure it if we tried).
As it turns out, you don't need to go out to infinity for this case. All you need to do is go about 5 units above the flat plate to observe a 99.999999% match with free-stream. So, in that case, infinity and 5 are effectively indistinguishable.
* ponders the shape of a knife capable of doing a 5 part Banach–Tarski decomposition... *