Why airplanes taxi

On Feb 10, 5:53*pm, Mxsmanic wrote:
writes:
Since there is no infinite volume, what would be the point?

The atmosphere of the Earth is an infinite volume, and in fact it demonstrates
rather well that the combined laws don't apply in that case. *Pressure is
largely uncorrelated with temperature, for example.

Half knowledge can be a dangerous thing in the wrong hands. The fact
that you cannot correlate pressure and temperature in the atmosphere
does not mean the gas laws do not apply. It just means that the
number of molecules in a given volume of the atmophere , ie density
varies with height with time, with winds etc. At any point in the
atmosphere ( or in space) the density of any block of space over which
the temperature and pressure can be considered to be constant, can be
calculated accurately using the gas equation

density =PM/RT

You see the fact that the volume of the atmophere or of space is
infinite is quite irrelvant because nobody wants to know what the
average density of the whole atmophere is ( which of course will
approach zero depending on your definition of where the atmsophere
actually ends) . But a pilot might want to know what the density is
in a particular layer of air where the temperature and pressure are
reasonably constant, say at an airport for example that he is going to
take off from and wants to know whether his aircrafts performance will
be sufficient to takeoff and clear a bunch of trees at the end of the
runway.

And in the case of the space example, you quoted a pressure of 1e-11
Pa, if the temperature is 3 deg C then again the density will be able
to be calculated perfectly well for that part of space for which that
temp and pressure apply, the fact that the temperature and pressure in
some other part of infinite space is different,is irrelevant , the gas
laws apply everywhere ( with appropriate modifications for non ideal
behaviour at very high temps and pressures but I wouldnt worry about
those if I were you, try to understand the basics first)
Terry
PPL Downunder
.