Here's another riddle that you may find entertaining and/or pedagogically
useful.
Short version: At the most basic physics level, what makes a turbojet engine
go the right way? If somebody asks you how a turbojet works, can you give
an answer that gets the basic physics right?
--------------
Long version: According to the usual explanation, the engine comprises
a) A compressor, which pumps air into the engine,
b) A combustion chamber, and
c) A turbine, which is driven by the hot gasses and which in turn drives
the compressor by means of a shaft.
Roughly (!) the picture is this:
-------------------------------------
| | | xxx | | |
jet out / / / / / / air in
<---- ==/=/=/================/=/=/== <--
/ / / / / /
| | | xxx | | |
------------------------------------- (what's wrong with
^ ^ ^ this picture?)
turbine flame compressor
Now, to a first approximation the primary effect of the flame is to
increase the temperature and pressure of the gas in the guts of the engine.
So the big question is, why does this high-pressure gas decide to go out
the back rather than going out the front? You cannot explain this in
terms of the foregoing picture, so there must be some omission or
oversimplification in the picture. What is it?
This question bothered me for years, starting when I was a little kid and
first heard the "explanation" of how a turbojet worked.
Hint: There is a 100% compelling answer in terms of basic physics. You
don't need to know anything about the Navier-Stokes equations of fluid
dynamics, and you don't need to know anything about supersonic shock waves
or anything like that.
Note that this question does not arise for a piston engine, which relies on
valves and valve-timing to force the air to go where it should. (The
turbojet doesn't have any such valves.)
Bonus question / corollary: What does your answer imply about various good
and/or bad ways of *starting* a turbojet engine?