On Sun, 11 Jul 2010 21:19:22 -0700, James H. wrote:

What is the logic of banging DOWN on a lawnmower crankshaft in order to
lift a flywheel UP?


They make a tool you can screw on the end of the crank to bang on so you
don't mess up the thread at the top. The physic is shock as in an impact
wrench will drive the screw tighter or looser with less effort and less
chance of breaking things. In the plumbing business there is the term
"warming up a pipe joint" when using steel pipe and fittings to loosen
the joint by banging on it when you can't wrench it apart. Same thing
with a lid on a glass jar that you can't loosen. Smacking it down on a
hard surface loosens it enough that you can then unscrew it. They are all
interrelated.