Why when a tall chimney falls does it often break into two pieces
before it hits the ground?

hi alexdrage,

When a chimney begins to fall, it rotates around its base, at first
slowly, then faster and faster. Its parts still accelerate, but it is
not so simple.
First of all, rotation around a pivot is not free fall. When the
chimney makes an angle A to the vertical, its parts move as if they
were on an inclined slope of angle A to the horizontal and therefore
their expected acceleration is only (g sinA), gradually increasing as
the motion becomes more and more vertical. Secondly, the different
parts of the chimney are not independent. If they were independent
(each on its own long thin rod connecting it to the center of
rotation, if you can imagine it), each would accelerate at this point
by (g sinA). But they are connected! That means, pieces near the
center accelerate by less than (g sinA) [zero at the center], and
pieces near the top, by more than (g sinA). Somewhere in the middle,
at one distance only, the acceleration is exactly (g sinA), the value
gravity alone would prescribe.
http://www-istp.gsfc.nasa.gov/stargaze/Sproblm2.htm
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realated links
"How Do Chimneys Fall ?", by Ian Stuart Murray, Mount Allison
University
http://aci.mta.ca/TheUmbrella/Physics/P3401/Investigations/ChimneyISM.html

Demonstration of how a chimney topples (includes mpeg)
http://www.physics.umd.edu/deptinfo/facilities/lecdem/services/demos/demosd2/d2-12.htm
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kewords used:
chimney falls two
chimney "breaks into two"
"chimney breaks" two
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hope this info helps you.

A pretty good answer. thank you