There is an old familiar rotation problem with which I am currently uneasy,
and about which I am hoping to get some advice. It involves an H-shaped
"frame" falling from a horizontal to a vertical position about an axis along
one side of the H. In Halliday, Resnick & Walker's Fundamentals, 5th ed.,
it is Problem 83 in Chapter 11. In order to solve for the angular speed in
the vertical position by a simple energy method it seems necessary to
introduce an ad hoc rule something like: "Any mass that contributes nothing
to the rotational inertia should be excluded from the center-of-mass
calculation." (Such a rule is not needed if the analysis is done for a
parallel axis a distance x along the crossbar of the H and then x is set
equal to zero.) Has anyone developed a satisfying way of dealing with this
problem? (Or can someone tell me that I'm just being obtuse and overlooking
something fundamental?)
______________________________________________________
Fred Lemmerhirt
flemmerhirt@mail.wcc.cc.il.us http://chat.wcc.cc.il.us/~flemmerh/physics.html
Waubonsee Community College Sugar Grove, Illinois