Re: Rotation problem

[brian whatcott <inet@INTELLISYS.NET>]

At 16:50 12/3/99 -0600, you wrote:
> 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


The well regarded Den Hartog, in his "Mechanics"
works a somewhat comparable problem: that of a horizontal
trap door, which when the latch is released, accelerates
its CofG at 3/4 g, and has 1/4 of its weight initially
supported at the hinge. This example assumes a uniform beam
in developing an equation for balancing moments.

P237, SBN 486-60754-2 Dover



brian whatcott <inet@intellisys.net>
Altus OK