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Re: [Phys-l] Principle of Virtual Work

Hi all-
I was an aero engineering undergrad, so the mechanics I learned after introductory physics was different from that of the physics majors. Engineering juniors took, and still take, courses called "Staics" and "Dynamics" (Timoshenko and Young has been widely used). PVW is usually introduced in the statics course.
I taught a statics course a few years ago and treated PVW as a very elementary introduction to the notion of variational principles. I wanted the students to have heard that there is an alternative to the statement of Newton's laws for formulating the principles of mechanics.
Regards,


On Tue, 10 Oct 2006, John Denker wrote:

On 10/10/2006 09:22 PM, Krishna Chowdary wrote:
.... Principle of Virtual Work. I don't teach
this (nor in fact did I learn this) in introductory mechanics; I'm not sure
if my experience is typical or not (i.e. if PVW is a tool that most physics
students get in their first course). As far as I can see by my quick survey
of the indices of intro calculus based physics texbooks in my office, none
of them talk about virtual work. It is in Goldstein, which must have been
the first place I saw it (so grad school). Do engineers see it sooner than
physicists?

I know of one physics text that mentions PVW on the 24th page or thereabouts.
It's a somewhat atypical text, by some guy named Feynman, with help from
Leighton and Sands. PVW is in the index.

Actually, my college classmates and I didn't need to delve as far as the
24th page. Our homework came from the Leighton/Vogt workbook, which has
the epitaph of Stevinus on the *cover*. So literally you couldn't come
home from the bookstore without being exposed to PVW, before classes even
started. We took that as a hint that PVW might be useful.

In the group I ran with, PVW was routine. Taken for granted. No big deal.

The weird thing is that I recall some other groups didn't get the hint.
I vividly recall that some students tried to solve the bridge problem
http://av8n.com/physics/img48/bridge.png
by using force vectors, rather than PVW. That group sure had a miserable
time.
(The problem asks: Which segments of the bridge are under tension and
can therefore be implemented using flexible cables, and which segments
must be implemented as rigid girders? Assume that the load W is huge,
so that the weight of each bridge component is negligible by comparison.
Hint/example: Segment GE can be a cable.)

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