Re: Work/Energy theorem ?

[John Mallinckrodt <ajmallinckro@CSUPomona.Edu>]

On Fri, 4 Apr 1997, Mark Shapiro wrote:

(referring to the experience of a person in an acceleraing train compared
to that of one standing on the ground,)
 
> 	Yeah..... Sure you can tell the difference.  If "what you feel" is
> too crude a measure for you, use an accelerometer.  My claim is that the
> guy on the ground will see a reading of zero on his accelerometer, the guy
> on the train will see his accelerometer read different from zero as the
> train stops.

If the person standing on the ground has an accelerometer that reads zero
then his accelerometer is either 1) broken or 2) tared for 9.8 m/s^2, up. 
I can't tell whether you guys are kidding me or not.  Do we or do we not
believe in the principle of equivalence?

> 	Here's a different work-energy problem for you.  Remember the old
> mail cars on railroad trains?  They could hook a mailbag from a station
> platform as the train rolled through the station.  Assume the train moves
> through the station at a constant velocity.  Mike is standing on the
> platform, Mary is riding in the mail car.  Calculate the change in K.E.
> for the mailbag (before hooking and after hooking) from Mike's point of
> view and from Mary's point of view.  Do they agree?

Of course not; it's simple to see that Mike "observes" a change of
+mv^2/2 (where m is the mass of the mailbag and v is the speed of the
train) where Mary "observes -mv^2/2.  Pick a value for the change in K.E. 
that you'd like to see and I'll find you a frame in which you'll observe
it.  But are you claiming this has something to do with the other problem?

Prost...

John
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