Re: Work/Energy theorem ?

[John Mallinckrodt <ajmallinckro@CSUPomona.Edu>]

On Fri, 4 Apr 1997, Mark Shapiro wrote:

> 	Since the observer on the train feels the forces from
> deceleration, he or she should realize that the train is not an inertial
> reference frame.  Thus, computing the change in the observed K.E. of the
> tree has no physical meaning.

But, wait a minute Mark.  We all feel the force of gravity which is
*exactly* analogous to the "force of deceleration."  Thus, *we* should
realize that we are not in an inertial (i.e., free fall) frame. 
Nevertheless we compute the change in the K.E. of falling objects just as
if there was a real force doing work on them.  Doing so, we get perfectly
reasonable results and never even blink.  Why shouldn't we accord the same
privileges to the person on the train? 

As I said in my March 20 posting: 

Since Charles [the observer on the train] is a "student of physics," he
shouldn't have any trouble figuring out what happened.  After all there
was a period of time when a uniform "gravitational" field in the "forward"
direction was clearly acting on everything in Charles' view including
himself.  Indeed, he personally experienced this gravitational field as
his seatbelt struggled to keep him from sliding off of his seat and as his
coffee slid "forward"  off his tray and onto his knees.  He should have no
trouble calculating that the "forward" force on the tree due to this field
did negative work as the tree moved "backward" and in an amount exactly
equal in magnitude to the initial kinetic energy of the tree.  The laws of
physics, therefore, *require* that the tree come to rest and Charles
should be *outraged* to see any other result.

John
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