Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: POE summary (was Re: Work/Energy theorem?)



John,

I'd like to get back to the original question for a moment. As I
understand it you have an observer on a moving train who is looking at
tree someplace on the ground. The observer on the train calculates the
K.E. of the tree. The train is then decelerates to a stop, and the
observer calculates the K.E. of the tree again and finds that it is zero.
Someone then wanted to know where the lost K.E. went.

My naively classical mind said that it didn't go anywhere - that
the observer on the train decelerated so the change in KE was not measured
in an "inertial" frame so it was a physically meaningless number.

I don't see how this changes if you view the problem from a GR
point of view. Give the guy on the train an accelerometer and give a guy
handcuffed to the tree an identical accelerometer. Let the accelerometers
be designed in such a way that the measure the "local" acceleration in
conformity with GR. The observer handcuffed to the tree will say that the
reading on his accelerometer never changed (and neither did his KE). The
observer on the train will say that the reading on his accelerometer did
change at least momentarily (and thus his calculation of the change in KE
for the tree needs to be interpreted vary carefully.)

He thinks for a minute and realizes that he did not measure a
change in KE for the tree, but rather his own change in KE as he was
decelerated.

Gentle comments are welcome!

Dr. Mark H. Shapiro
Physics Department
California State University, Fullerton
P.O. Box 6866
Fullerton, California 92834-6866

Phone: ++ (714) 773-3884
Fax: ++ (714) 449-5810
e-mail: mshapiro@fullerton.edu