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From: bhuber@sjuvax.UUCP (B. Huber)
Newsgroups: net.math
Subject: Re: Re: Fly and train
Message-ID: <2384@sjuvax.UUCP>
Date: Mon, 14-Oct-85 16:22:39 EDT
Article-I.D.: sjuvax.2384
Posted: Mon Oct 14 16:22:39 1985
Date-Received: Thu, 17-Oct-85 01:32:31 EDT
References: <508@runx.OZ> <2092@brl-tgr.ARPA>
Organization: St. Joseph's University, Phila. PA.
Lines: 19

> > In order to "turn around" (change from a positive velocity to a negative one)
> > the fly must have had a zero velocity at some point in time (my assumption).
> > Having a velocity of zero, the fly must have been stopped.
> >  **** At that time, the fly would have been in contact with the front of the train.
> > As the two bodies were not rotating, would not the train have been stopped as 
> > well?  If not, then why not?
> 
> In all such problems, beware of taking extreme
> limits as actualities; consider systems that are
> not quite so ideal and you will find that the
> analysis is much easier.

Discontinuous momentum is an example of such an extreme limit.  
Actually, if the fly decelerated to zero speed, stopping sufficiently far
from the train, then the train would just catch up with it as it began its
acceleration backwards.  At the time the two met, they would have identical
velocities; no momentum transfer need occur.  From that moment on, the fly
would be moving faster, not slower, than the train, and so would speed on
ahead.