Re: [Phys-l] FCI answer?

[John Denker <jsd@av8n.com>]

I think y'all have identified a rather tricky issue.

On 02/14/2009 12:38 PM, Bill Nettles wrote:

> That's what I thought, too, initially. Then I began thinking, "If
> this is an implusive force directed perpendicular to the
> instantaneous velocity, then it does zero work. That means the speed
> doesn't change, only the direction." Am I missing something?

If the kick is perpendicular to the _initial_ instantaneous
velocity, it will not be perpendicular to the _final_ 
instantaneous velocity.

You have to use some "average" orientation ... or do the
calculation some other way entirely.

In fact the work is nonzero, as you can verify by simply
calculating the KE before and after.

==========

This demonstrates that the notion of "impulse" must be used
with care.  The problem is that it introduces an artificial
singularity.  It uses a singular construct to approximate the
underlying nonsingular physics.  This makes some parts of
the calculation easier, but makes other parts more difficult
and/or more error-prone.