Thanks, Jecel!  This is great.  I still  have exercise answers and 
teachers notes to write, so with your permission, I'll pass this on 
to teachers that way.

Mark

>Mark,
>
>great draft chapter! By coincidence, I have always used the falling
>object example to explain Morphic (in Self). I don't know what your
>intended audience is, but when creating the example for people who have
>had some high school physics I like to use points for acceleration and
>velocity. The code is just as simple as what you presented (to the
>amazement of those who have tried something like this in some other
>language):
>
>
>    step
>                     velocity _ velocity + gravity.
>                     self bounds: (self bounds translateBy: velocity).
>
>where you have set velocity to 0 at 1 or something. Now you can start with
>a horizontal velocity and get the nice parabola with no extra coding!
>In fact, with a slight patch in the above method to always have gravity
>pointing to some object (instead of straight down) you can plunge
>directly into celestial mechanics (and no - I don't mean SMTP ;-)
>
>-- Jecel

--------------------------
Mark Guzdial : Georgia Tech : College of Computing : Atlanta, GA 30332-0280
Associate Professor - Learning Sciences & Technologies.
Collaborative Software Lab - http://coweb.cc.gatech.edu/csl/
(404) 894-5618 : Fax (404) 894-0673 : guzdial at cc.gatech.edu
http://www.cc.gatech.edu/gvu/people/Faculty/Mark.Guzdial.html