In the lecture of Wed 2/22 we did a problem of a person
stretching a perfect spring by pulling a mass at the end
of the spring at constant speed.  In this example, I did
a very bad job of explaining why the force exerted by the
person is equal in magnitude and opposite in direction to
the force exerted by the spring.  Here is a correct 
explanation.  This is also available as a pdf file 
and at the Noble Library Copy Center.  I apologize for any 
confusion this might have caused.  GBA.