I have been following the thread on websites and want to add a word
of caution. Here is an excerpt from the site:
<http://library.advanced.org/11902/physics/momentum.html>
The discussion is about a bat hitting a baseball and concludes:
"Now, you may have noticed that I said momentum is "almost"
conserved. Why isn't it if the equations say it should be? Well,
momentum is only conserved if the materials involved in the
collision are inelastic or inflexible (perfectly solid). Although you
might assume that bat and the ball are inelastic materials, they
are not. When the ball hits the bat, the ball will be squished to a
certain degree. After few milli-seconds, it rebounds back. This
contraction action will use up some energy, making conservation
of momentum false. There are also factors that can use up
energy, however, the concept of conservation of momentum is still
relevant in predicting the range of a baseball."
This is about as wrong as an explanation can be and yet it lives on a
site that has won *many* awards. This is just one example and it is
impossible to edit the web; so we must be cautious in directing our
students to avail themselves of web resources.
**********************
* Nick C. Steph *
* Franklin College *
* Department of Physics *
* Franklin, IN 46131 *
* Phone: 317-738-8308 *
* Fax: 317-738-8310 *
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