Re: Impulse

["Carl E. Mungan" <mungan@USNA.EDU>]

Larry Smith asked:

> I guess I've always taught impulse
> because it is in the book.  Is John right that we don't need it?  Can we
> forego the new terminology and just talk about momentum transfer?  Even in
> the dashboard example?  Others: please opine.

At the risk of needlessly provoking others, I offer up the following thoughts:

1. I see an interesting parallel between the work-kinetic energy
theorem and the impulse-momentum theorem.

net work = integral of force dot space = change in KE
net impulse = integral of force dot time = change in momentum

To highlight this parallel, I usually have my students work the
following problem in pairs. Nothing particularly profound, but it
does allow us to discuss the connections between KE, p, and F from a
few different angles:

Suppose that a ping-pong ball and a bowling ball are flying
horizontally toward you. They have equal momenta and you exert the
same average force to stop them.
(a) Which ball initially has more KE?
(b) Which ball requires a larger distance to stop it?
(c) Which ball requires more time to stop it?
(d) How can it be that it takes the same time to stop both balls and
yet a larger distance for the ping-pong ball?

2. Essentially all of mechanics in principle can be done with the
definition of velocity and acceleration and Newton's 2nd and 3rd
laws. This is more or less the basis of the course called
"Engineering Mechanics." All other nomenclature is more or less
introduced for convenience. So at your convenience, you are free to
add/delete terms, textbook sections, etc.

3. Impulse is a pretty common term and therefore perhaps worth some
mention. But if you're only teaching elementary education majors, you
could probably delete it. If you're teaching engineers, I would be
more reluctant to do that.
--
Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
      U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu    http://physics.usna.edu/physics/faculty/mungan/