At this point neither the text nor I have mentioned that KE can be written
as p^2/2m. Many texts don't point that out until modern physics, so I
sometimes forget it also. I suppose the reason they don't point it out is
because learning about KE comes a chapter or two before momentum in the
typical textbook sequence. I did a quick examination of the text I am
currently using (Serway/Jewett) and I did not see KE = p^2/2m until chapter
40 which is the start of quantum mechanics. I personally try to remember to
point this relation out when we are in the momentum chapter (chapter 9), but
I am certain I forgot to point it out to this class. I think this is an
error on the part of many authors. It was also an error for me to forget it
for this class, but I did forget it.
Not only had I not personally made the connection that KEf/KEi = v1i/vf, I
also had not made the connection Krishna pointed out that KE retained is
m1/(m1+m2), so if a student had written that without showing the work, that
also would have surprised me. As Krishna pointed out, this is a special
case, so we don't spend much time on it. However, we do a lab on 1-D
collisions on the air track, and this particular case is one of five
different types of collisions they do in that lab.
In this case, the student did not show his work, so I already have to take
off points for that. In addition, I know the students well since I have a
small class, and this student generally did not hand in assigned problem
sets. Therefore it is unlikely he ran into this as he was working similar
problems. I still think he didn't know what he was doing.
Here is what I decided to do. I did not give any credit for the answer, and
I wrote this comment... "This is not how I expected you to solve this
problem, and you did not show how you got this relation. If you show me how
you got this relation, I will give you full credit for your answer."
It would be my guess he will not take the to time find out why his solution
was actually correct, and getting or not getting the points on this problem
will not change his final grade for the course. But I am willing to be
pleasantly surprised if he comes in and shows me why his method is correct.
Perhaps my challenge will motivate him to work harder second semester.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270
edmiston@bluffton.edu