In reading an interesting article of G.E. Smith in the October 2006
issue of Physics Today I was thinking about the following
lab-demo-based set of questions. I am assuming a tool for measuring the
speed of a freely falling body, for example, a motion detector or
photogate, is available.
a) Suppose we double or triple the distance, what will happen to the
final speed?
I expect that someone will say: "v will also double, triple, etc." A
student who has already been exposed to the subject might mention what
Galileo discovered -- the speed will be 4, and 9 times larger. The "who
is right and who is wrong" strategy always helps to generate interest.
b) How do we know this? What kind of experiment can we performed to get
an authoritative answer from mother nature?
Collect data and plot the v versus h relation based on 5 to 10 data
points. Help students to discover that it is v^2 and not v that is
proportional to h.
c) Why do we conclude that it is v^2, and not, for example, v to the
power of 2.1, that is proportional to h?
Then discuss bars of errors, significant figures, number of data
points, etc.
Yes, I know that many on this list have similar projects. But how can a
retired teacher resist sharing old ideas? Most often it is not what we
do but how we do it that helps to make demos more effective. Try to get
students involved. Tell that you expect short individual reports about
"what we did and what we learned" in this activity at the next meeting.
Students often pay more attention when they know that a report is
expected.
The title of Smoth's article, by the way, is "The vis viva dispute: A
controversy at the dawn of dynamics." I strongly recommend reading this
article.
Ludwik Kowalski
Let the perfect not be the enemy of the good.