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Re: free fall data



In a message dated 97-09-16 10:07:57 EDT, you write:

<< The class analysis will be limited to the speed-time line approach.
I will say something like this. "g is the slope of the v-t line. Thus to
verify its constancy we must verify that the v(t) curve is a straight line.
It is not a perfect linear graph but we will find the average slope. How
close is it to 9.8?" What else can I do with data on displacements which
are not accurate enough? >>

Ludwik,

Please don't say that g is the slope of the v-t plot. Say acceleration is
the slope of a v-t plot. After all, isn't this the definition? And what's
wrong with analyzing the v-t graph?

If the slope of a v-t plot is positive and constant, then the acceleration is
positive and constant. Why? Because, by definition it is the slope, and
something we will all agree upon.

If the slope of a v-t plot is negative and constant, then the acceleration is
negative and constant. Why? Because, by definition it is the slope, and
something we will all agree upon.

Now, the area under a v-t graph is displacement. Why? Check the dimensions.
If the v-t graph is a straight line, then we can break it into a triangle
and a rectangle and add their areas. What will the equation for the combined
area be? Why won't these equations be good if v-t is not straight?

I think you're missing an opportunity here. There are so many other constant
acceleration experiments you can be doing rather than focusing your students
on the most accurate value for g. And by doing other acceleration
experiments you will find that maybe your students won't always put g in for
a in equations. You could be having your students recognize what v-t looks
like for constant a, and stessing why we use the equations, that we do, for
constant acceleration (no dark rooms where equations fly into empty minds).
You have an opportunity to stess the approach here, and it will be of much
more value to your students.

Bob Carlson