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Re: CBR to measure g? (fwd)



Mon, 27 Nov 2000 10:45:04 -0600
From: John Clement <clementj@stpiusx.org>
Subject: RE: CBR to measure g?

The CBR is easily used to get an acceleration graph, which could be used to
determine g. The easy way is to mount a motion detector on the ceiling and
toss a basketball about a ft or 2 into the air under it. Alternately you
can use the slope of the v-t graph. The standard CBR program can display
x-t, v-t, or a-t. Unfortunately I do not believe that they can easily be
displayed simultaneously for the same data.

Of course you can also determine g with a spring scale and masses. This
brings up the philosophical question of what is meant by g. Students are
extremely confused when g (9.8m/s^2) is used to determine the weight from
the mass using F_g = m g (use the _ to indicate a subscript). This
confusion is compounded if they are familiar with F=ma. Since weight is
what a spring scale measures, the object is not accelerating, so how can
F=ma be used to calculate it from mass? Properly speaking g is actually 9.8
N/kg when this equation is used, and it comes from the NTN gravitational
force law. The gravitational acceleration should then be referred to as a_g
or 9.8m/s^2. Pedagogically speaking this works a lot better when dealing
with first year students. The fact that the a_g = g is not worth pushing
with beginning students, as it tends to promote memorization rather than
understanding. There is certainly no harm in mentioning it once they have a
firm grasp of force, weight, mass, and acceleration. I would recommend
reserving g for the equation F_g = m g, and using the subscript g on a for
acceleration due to gravity and on F for the force due to gravity (weight).
Using w for weight creates even more confusion later when work is
introduced.

John M. Clement

-----Original Message-----


At 8:36 AM +0100 11/27/00, William J. Larson, you wrote about CBR to
measure g?:



Has anyone successfully used the CBR to measure g? If so, how?