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Re: CBR to measure g?



-----Original Message-----
From: phys-l@lists.nau.edu: Forum for Physics Educators
[mailto:PHYS-L@lists.nau.edu]On Behalf Of William J. Larson
Sent: Monday, November 27, 2000 2:37 AM
To: PHYS-L@lists.nau.edu
Subject: CBR to measure g?

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

Dr. William J. Larson


The CBR just isn't the right tool to use to measure g. It's kind of like
using a screwdriver to change a bicycle tire. You can do it, but you won't
be pleased with the results.

(For those of you wondering about the acronym CBR, a Calculator-Based-Ranger
is just a motion detector designed to connect directly to a TI calculator
with no CBL or other interface required. As far as its sensor capabilities,
it is just a standard motion detector.)

The problem is that objects that are "seen" well by the CBR also tend to be
objects that suffer significant air resistance, disturbing the measurement.
Also, a CBR doesn't really have the resolution needed to produce a high
quality acceleration measurement with the few points generated by a falling
object.

So what to do? I suggest you measure some other acceleration, such as a cart
on a ramp. The physics is identical, but the measurement will be much better
quality. The slower motion means air resistance won't be as large a factor,
and you'll get far more points. You can accomplish the same pedagogical
things with a cart/ramp lab.

If you insist on measuring g, use a picket fence and a photogate. Such labs
are much more difficult for a student to understand what is happening, but
the photogate is the better tool for freefall.

JEG

--
John E. Gastineau john@gastineau.org
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