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Re: Bad textbooks (an example for the skeptical)



You probably can find even more in the Hubisz Report
www.psrc-online.org/curriculum/hubisz.htm
Of course publishers change the names of the books and the authors names
just to disguise these sorts of things. The only middle school textbook
that received a passing grade was Introductory Physical Science , - Highly
recommended by an article in Science News "Where's the Book" by Janet
Ratloff, and also by John Hubisz in The Physics Teacher, 39(5) May 2001 pp
304-309. It has the virtue of being written and published by one author (Uri
Haber-Schaim), so it will be nearly error free. It has been extensively
classroom tested, and is now in 7th edition. It is consistent with many of
the ideas found by research to promote better learning. It may be purchased
from Science Curriculum Inc., Belmont, Mass. www.sci-ips.com

John M. Clement
Houston, TX

-----Original Message-----
From: phys-l@lists.nau.edu: Forum for Physics Educators
[mailto:PHYS-L@lists.nau.edu]On Behalf Of Brian Whatcott
Sent: Saturday, February 16, 2002 10:30 PM
To: PHYS-L@lists.nau.edu
Subject: Re: Bad textbooks (an example for the skeptical)


At 07:59 PM 2/16/02, you wrote:
...two problems both appear on the same page of the
textbook (thankfully, in an appendix, not the main text).

1. "A peach falls from a tree with an acceleration of 9.8
m/s^2 downward.
The peach has a mass of 7.4 g. [Editorial comment: not a very big peach?
But wait; here comes the good part!] With what force does the
peach strike
the ground?"

2. "A person steps off a diving board and falls into a pool with an
acceleration of 9.8 m/s^2, which causes the person to hit the
water with a
force of 637 N. What is the mass of the person?"

Apparently neither the height of fall nor the duration of impact have any
effect on the results.
--MB


It is clear that the questions are modeling a rather special universe.
Taking question 2) first, if the height of fall is not given, we
can suppose
it is not material: This leads to the conclusion that one
reaches terminal
velocity for any reasonable height of diving board there.
If the impact force can be given, and related only to the mass
of the person
then this drag force is (0.5 rho v^2 A Cd) and we suppose that v^2 A
is proportional to mass. But mass could be modeled as proportional
to the volume of a cylinder of cross section A times height,
which leads us to deduce that here, the terminal velocity v is
proportional to
sqrt(jumper's height)

This is the key to answering question 1), apparently:
a peach has a height sensibly equal to its cross sectional diameter.
Extending the deductions for prob 2) to prob 1) are now left as
an exercise
for the student.


Brian Whatcott
Altus OK Eureka!