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Re: [Phys-l] 9.80 vs. 9.81

This business of an accepted value is nonsense. Students are often asked to
calculate error by subtracting the accepted value from their value, when
they should be looking at how their values spread. Notice that the idea of
an accepted value makes error seem like something that is "wrong". And how
is the accepted value calculated in this case? Is it measured at a
"standard" location, or is it just an average over the entire surface of the
Earth? Or is it an average by latitude, which will give a different value.

That being said, the value that students should use is the value they
measure in the lab. Since my students do it in a very approximate fashion,
they measure 10N/kg, so that is what we use. Since the text I use also uses
this value, it is quite good enough.

One of the important reasons for using 10 is that they have previously been
told that the gravitational acceleration is 9.81, but they do not understand
this, and they are very confused when calculating F_g = m g where g is
quoted as an acceleration. After all a book on a table is not accelerating
so why use an acceleration in the calculation. But encountering the idea
that the force is 10N/kg x the mass is fairly natural. The fact that we use
a different number somewhat decouples the previous memorized knowledge from
the new understanding.

There is another problem in that g can be interpreted either as the actual
acceleration or the gravitational field constant. These yield different
values. One must make a choice there, and all too often this is glossed
over. Since my students are at a fairly low level, this distinction is
never brought out, but at all times g is treated as the gravitational field
constant.

While Knight certainly has many good things, the big misconceptions are not
always treated well. By doing Newton's laws in the numerically correct
order, they are done in the pedagogically wrong order. NTN3 needs to come
first with the idea of interactions. By deriving the local gravitational
field constant from NTN2, students are then confused. He introduces energy
bar charts, but then does not have students use them in textbook problems.
He also uses 2 types of bar charts, which can be confusing. So the research
based ideas are sometimes decorations rather than a solid part of the book.
His kinematics section is better in this regard as he does use motion maps
and graphs in textbook problems. The student workbook, however, does look
good, as it resembles McDermott tutorials.

So I would not worry about the difference between 9.80 and 9.81. This is a
small thing compared to the many other issues.

John M. Clement
Houston, TX

The value of g at my location is less than 9.80 m/s^2, and Randy Knight
uses 9.80 in his textbook, but NIST says the accepted value for standard
gravity is 9.80665 which rounds to 9.81. Most textbooks use 9.81 when
they
want three sig-figs (but not Knight).