Communication Guidelines
Physics 2325 and 2326
| Dr. Cox | rev. Aug. 2003 |
| "The French never care what they do, actually, as long as they pronounce it properly." | ||
| - Professor Higgins, in My Fair Lady |
1. Instructions for a particular job, test, or problem take precedence over general guidelines such as these. Applicable laws, university regulations, and the like, as well as ethical considerations, are still higher priority (but should not be in conflict).
2. Significant digits:
Physical quantities are expressed by giving a value (number),
the appropriate unit, and an indication of accuracy or precision. In the
laboratory or on the job, the appropriate level of accuracy should be
evident from the nature of the data and/or the procedure leading to
the result.
However, in any calculations that are intermediate between
raw or input data and final answers, arithmetic results should be
carried through as if the data were considerably more exact than
they are, if possible; this will reduce roundoff errors. Intermediate
arithmetic results should always be carried to at least one
non-significant digit, to reduce the possibility of significant
roundoff error.
In the lecture portion, only, of my
courses, the following apply:
All given quantities are valid to three significant digits, unless
more are shown. (Trailing zeros that would be needed to make that
precision apparent will often be omitted to save some space.) If more
than three significant digits are given for a quantity, it is to be
treated as being more precise. Answers are to be given to the precision
justified by the inputs, normally three significant digits, with the
following exceptions: trailing zeros after a decimal point may be dropped,
and one extra digit will be accepted if that allows
all roundoffs (including intermediate) to be avoided.
3. Using power-of-ten notation ("scientific notation"):
Notations such as '**' or "E' for giving exponents are
computerese; they are necessary in computer programs, e-mail, and other
contexts where superscript is not available, but they are not appropriate
otherwise. The same applies to notations simply copied from a calculator
display, if they differ from standard notations for the same quantity.
In final answers, a number multiplying a power of 10 should
have exactly one significant digit before the decimal point.
In final answers, 10-1, 100, and
101 should not be used (unless to maintain uniformity of format,
as in a table); use ordinary notation for these cases.
4. Since metric prefixes were introduced in order to avoid large or tiny
numbers, there is seldom a good reason to combine powers of ten and metric
prefixes (following instructions in a test of ability to do conversions or
to use powers, is the only good reason I can think of).
In final answers, instead of a power of ten multiplying a
prefixed unit, I expect you to convert either to a prefix indicating near
the actual size with the appropriate number (not using power of 10), or to
the appropriate power with the unprefixed unit. However, there are some
exceptions: (a) Since SI units are built with kilogram instead of gram,
therefore for purposes of this rule kilogram counts as an unprefixed unit
while gram counts as a prefixed unit. (b) The list of metric prefixes has
not always been as extensive as it now is; a number of usages became common,
and are still accepted, involving using a power of ten either for still
larger multiples of the largest, or for still smaller multiples of the
smallest. then-current prefixed unit; thus, for example, large distances
often use power-of-10 with km, despite the prefix, since the larger
prefixes are not commonly used with meter.
5. If an equals sign appears connecting two expressions, the expressions
must be equal, not just related. A common type of violation of this rule
is of the form (the example being in a calculation of the volume of a
rectangular box which is 2 m by 3 m by 4 m)
2 m x 3 m = 6 m2 x 4 m = 24 m3
Here the second equals sign is legitimate but the first one claims that
6 square meters (value on left) equals 24 cubic meters (value on right).
For the same problem, the possible sequence
l w h = 2 x 3 x 4 = 24 m3
has two violations: l w h denotes a product of lengths, which does
equal 24 m3, but neither of them equals a number, which
the middle expression is.
6. In any normal physics formula, the labels denote physical quantities
which have appropriate units. (For a few labels "appropriate units" is
no unit; for these, this procedural rule will be hard to violate.) In
any instance of the formula, some (occasionally none) of the quantities
will have values substituted for the labels: these values must have
appropriate units. The other quantities (on different occasions none)
will still be denoted by the label, which indicates a quantity with
units: any unit stated with those is therefore redundant and thereby
incorrect.
Thus when the formula is F = m a, the appearance of either
(m) (4.5)
or
(5 kg) (a m/s2)
as an intermediate stage is incorrect in its units. (However, writing
down 5 x 4.5, without any units, off to the side, if you prefer to do
so to do the arithmetic, is permissible. The arithmetic part is a
legitimate part of the problem which you may separate from the rest.
The flaw in the objectionable forms is that the use of some units or
of some physical labels indicates that you are still using the physics
formula which involves the physical quantities; you are not yet just
doing the arithmetic.)
7. Remember that in essentially all cases capital and lower-case letters have different meanings: be sure your penmanship distinguishes them to others' eyes, not just in your own eyes.
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