Ask A Scientist General Science Archive

name         Jami M.
status       educator
age          30s

Question -   I am looking for ideas on how to present significant
figures to my 7th grade science class?
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Jami,

Ask your students what they think of the accuracy and usefulness of a
measurement result that is the sum of one that's very precise and one which
was crudely made.

For example, what's the point of measuring the distance from the edge of
their desk to their home by measuring the desk length to within a hundredth
of and inch and then adding on to it their guess (estimate) as to the rest
of the distance? They'll easily see the futility (waste of time) associated
with all the pains taken to obtain the first measurement if that result is
to be added to the very imprecise (and possibly inaccurate) "guesstimate" of
the second distance.

You could then remind them that when a collection of different measurements
are used to obtain a calculated result, that result can be no more accurate
(useful) than the least accurate piece of data which was part of the
calculation. Remember the Boy Scout motto, " A chain is as strong as its
weakest link."

This little mental exercise will also afford you an opportunity to enhance
the rigor of student vocabulary by establishing the important difference
between the terms, "precision" (repeatability) and "accuracy" (nearness to
accepted truth). A measuring instrument that is out of calibration may still
be able to provide repeatable data that is not correct -- and therefore
useless.

You can demonstrate this by using a simple lab balance -- assuming you have
access to one. Prepare and zero the balance, then have several students use
it to weigh a small object. After several students have obtained a result,
compare them to each other. If all were made with care, the results should
be in close agreement. Let the average of those results represent the
"accurate" data.

1. Remove the object; then place a coin on the balance pan. Let the coin
represent "a contaminant" that was carelessly left behind on the balance
pan.

2. With the coin in place, add the object to be weighed and ask a student to
make a measurement.

3. Remove the object (leave the coin in place), move the balance reference
masses out of equilibrium, then hand the object to another student. Ask
him/her to reweigh the object and record the result.

4. Repeat step 3.

5. After several students have obtained a result, compare them to each
other. Once again, if all were made with care, the results should be in
close agreement -- thus, the balance was performing with precision.

All the students should then easily understand that the result was not at
all accurate because the presence of the coin (contaminant) destroyed the
accuracy of the measurements. The point? Measurements that are precise are
not necessarily accurate.

Regards,
ProfHoff 509
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