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Name: Jami M.
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: Saturday, October 12, 2002


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


Replies:
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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