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Michael Phelps by a fraction!
This article contains a mini-lesson to help you help your students understand the significance of roundiing to the correct significant digits.
Start by having students watch this short you-tube video: Michael Phelps Wins!
Recap the event, highlighting the fact that Phelps won by only 0.01 seconds, one one-hundreth of a second.
Next, give the students the following instructions: Round each number to 1 significant digit. (Answers are in parenthesis.)
If your students are anything like mine, some of them will round the last example to 0. The idea that the zero after the decimal but before an integer is just a place holder is difficult for some to understand. This is when you play the Michael Phelps card. Point out that he won the race by 0.01 seconds. If that zero after the decimal was considered significant, we'd have to round the amount of his victory off to 0.0, and that's no margin to win by.
Those placeholder zeros can confuse students. By giving them a real-life example to relate back to it will be easier for them to comprehend and remember.
An additional point to consider:
The touchpad that Michael Phelps hit at the end of his race was calibrated to read to the hundreths place. Therefore, the results of his race could only accurately be reported to two decimal places. It would be inaccurate and dishonest to claim that he won by 0.005 seconds because the touchpad can't measure with that precision. Remind students that the way they record and report their measured numbers reflects the quality of the tools they used. "Liar, Liar, pants on fire" is an appropriate song for any who disregard their significant figures.