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Tuesday, August 20, 2013

Significant Digits Help!


What can you do when kids don't get it?

Each year when I teach significant digits to my chemistry classes, I feel like I am banging my head against a wall!  Most of my students will diligently learn the rules for determining the number of significant digits in a measurement.   They can look at 50.00 and tell me that it has 4 significant digits.  But I always get the feeling that some of them never really understand why they are learning these rules and what they mean.

In an effort to help my students grasp the concept, I started doing this significant digit lab each year.  The materials list is super simple: a wood block, three different balances, and three different rulers.  Students begin by measuring the length, width and height of their wood block using a diagram of three different rulers.



They are frustrated by the first ruler.  They don't like having to estimate!  I quickly get the students to understand that the first ruler has only one significant digit, the second ruler has two, and the third ruler has 3.  They use the measured length, width, and height to determine the volume.

Next students get the mass of their wood block using three different balances.


Now that the volume and the mass is known, students must calculate the density of the wood block to one significant digit, two significant digits, and three significant digits.  We then calculate the percent error, using our three-significant-digit density as the true value.  Wow!  Kids quickly realize how important significant digits are to a measurement.

Throw in an additional page of practice problems on rounding and calculating with significant digits, and I'll call this day a success!  I hope you are off to a great school year.  I'd love to hear your ideas on significant digits!



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