I want students to think about the purpose of a ruler and how it is designed. Specifically, I want them to realize that there are 10 millimeters in a centimeter, based on the work that they have done with converting metric units. This do now activity helps to tie what they already know to something that they have done before, measuring, just with different units.
I ask students to draw a picture of how you think a metric ruler should look if it contains only centimeters and millimeters.
I give students time to draw their rulers in their notebooks.
We then review and I ask students to share their ideas:
Most students realize that there should be 10 millimeters that make up 1 centimeter. I then model to mathematical logic behind their reasoning (see photo):
I model how you can convert 1 centimeter to millimeters by either multiplying by 10 or using the staircase method to move the decimal one place to the right and adding a 0.
I then draw a ruler on the board and show that there are 10 millimeters (see photo).
I like to teach kids how to read measurements off of a piece of paper before just giving them a ruler and having the measure. I've found that students are more successful after using a number line to visualize what measuring length means from a mathematical standpoint.
I first ask students, "Based on our ruler that we just created on the board, what is the smallest and largest decimals that we will have when measuring to the nearest millimeter?"
Answers: .1 is the smallest decimal and .9 is the largest decimal
This strategy helps students when reading their ruler. I then review the first measuring problem and model how to read the ruler.
I tell students to decide the whole number they measuring to and then add the decimals. This helps assure that they are accounting for all numbers in each length measurement.
To add a sense of humor to the lesson, while getting across the message of proper ruler use, I have students take an oath.
I have students raise their right hands and then repeat after me:
"I, (state your name), promise to follow rules for rulerr use. A ruler is not: a back scratcher, catapult, comb or anything else that does not include drawing a straight line or measuring the length of an object. So help me science!"
If kids are misusing a ruler, I simply state that they are violating the oath that they took and they know to stop. Well, some know to stop :)
I then go over tips to follow to measure accurately:
1) Start at the zero line or the end, depending on your ruler
2) Take your time and include units
We then review our answers as a class.