How Well Can You Measure?
5 min
Narrative
In this activity, students measure the side lengths of a triangle and calculate its perimeter. This serves as an opportunity to review the meaning of perimeter as well as how to use a ruler to measure to the nearest tenth of a centimeter, in preparation for the next activity.
Launch
Ask students to describe in their own words what is the perimeter of a shape. If needed, display the triangle for all to see, and annotate the image as a student describes the perimeter. Distribute a ruler to each student. Give students 2 minutes of quiet work time.
Monitor for how students are measuring and calculating perimeter. If any student has a value for the perimeter that is very far from the correct value, help them identify their error in measurement or calculation.
Student Task
Measure the perimeter of the triangle to the nearest tenth of a centimeter.
Sample Response
Answers vary depending on the printed size of the figure.
Sample response: About 13.4 cm
Synthesis
The purpose of this discussion is to highlight variability due to measurement error. Poll the class on their answers for the perimeter of the triangle. Point out that many of the answers are pretty close but there is some variation.
Ask students:
- “Why are these answers not all exactly the same?” (When measuring the same thing, people may get slightly different answers due to errors in the measurement.)
- “How do you know if your measurement is actually incorrect?” (If your answer is more than just a little bit different from the answer that others have gotten when they measure the same thing, then you may have measured incorrectly.)
Standards
- 3.MD.8·Solve real world and mathematical problems involving perimeters of polygons, including finding the perimeter given the side lengths, finding an unknown side length, and exhibiting rectangles with the same perimeter and different areas or with the same area and different perimeters.
- 3.MD.D.8·Solve real world and mathematical problems involving perimeters of polygons, including finding the perimeter given the side lengths, finding an unknown side length, and exhibiting rectangles with the same perimeter and different areas or with the same area and different perimeters.
- 7.G.4·Know the formulas for the area and circumference of a circle and use them to solve problems; give an informal derivation of the relationship between the circumference and area of a circle.
- 7.G.B.4·Know the formulas for the area and circumference of a circle and use them to solve problems; give an informal derivation of the relationship between the circumference and area of a circle.