# Lesson 3

Hagamos y usemos una regla

### Lesson Purpose

The purpose of this lesson is for students to use a ruler to measure and compare lengths in centimeters.

### Lesson Narrative

In earlier lessons, students measured with centimeters using centimeter cubes and 10-centimeter tools (base-ten blocks). They used multiple copies of the units to measure objects and line segments of different lengths.

In this lesson, students transition from measuring length with physical objects to measuring length with more abstract tools such as rulers, measuring tapes, and meter sticks. They create rulers and describe how a centimeter is represented on the tool (MP2). They learn that a centimeter is represented by the length between two tick-marks on a ruler and that each number on the centimeter ruler represents the distance in centimeters from zero.

Students use the ruler they create in the first activity throughout the lesson, including the cool-down. They should store their rulers for easy access in upcoming lessons.

• Representation

### Learning Goals

Teacher Facing

• Create and use a ruler with centimeter units.
• Measure to determine how much longer one object is than another.

### Student Facing

• Hagamos reglas y usémoslas para medir y comparar longitudes en centímetros.

### Required Materials

Materials to Gather

Materials to Copy

• Centimeter Ruler Template

### Lesson Timeline

 Warm-up 10 min Activity 1 20 min Activity 2 15 min Lesson Synthesis 10 min Cool-down 5 min

### Teacher Reflection Questions

In future lessons, students will learn how to represent whole numbers as lengths from 0 on a number line. How does their work today with represent length units on a ruler help lay the foundation for the more abstract work with the number line? How can you continue to reinforce how measuring tools represent length units in upcoming lessons to prepare for student work with number lines?

### Suggested Centers

• Target Numbers (1–5), Stage 5: Subtract Two-digit Numbers (Addressing)
• Five in a Row: Addition and Subtraction (1–2), Stage 6: Add within 100 with Composing (Supporting)