Lesson objective: Measure objects with precision by partitioning into smaller parts.

Students bring prior knowledge of measuring fractional lengths on number lines and rulers to wholes, halves, and fourths from 3.MD.B.4. This prior knowledge is extended to measuring with precision to smaller units as students partition number lines and rulers to measure objects with precision. A conceptual challenge students may encounter is thinking that the lines or markings on a number line or ruler are final and they can not be partitioned further to find a more precise unit.

The concept is developed through work with number lines and rulers, which can be partitioned in the same way.

This work helps students deepen their understanding of equivalence because students learn that the length of an object or figure can be named with different measurement units. This does not change the physical length of the object or figure, so the measurements must be equivalent.

Students engage in Mathematical Practice 6 (attend to precision) as they partition number lines into smaller pieces in order to find more precise measurements of objects.

**Key vocabulary:**

- exact
- invariant
- length
- number line
- partition
- precise
- ruler