Study guide: Measure Standard Lengths and Number-Line Changes
Read ruler intervals, compare unit sizes, find differences, and model changes on a line.
Ideas to explain in your own words
- A ruler gives positions along a scale.
- Different units produce different counts for the same length.
- A length comparison asks for the extra distance beyond the shorter object.
- A number line measures position from zero using equal intervals.
Useful vocabulary
- centimeter
- A standard metric unit of length.
- interval
- The space between two neighboring marked values.
Watch your reasoning
- The end label equals length only when the start is zero.
- A smaller written count may describe a longer unit.
- How much longer requests a difference, not a combined length.
- Subtraction moves left on a usual increasing number line.
A worked review example
A strip starts at 2 cm and ends at 11 cm. Its length is 11−2=9 cm. Nine one-centimeter intervals lie under the strip. The endpoint eleven includes two centimeters before the strip’s start, so it is not the strip’s length. Count spaces between marks, not both endpoint labels. The end label equals length only when the start is zero. Draw a ruler segment and emphasize the two endpoints of the object.
Make a practice plan
- Measure When an Object Does Not Start at Zero: Start 5 cm, end 18 cm. Expected: 13 cm.
- Compare Measurement Counts Using a Known Unit Relation: One long unit equals five short units; seven long units equal 35 short units.
- Find How Much Longer in the Same Unit: Compare 58 cm and 39 cm. Expected: 19 cm longer.
- Model Addition and Subtraction as Number-Line Jumps: Start at 46 and move right 27. Expected endpoint: 73.