Measure length with equal units
Align starting points and measure with equal units placed without gaps.
Start with the activity ↓Can the count of units change while the length stays the same?
Align one end to compare lengths. To measure, lay equal rods end to end and count how many cover the length.
Two ribbons starting in different places can be hard to compare. Align their left ends. The one reaching farther is longer. Moving a ribbon is not the same as stretching it.
A gesture may not tell a friend the length clearly. Place equal small rods end to end. If six fit exactly, say the length is six small rods.
Gaps leave parts unmeasured. Overlaps count parts more than once. Mixing unequal rods also changes what ‘one rod’ means.
Now use a large rod as long as two small rods. The same ribbon fits six small rods or three large rods. The counts differ, but the ribbon was neither cut nor stretched. Always name the unit.
A ruler repeats equal intervals. From 0 to 5 there are five intervals; from 2 to 7 there are also five. Read the distance between the endpoints, not just the final label. The symbol cm names a standard unit called a centimetre.
Listen one paragraph at a time
1 / 5
2 / 5
3 / 5
4 / 5
5 / 5
Now check it yourself
Move the ribbon's starting point or change the unit rod. Predict what changes while the ribbon length stays fixed, then reveal the result.
Only the position and unit rod change; the ribbon stays the same length.
Unit: one small rod
This activity uses lengths that fit whole rods exactly. Partial units and more detailed unit conversions come later.
Try it away from the screen
Measure a ribbon with equal blocks. When using a real ruler, align the start with zero. The screen diagram is not drawn at a physical ruler's scale.
Can you explain another example?
Check your idea 1
Check your idea 2
Can you explain it in a new setting?
Explain why the same length can be reported with different numbers.
Speak, point or draw on paper. Check whether your own explanation includes these ideas.
Continue reading, or pause here
Is a longer object necessarily heavier? Next, distinguish length from weight.