Where do you start measuring?
Slide the pencil along the ruler. Start it at 0 and the number under its tip is the length. Slide it up to 3 and that number changes, though the pencil is exactly as long as it was.
4 cm
It starts at 0, so the number at the end is the length.
- Starts at
- 0
- Ends at
- 4
- Length
- 4
Drag the pencil to slide it, or drag its right-hand end to change its length.
Same pencil, moved
Ready to try some? Measuring worksheets
Where it usually goes wrong
The number under the far end gets copied down as the length, however the object was placed. Start a 4 cm pencil at the 3 mark and the ruler says 7, and the child writes 7. Length is a difference between two marks, and it only equals the end number when you started at zero.
Try this
- Press From 0, then From 3. The pencil never changes size and the number at its tip changes by three.
- Slide the pencil to a random place and work out the length before looking. End take away start, every time.
- Press Half marks and read a length that is not a whole number. Most real measurements are not.
Where this goes next
Elapsed time is this same subtraction on a dial instead of a strip, and so is every reading taken off a scale in science.