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Ruler detective: how long is the segment really?

A grade 4 pupil first estimates by eye, then uses a ruler to measure three printed template segments (40, 70 and 100 mm), and uses a 3D pen over that same template to make three separate, flat PLA measuring strips of those lengths. After cooling, the pupil measures each strip to 1 mm, calculates the difference between the design and finished dimensions, and identifies the largest difference.

Age
9–11 years
Length
approx. 45 min
Level
Easy
Where you teach
School
Purpose
Educational
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How to run this session

Stages and timing
  1. Three strips: A 40 mm, B 70 mm, C 100 mm
  2. Cold strips and a prediction
  3. Measure strip A
  4. Measure B and C and complete the table
  5. One sentence and tidying up
  6. Optional: two strips in one row

What gets built

The pupil measures a segment to 1 mm and records the result in millimetres, estimating its length by eye before measuring. After the lesson, the pupil can explain: (1) the design number and measured number describe different things: the design says how long the segment WAS MEANT to be, while measurement says how long it REALLY is; (2) the ends must be clearly defined, and the ruler is aligned using its 0 mark rather than its edge.

What it practises
  • Estimating and measuring lengths to 1 mm.
  • Making three PLA strips and comparing design with measurement.
  • Calculating differences and explaining the result.

Success criteria
  • The pupil estimates before measuring, then records three measurements in mm, read to 1 mm.
  • Aligns the 0 mark with the outer end of the strip and compares the measurement with the design.
  • Calculates differences, identifies the largest and explains why a design does not replace measurement.

What you need
Filament
PLA
Colours
colour shown in the illustration
Also
3D pen for PLA: 1 per pupil., PLA filament, 1.75 mm: 0.8 m (2.4 g) per pupil (a complete set of three strips A+B+C), in one dark, saturated colour., Pen power supply: 1 set., Working mat resistant to the PLA operating temperature: 1., 1:1 template MAT01-LISTWY (A4): 1 sheet per pupil., Measurement sheet W2 (A4): 1 sheet per pupil., Pencil: 1 per pupil., Ruler with millimetre divisions, at least 15 cm long: 1 per pupil + 2-3 spares for the teacher.

Files

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Version 2f2c329e-4cb2-4db1-b56c-8f470c91691e Updated 2026-10-12

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