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A boat made of the same material: the condition for floating

Year VII pupils make a solid PLA token and check that it sinks. They build a shallow tray from the same PLA: a densely filled 60 by 40 mm base and four wall strips welded on the inside with a book as support. The tray floats, sits lower with tokens, and the teacher's tray sinks under the class load. Pupils explain floating through the weight of the whole object and displaced water.

Age
12–14 years
Length
approx. 45 min
Level
Medium
Where you teach
School
Purpose
Educational
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How to run this session

Stages and timing
  1. Solid token
  2. Token in water
  3. Tray base
  4. Four wall strips
  5. First long side: weld
  6. Second long side and first short side
  7. Close the perimeter and corners
  8. Tray on water
  9. Two tokens aboard
  10. Stage 10: class result and extension

What gets built

A sinking token and a watertight floating tray made of the same PLA; sheet K records sinking / floating, immersion in bead rows, the class demonstration and an explanation.

What it practises
  • Comparing two objects of the same material: solid and hollow.
  • Comparing immersion using whole wall bead rows, with eyes at water level.
  • Drawing dense, watertight infill and a continuous weld on a supported joint.
  • Safe work with water and an electrical device in the same room.

Success criteria
  • The pupil shows that a solid PLA token sinks while a tray made of the same PLA floats and remains dry inside after 30 s.
  • The pupil predicts and records that the tray with two tokens will still float but sit lower (less wall above the water), then observes it.
  • The pupil records a prediction of the number of tokens needed to sink the teacher's tray alongside the class result.
  • The pupil explains floating and sinking through the whole object's weight and the amount of displaced water, without claiming that air inside lifts the tray.

What you need
Filament
PLA
Colours
colour shown in the illustration
Also
3D pen (one per pupil), transparent work mat, pen stand, W1: full-size 1:1 PLA part templates (printed from the package, per pupil), K: observation sheet (printed from the package, per pupil; kept at the water container), a book, such as a hardback textbook (per pupil; supports the strip during welding), a water container or bowl at least 15 cm wide, water about 5 cm deep (1 per two pairs), plus a transparent water container on the teacher's desk; these are not illustrated, cloth (1 per two pairs, at the container), pencil and ruler (per pupil)

Files

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Version 0b1a8ce8-00f9-47ee-8899-9547108ea975 Updated 2026-10-12

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