The big idea: A physical prototype answers questions a screen cannot: weight, reach, grip, balance and fit in a real space.
The guide names three considerations for building one — scale, shape and space — and two purposes, aesthetic and functional.
Each consideration drawn as the failure a prototype catches and the failure that slips past without one.
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Aesthetic prototype
- Tests how it looks and feels in the hand
- Needs the right surface, colour, proportion and weight distribution
- Need not work at all — often a solid block
- Answers: would a user pick this up? does it read as a quality tool?
Functional prototype
- Tests whether it does the job
- Needs the right mechanism, fixings and materials
- May look like nothing — a rig on a board is fine
- Answers: does the catch hold? does the grip overheat? does it tip?
One prototype doing both usually does neither: Give users a finished-looking working model and they comment on the finish. Build a working mechanism inside a styled shell and the mechanism gets compromised to fit.
Build two, each answering its own question, and each can be rough where it does not matter.
Cost rises with fidelity and willingness to change falls — the argument for low early and high late.
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| Consideration | What you build | What it catches |
|---|---|---|
| Scale | Full size wherever a body is involved; reduced scale only for layout | That the drill's grip circumference is too large for a 5th-percentile hand |
| Shape | Shaped foam or carved block, weighted to the real mass | That the trigger is comfortable but the tool rolls when you set it down |
| Space | The model taken to the real place, with the real obstructions | That the body will not fit between a joist and a wall at the angle needed |
Full size, or it is not testing the body: A quarter-scale model looks perfectly proportioned and tells you nothing about reach, grip or weight.
Reduced scale is for layout and space planning. Anything a hand or a body judges must be built full size.
Weight the model: A foam drill body weighs almost nothing, so every user says it is comfortable.
Add lead shot or steel to bring it to the real mass and the same users find the balance wrong within seconds. Unweighted models flatter every design.
How this is tested — constructing physical prototypes at a fidelity that answers a stated question. It comes up two ways:
Paper 1 — multiple choice
- Identify whether a prototype is aesthetic or functional.
- Pick the consideration a described failure relates to.
Paper 2 — analysing a product
- Justify the prototypes to build for a named product.
- Explain what a physical prototype tests that CAD cannot.
The trap: Building one impressive model. It flatters the design, it cannot be changed, and it answers whichever question it happens to suit rather than the one you need answered.
A team is developing a cordless drill for users with reduced hand strength. Justify the physical prototypes they should build, referring to scale, shape and space.
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