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The science
Sorting by material. Each part has a material with a typical handbook density (steel 7850, aluminium 2700, brass 8500, ABS 1050, nylon 1140, rubber 1150 kg/m³). Mass is m = ρ V with a rounded volume per part; the volumes are my estimates of the modelled objects, not measurements. The Magnet button marks ferromagnetic parts (iron and steel are attracted; aluminium, brass, copper and plastics are not), which is how you tell a steel bracket from an aluminium one. A relay is only weakly attracted (steel core).
Fall into a bin. A released part falls with h = ½ g t², so t = √(2h/g) and the impact speed is v = √(2 g h). The page integrates with a fixed 1/240 s velocity-Verlet step (exact for constant g) and solves for the contact instant inside the last step, so this check confirms the implementation rather than testing new physics. On the Moon (1.62 m/s²) the same drop takes 2.46 times longer than on Earth. Restitution (0.25 for metal, 0.4 plastic, 0.6 rubber) is an assumed illustrative value.
Bin heap. The heap height is H = ΣV / (A φ) with the bin floor A = 0.44 x 0.30 m and an assumed loose-packing fraction φ = 0.12 for irregular parts.
Wiping. The rag removes a swept disc; a stroke of length L and radius r clears A = π r² + 2 r L (a stadium shape). The bench counts as clean when under 6 % of the starting dirt remains.
Recycling energy. Making metal and plastic from recycled scrap needs far less energy than from ore or oil; the table (steel 18, aluminium 150, brass 60, plastics 45–60, rubber 25 MJ/kg saved) holds illustrative round numbers of the right order of magnitude, not audited data. 3.6 MJ = 1 kWh.
Scoring. +10 points per correct part, +10 % per consecutive correct part (capped at x2), −5 for a wrong bin (the part is thrown back). Cleaning the bench gives 100, each tool on the pegboard 25, and finishing everything adds 2 points per second left. Levels need 100 + 50 L experience. These numbers are game design, not physics.
Checks. (1) fall time and impact speed against the closed form on Earth, Moon and Mars; (2) a wipe stroke against πr² + 2rL by pixel counting; (3) scoring against the closed-form sum for 40 correct parts; (4) a cylinder volume by lattice counting against πd²l/4 and the bolt mass; (5) the clock over 600 s at irregular frame rates; (6) sorting all 86 sortable parts and comparing bin masses with a direct tally. Simplifications: parts are shown enlarged (x1.35 by default), no rotation-dependent contacts, no bin overflow, the dirt is a 2D texture, prices and energy numbers are illustrative.
The workshop, bins, tools, 26 part types, bicycle, mower and street are models made in Blender; the model and the game logic run in the page.