A sack is a bag of flour: if the cloth and the grains are simulated, weight, drag and volume come out of the physics, and the only acting left is when the sack chooses to pull a muscle.
The sack is a closed ring of 48 Verlet points with near-inextensible stretch links, a weak bend link and an area constraint, plus two corner flaps that are 4-point spring chains; inside it are 327 simulated flour grains (1.2 cm discs with collision, friction and gravity; grains at rest are drawn merged into one powder surface) that pool at the bottom and press the cloth out. The tower, the pile and the sack's filling are the same 753 grains: the 426 tower grains are held by a packing frame until the knife (a kinematic blade) cuts them loose, and the cloth's rest lengths grow with the count of grains inside, so the sack is only as big as the flour it holds (the pale puffs are uncounted dust motes).
The score sets only muscles: a net-force-free pull of the ring toward a pose (squash, lean, a mid-height bend that carries the line of action, spread, crouch, stretch), ear stiffness, and push-offs that act only while the cloth touches the table; everything after is integrated at a fixed 1/144 s step (gravity is set to 1.8 m/s² as a style choice: slow, weighty falls that read at this scale over 32 s) and cached per frame, so scrubbing replays the run. Travel is hops: a cloth point touching the table keeps its x (static friction) until the next push-off; while bracing, the table's friction holds the body's centre so it does not give ground.
Scored world events, not simulated: the tower is a rigid packed frame whose lean is a scheduled creep (accelerating as it is undermined) that the sack's measured contact push slows to zero; while the sack braces, a push beyond holding rights the tower at no more than 2° per second; the knife's strokes are timed; the fourth stroke flicks the penny straight up with a fixed small drift that knows nothing of the sack, and the tower releases all its grains at once. The leap reads the falling penny and aims to meet it; once the penny touches the sack's top it rides that point of cloth, sliding along the top seam only where the seam is steeper than its friction (the ear flaps stop it at the seam's ends); the sack then balances it, tipping its top so the penny's side rises, and it settles in the seam's dip. In the gather a flow field at the open top draws loose grains in; a grain that reaches the mouth is moved 12% of the way toward the centre and counted inside.
| choice | score | measured |
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