FA-87223 / Physics integrator stability / Member archive

Friction clamps each increment rather than the accumulated tangent impulse · case 03

Sliding contacts accumulate unbounded friction over iterations.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

Case contract

solve(vn0, vt0, w, mu, iterations, cached, warm): several contact points share one relative normal and tangent velocity with inverse mass w. Warm start each point with warm*cached [jn, jt] and apply it to the velocities. Each iteration visits points in order: friction first (accumulated jt clamped to |jt|<=mu*jn), then normal (accumulated jn clamped >=0). Return [jn list, jt list, vn, vt] rounded to 6.

Why this case matters

Game and robotics physics loops depend on integrator update order, step control and stabilization terms; a wrong decision point turns a stable simulation into drifting or exploding motion.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
case 4	`[[0.0, 0.0, 0.422492], [-0.385211, -0.488, -0.0584], -0.0, 0.0]`	`[[0.0, 0.0, 0.422492], [-0.0, -0.0, -0.392495], -0.0, 0.354738]`	Failed

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