Angle of repose θ_r = tan⁻¹(μ): the steepest incline at which the object just stays at rest. If θ ≤ θ_r it stays (static friction f = mg·sinθ); if θ > θ_r it slides with a = g(sinθ − μ·cosθ). The mass cancels out, so it changes the forces but not whether the object slides. Arrows are drawn in units of mg.