Center of Mass 2D — Balancing a Multi-Mass Seesaw

Where is the center of mass of a body made of several parts? This 2D simulator makes that question concrete — attach point masses anywhere along a rigid seesaw, drag them to new positions or masses, and watch the center of mass x̄ = Σmᵢxᵢ / Σmᵢ recompute live. Move a pivot support under the rod and release it: because every mass rides exactly on the rotating rod, the moment of inertia about the pivot is exact, not approximate, and the live torque τ = −M g cosθ (x̄ − x_pivot) integrates forward every frame as real angular acceleration. Line the pivot up under the center of mass and the rod can rest tilted at any angle — a neutral equilibrium; miss it, and the rod swings until the heavier side hangs straight down, exactly like a real compound pendulum.