counterpoise

A hanging mobile, every bar tilted. Hang weighted shapes on its hooks and watch the whole mobile swing and settle into its new tilt. A bar hangs level when weight × distance matches on both sides, and everything hanging below a hook counts toward its weight.

Nothing falls and nothing rushes you: the swing just shows your sum. On the bigger mobiles the trick is the order. Solve the bottom first, because those totals decide what the bars above them need.