Moments and balance

The rule is uniform and exactly 100 g, and every mass is exact. Each mass hangs at a whole centimetre mark on a thread of no mass; g = 9.8 N/kg.

The rule tips only a few degrees, so its marks can still be read; a real rule would tip until it touched the bench. It counts as level when the moments differ by less than 0.004 N m, so each slide balances at one mark.

The card’s centre of gravity is worked out from its shape. Real readings are taken in a lab; this is simulated apparatus.

With reduced motion the rule and the masses go straight to their new places, and the card hangs still at once.