AS Daily A Level Physics question
In a lab, a uniform beam is pivoted at its centre and held horizontal. A 20 N weight is hung 0.60 m from the pivot on the left. To balance the beam, at what distance from the pivot on the right should a 15 N weight be hung?
Answer
The correct answer is D.
Correct: D — 0.80 m. Balancing requires equal and opposite moments: 20 × 0.60 = 15 × d, so d = 12/15 = 0.80 m. A uses equal distances, which only balances if the forces are equal; 20 N ≠ 15 N. B applies direct rather than inverse proportion: 0.60 × (15/20) = 0.45 m gives 15 × 0.45 = 6.75 N m, too small to balance 12 N m. C overestimates the needed distance: 15 × 1.00 = 15 N m, which would tip the beam the other way. D equates the moments correctly to give 0.80 m.