Principle of moments for balance

Steps:

• Uniform rule's weight acts at center (50 cm), 16 cm right of pivot (34 cm).
• 64 g mass at 4 cm is 30 cm left of pivot.
• Balance: anticlockwise moment (64 g × 30 cm) = clockwise moment (M × 16 cm).
• Solve: M = (64 × 30) / 16 = 120 g.

Why C is correct:

• Matches torque equilibrium from principle of moments: ∑ moments = 0 at pivot.

Why the others are wrong:

• A: Too low; ignores full lever arm distances.
• B: Too low; likely miscalculates center of mass position.
• D: Too high; possibly uses 50 cm as pivot distance.

Final answer: C