JAMB Physics 2016 Paper

Concept: For a balanced ruler, the total clockwise moment equals the total anti-clockwise moment about the pivot.Explanation:Moment is the product of force and perpendicular distance from the pivot.Let the 10 N force produce a moment on one side: $10 \times 30 = 300 \, \text{Ncm}$.Let the 15 N force be placed at distance $x$ on the other side to balance it.For equilibrium: $10 \times 30 = 15 \times x$$300 = 15x$$x = \frac{300}{15} = 20 \, \text{cm}$.So the 15 N force must hang at a distance of 20 cm from the midpoint on the opposite side.Answer: C. 20 cm.