WAEC Physics 2015 Paper

Concept:A uniform bar has its whole weight acting at its midpoint, called its centre of gravity.For equilibrium, the total clockwise moment about the knife-edge equals the total anticlockwise moment.Explanation:Let the mass of the uniform bar be $M$.The weight of the bar acts at its centre, which is $10\ \text{cm}$ from the knife-edge.The $22\ \text{g}$ mass hangs at $C$, which is $30\ \text{cm}$ from the knife-edge on the other side.Taking moments about the knife-edge:$$M \times 10 = 22 \times 30$$$$10M = 660$$$$M = \frac{660}{10} = 66.0\ \text{g}$$Since the same value of $g$ acts on both sides, mass may be used directly in the moment equation.Answer:The mass of the bar is $66.0\ \text{g}$.The correct option is B.