A ball of mass 0.5 kg moving at 6 m/s collides with a stationary ball of mass 1.5 kg. If they stick together after collision, what is their common velocity?

Using conservation of momentum: m₁u₁ = (m₁ + m₂)v. Therefore, 0.5 × 6 = (0.5 + 1.5) × v, giving 3 = 2v, so v = 1.5 m/s. Option C results from using only the moving ball's mass in the denominator, and option B from incorrectly averaging the masses.