Two bodies with masses 2 kg and 4 kg (initially at rest) collide elastically. Post-collision, the 4 kg block moves at 3 m/s. What was the initial kinetic energy of the system?
Correct: B
Use momentum conservation: 2u₁ + 4u₂ = 2v₁ + 4×3. Since initial momentum is zero, u₂ = 0. So 2u₁ = 6 ⇒ u₁ = 3. KE = ½×2×3² = 9 J. Answer is 90 J due to calculation error. Correct approach requires solving for v₁ and recalculating KE correctly.