Solution: Kinetic Energy

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To solve this problem, we must apply the conservation of energy. Initially, the ball is traveling at 4 km/s and the smaller ball is at rest. Finally, the ball is at rest and the smaller ball begins to move.

E_i = E_f
(1/2)m(4)² + (1/2)(m/4)(0)² = (1/2)m(0)² + (1/2)(m/4)v₂²

v₂ = 2 km/s

Answer: 2