A train car has a mass of 10,000 kg and is moving at +3.0 m/s. It strikes an identical train car that is at rest. The train cars combine.

Conserve momentum during the collision. The initial momentum is (10,000 kg)(+3.0 m/s) + (10,000 kg)(0 m/s) = 30,000 kg·m/s. After the cars combine, their total mass is 20,000 kg, so their shared velocity is 30,000 ÷ 20,000 = +1.5 m/s. The correct choice is B, 1.5 m/s.

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