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Ball A (mass 5.0 kg) moving at 20 m/s collides with ball B moving at 10 m/s in the same direction. After the collision A moves at 10 m/s and B at 15 m/s, both still in the same direction. Find the mass of ball B.

1. 6.0 kg
2. 10. kg
3. 2.0 kg
4. 12 kg

Correct answer: 10. kg

Solution

Momentum conservation gives 5*20 + m*10 = 5*10 + m*15, so m = 10 kg.

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