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A body of mass 2 kg makes an elastic collision with a second body at rest and continues to move in the original direction but with one fourth of its original speed. What is the mass of the second body?

1. 1.0 kg
2. 1.5 kg
3. 1.8 kg
4. 1.2 kg

Correct answer: 1.2 kg

Solution

For an elastic collision with a stationary target, v1' = (m1-m2)/(m1+m2)*v = v/4. So (2-m2)/(2+m2)=1/4 -> 8-4 m2 = 2 + m2 -> m2 = 1.2 kg.

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