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A 2 kg body moving at 4 m/s collides elastically with a stationary body and afterwards keeps moving forward at one-fourth of its original speed. The speed of the centre of mass of the two-body system is x/10 m/s. Find x.

1. 20
2. 25
3. 30
4. 40

Correct answer: 25

Solution

In an elastic collision the centre of mass moves at constant velocity equal to total initial momentum divided by total mass; here it is 2.5 m/s, so x = 25.

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