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A bomb of mass 1 kg is thrown vertically upwards with a speed of 100 m/s. After 5 seconds it explodes into two fragments. One fragment of mass 400 g is found to go down with a speed of 25 m/s. What will happen to the second fragment just after the explosion? (g = 10 m/s²)

1. It will go upward with speed 80 m/s
2. It will go upward with speed 100 m/s
3. It will go upward with speed 60 m/s
4. It will also go downward with speed 40 m/s

Correct answer: It will go upward with speed 80 m/s

Solution

Using the law of conservation of momentum, the total momentum before and after the explosion must be equal. Before the explosion, the bomb's velocity is 50 m/s (100 - 10×5). After the explosion, the momentum of the first fragment (0.4 kg × -25 m/s) and the second fragment (0.6 kg × v) must sum to the initial momentum (1 kg × 50 m/s). Solving gives v = 80 m/s upward.

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