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A 50 kg object is thrown straight up with an initial speed of 100 m/s. Five seconds later, it splits into two fragments of masses 20 kg and 30 kg. If the 20 kg fragment moves upward at 150 m/s, what is the velocity of the other fragment?

1. 15 m/s downward
2. 15 m/s upward
3. 51 m/s downward
4. 51 m/s upward

Correct answer: 15 m/s downward

Solution

Using g = 9.8, after 5 s the body's velocity is 100 - 9.8(5) = 51 m/s upward, so momentum = 50(51) = 2550 kg m/s up. Conservation: 20(150) + 30 v2 = 2550, giving v2 = (2550 - 3000)/30 = -15 m/s, i.e. 15 m/s downward.

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