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Question
A boy of mass 50 kg running at 5 m/s jumps on to a 20 kg trolley travelling in the same direction at 1.5 m/s. What is their common velocity ?
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Solution
Momentum = Momentum of the boy + Momentum of the trolley
= Mass of boy × velocity of boy +Mass of trolley × Velocity of trolley
Momentum = Mass of the boy and trolley × common velocity
⇒ 280 = (50+20) × common velocity
⇒ 280 = 70 × common velocity
⇒ `280/70` = common velocity
⇒ 4 = common velocity
Hence, common velocity is 4 m/s.
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