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A balloon was moving upwards with a uniform velocity of 10 m/s. An object of finite mass is dropped from the balloon when it was at a height of 75 m from the ground level. The height of the balloon from the ground when object strikes the ground was around: (takes the value of g as 10 m/s²)

1. 300 m
2. 200 m
3. 125 m
4. 250 m

Correct answer: 125 m

Solution

The dropped object has u = +10 m/s upward from 75 m: 75 + 10t - 5t^2 = 0 -> t^2 - 2t - 15 = 0 -> t = 5 s. The balloon keeps rising at 10 m/s, so its height = 75 + 10x5 = 125 m.

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