A student determined to test the law of gravity for himself walks off a sky scraper 320 m high with a stopwatch in hand and starts his free fall (zero initial velocity). 5 second later, superman arrives at the scene and dives off the roof to save the student. What must be the maximum height of the skyscraper so that even superman cannot save him.

65 m

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85 m

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125 m

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145 m

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Solution

The correct option is C 125 m
Assuming superman can move at infinite speed, he would be able to save the student unless he has already fallen onto the pavement in 5 seconds.
Maximum height of the skyscraper so that the student falls on to the pavement in 5 seconds is:
$h = \frac{1}{2} g t^{2} = \frac{1}{2} (10) (25) = 125 m$
Hence, option C is correct.

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Similar questions

Q. A student determined to test the law of gravity for himself walks off a sky scraper 320 m high with a stopwatch in hand and starts his free fall (zero initial velocity). 5 second later, superman arrives at the scene and dives off the roof to save the student. What must be superman's initial velocity in order that he can save the student. [Assume that the superman's acceleration is that of any freely falling body.]

(g = 10 m / s^{2})

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