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/s²)
