Question

A ball is thrown with an inital velocity of 100 m\s. after how much time will it take to return

Answer 1

You should specify the direction with horizontal or vertical, but assuming that we throw it vertically upwards it will experience acceleration due to gravity of 10 m/s^2 so it will return to ground with same speed of 100 m/s due to conservation of energy, applying eq. v = u + at

-100 = 100 + (-10)*t
-> t = 20 sec