A body is thrown vertically upward or downward from an elevated point (tower, building, helicopter), and the question asks for time to reach the ground, maximum height, meeting point, or a relationship between given quantities.
From the top of a tower, a ball is thrown vertically upward which reaches the ground in 6 s . A second ball thrown vertically downward from the same position with the same speed reaches the ground in 1.5 s . A third ball released, from the rest from the same location, will reach the ground in $\_\_\_\_$ s.