Question

Two tall buildings are 40 m apart. With what speed must a ball be
thrown horizontally from a window 120 m above the ground in one
building so that it will enter a window 40 m from the ground in the
other.​

Answer 1

Ball is thrown horizontally with speed u .

Let it take t time to reach other building.

Height of descend for ball = 120 - 40  = 80 m

use S = ut + 1/2gt^2 in y direction; also uy = 0

80 = 1/2 g t^2 ..... take g = 10

t = 4 sec

so Ball takes 4 sec to descend 80 meters and reach 2nd building

This much time ball should take horizontally to cover 40 m.

now use ux t = 40  

this will give ux = 10 m/s , hence ux = u = 10m/s

Answer 2

10m/s is the right answer