Question

A ball is thrown horizontally from the top of tower
100 m high, with speed 10 m/s. After 2 seconds of
fall, gravity disappears. Then total time taken by ball
to reach the ground is (g = 10 m/s2)
(1) 10 s
(2) 8s
(3) 6 s
(4) 4 s​

Answer 1

Answer:

Option (3) 6s

Explanation:

Given,

The height of the tower = 100 m

Initial horizontal speed = 10 m/s

Ball is thrown horizontally from the top of the tower therefore its initial vertical speed will be zero

Thus using the second equation of motion

The vertical distance h covered in 2 seconds

$h=\frac{1}{2}gt^2$

or, $h=\frac{1}{2}\times 10\times 2^2=20$ m

Therefore the height left = 100 - 20 = 80 m

The vertical velocity after 2 seconds

using the first equation of motion

$v=0+gt$

$\implies v=10\times2=20$ m/s

Since the gravity disappears after 2 seconds

therefore there will be no acceleration in the vertical direction

time taken to cover 80 m distance = distance/velocity

                                                          = 80/20 = 4 seconds

Therefore, total time taken by the ball to reach the ground = 2 + 4  =6 seconds