Question

A boy is standing on top of a tower of height 85m and throws a ball in the vertically upward direction with a certain speed. If 5.25 seconds later he hears the ball hitting the ground, then the speed with which the boy threw the ball is (take g = 10 m/s2 , speed of sound in air = 340 m/s)

• A. 6 m/s
• B. 8 m/s
• C. 10 m/s
• D. 12 m/s

Answer 1

The correct option is B 8 m/s

Time taken by sound =$\frac{85}{340}=0.25sec$
Time taken by the ball = 5.25 – 0.25 = 5 sec.
Now
$-85=u\times 5-\frac{1}{2}\times 10\times 25$
U = 8 m/sec