KVPY SX SB Exam 2015 Question Paper

Question : 26 of 120

Marks: +1, -0

A boy is standing on top of a tower of height 85 m 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/s

^{2 }

, speed of sound in air = 340 m/s)

6 m/s
8 m/s
10 m/s
12 m/s

Validate

Solution:

Time taken to reach sound after hit

t_s =H/V_\s

t_s=(85/340)\sec ; t_s = 0.25 \sec

For ball time of flight

T_f

T_f + t_s = 5.25 \sec

T_f = 5.25- 0.25

T_f= 5 \sec

For ball

V_0t-1/2gt^2=-H

⇒

V_0 (5) -1/2 10(5)^2 = - 85

5V_0 = 40

V_0 = 8

m/s

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