Question

11. A person throws a ball vertically upwards with velocity 20 m sfrom the top of a building of heighth and simultaneously throws another ball with same velocity in downward direction. If the ratio of the time taken by two balls to reach the ground is 5: 2, then the height of the building is m. (Take g = 10 m ) (a)800/9(b)9/800(c)400/3(d)3/400

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Answer 1

Answer:

option (a) is correct 800/9 m

Explanation:

t₁ /t₂ = 5/2

Since final velocity at ground is same

v = -20 + 10t₁ = 20 + 10 x t₂

-2 + t₁ = 2 + t₂

t₁ - t₂ = 4

5/2 x t₂ - t₂ = 4

t₂(5/2 - 1) = 4

t₂ = 4x 2/3 = 8/3 s

for second ball

s = ut + 1/2 gt²

h = 20 x 8/3 + 1/2 x 10 x 64/9

= (480 + 320)/9

= 800/9 m