Question

A balloon is going vertically up with velocity 12 m/s. When it is at height of 65 m above the ground, it releases a stone. In how much time the stone will reach the ground. 

(A) √13 s 

(B) 10 s 

(C) 5 s 

(D) 6 s

Answer 1

Correct option: (C) 5 s

Explanation:

given: v₀ = 12 m/s 

s = 65 m/s 

we know s = v₀t + (1/2) at² 

Taking v₀ = – 12 m/s (stone moving down) 

hence 65 = – 12 t + (1/2) × 10 × t² 

∴ 65 = – 12t + 5t² 

∴ 5t² – 12t – 65 = 0 

∴ 5t² – 25t + 13t – 65 = 0 

∴ 5t(t – 5) + 13(t – 5) = 0 

∴ 5t + 13t = 0 OR t = 5 = 0 

∴ t = 5 sec