2.Motion in Straight Line
medium

A balloon starts rising from ground from rest with an upward acceleration $2 \,m / s ^2$. Just after $1 \,s$, a stone is dropped from it. The time taken by stone to strike the ground is nearly ....... $s$

A

$0.3$

B

$0.7$

C

$1$

D

$1.4$

Solution

(b)

$u=0, a=2 \,ms ^{-2}$

The velocity of object after one second

$v=u+a t$

$\Rightarrow v=2 \,ms ^{-1}$

$s=\frac{1}{2} \times 2 \times 1^2=1 \,m$

Now after separating from the balloon it will move under the effect of gravity alone.

$-h=v t-\frac{1}{2} \times 9.8 \times t^2$

$\Rightarrow-1=2 t-4.9 t^2$

$\Rightarrow 4.9 t^2-2 t-1=0$

$\Rightarrow t=0.7 \,s$

Standard 11
Physics

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