A packet is dropped from a balloon rising up with uniform velocity 9.8 $m / s^{- 1} .$ . If the balloon is at a height of 39.2 m from the ground at the time of dropping the stone, the stone reaches the ground after.

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6 s

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4 s

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8 s

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Solution

The correct option is B 4 s
Given,
$u = - 9.8 m / s^{2}$
$a = + g = 9.8 m / s^{2}$
$s = 39.2 m$
We know the formula,
$S = u t + \frac{1}{2} a t^{2}$
$- 39.2 = 9.8 \times t - 4.9 t^{2}$
$4.9 t^{2} - 9.8 t - 39.2 = 0$
$t^{2} - 2 t - 8 = 0$
$t^{2} - 4 t + 2 t - 8 = 0$
$t (t - 4) + 2 (t - 4) = 0$
$(t - 4) (t + 2) = 0$
$t = 4, t = - 2$

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A packet is dropped from a balloon rising up with uniform velocity 9.8m/s−1.. If the balloon is at a height of 39.2 m from the ground at the time of dropping the stone, the stone reaches the ground after.

The correct option is B 4 sGiven,u=−9.8m/s2a=+g=9.8m/s2s=39.2mWe know the formula,S=ut+12at2−39.2=9.8×t−4.9t24.9t2−9.8t−39.2=0t2−2t−8=0t2−4t+2t−8=0t(t−4)+2(t−4)=0(t−4)(t+2)=0t=4,t=−2

A packet is dropped from a balloon rising up with uniform velocity 9.8 $m / s^{- 1} .$ . If the balloon is at a height of 39.2 m from the ground at the time of dropping the stone, the stone reaches the ground after.