$A$ ball is dropped downwards. After $1$ second,another ball is dropped downwards from the same point. What is the distance between them after $3$ seconds?

Due to the presence of air resistance, if a body dropped from a height of $20 \ m$ reaches the ground with a speed of $18 \ m/s$, then the time taken by the body to reach the ground is nearly (in $s$)

$A$ paratrooper jumps from an aeroplane and opens a parachute after $2 \ s$ of free fall and starts decelerating with $3 \ m/s^2$. At $10 \ m$ height from the ground, while descending with the help of the parachute, the speed of the paratrooper is $5 \ m/s$. The initial height of the aeroplane is . . . . . . $m$. $(g = 10 \ m/s^2)$

$A$ balloon rises from rest with a constant acceleration $g/8$. $A$ stone is released from it when it has risen to height $h$. The time taken by the stone to reach the ground is

$A$ helicopter rises from rest on the ground vertically upwards with a constant acceleration $g$. $A$ food packet is dropped from the helicopter when it is at a height $h$. The time taken by the packet to reach the ground is close to [$g$ is the acceleration due to gravity].

From $18 \text{ m}$ height above the ground, a ball is dropped from rest. The height above the ground at which the magnitude of velocity equals the magnitude of acceleration (in the same set of units) due to gravity is . . . . . . $\text{m}$. (Take $g = 10 \text{ m/s}^2$ and neglect the air resistance)