$A$ ball is thrown vertically upwards with a velocity of $15 \ m/s$. To what maximum height will it reach? (Take $g = 10 \ m/s^2$)

$P, Q$ and $R$ are three balloons ascending with velocities $U, 4U$ and $8U$ respectively. If stones of the same mass are dropped from each when they are at the same height $h$,which stone reaches the ground first?

The acceleration of a vertically projected body at its highest reaching position is

$A$ 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 . . . . . . $m$. $(g = 10 \ m/s^2)$

When a ruler falls vertically, $5$ different persons catch it with different reaction times. $(g = 9.8 \text{ ms}^{-2})$
$A$. Person $A$ has reaction time of $0.20 \text{ s}$
$B$. Person $B$ has reaction time of $0.22 \text{ s}$
$C$. Person $C$ has reaction time of $0.18 \text{ s}$
$D$. Person $D$ has reaction time of $0.19 \text{ s}$
$E$. Person $E$ has reaction time of $0.21 \text{ s}$
What is the correct order of the distance travelled by the ruler for each person?

$A$ ball is thrown vertically upwards from the ground $G$ with a speed $u$. It reaches a point $B$ at a height $h$ (lower than the maximum height) after time $t_1$. It returns to the ground after time $t_2$ from the instant it was at $B$ during the upward journey. Then $t_1t_2$ is equal to