$A$ window is $50\,cm$ long. $A$ stone is falling from a height of $40\,cm$ above the window. It crosses the window in

Two balls $A$ and $B$ are thrown with the same velocity $u$ from the top of a tower. Ball $A$ is thrown vertically upwards and ball $B$ is thrown vertically downwards. If $t_A = 6 \ s$ and $t_B = 2 \ s$,then the height of the tower is ......... $m$.

$A$ 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].

Assertion: The zero velocity of a particle at any instant always implies zero acceleration at that instant.
Reason: $A$ body is momentarily at rest when it reverses its direction of motion.
The correct option among the following is:

$A$ stone is dropped from a height $h$. Simultaneously,another stone is thrown up from the ground which reaches a height $4h$. The two stones cross each other after time:

$A$ ball is thrown upward with speed $10 \, m/s$ from the top of a tower and reaches the ground with a speed $20 \, m/s$. The height of the tower is ......... $m$. (Take $g = 10 \, m/s^2$)