$A$ train is running at a constant speed of $90 \,km/h$ on a straight track. $A$ person standing on the top of a bogie moves in the direction of motion of the train such that he covers $1 \,m$ on the train each second. The speed of the person with respect to the ground is .........

Ball $A$ is thrown vertically upward with a speed of $10 \, m/s$. At the same instant,another ball $B$ is released from rest at a height $h$. At time $t$,the speed of $A$ relative to $B$ is ...........

$A$ particle is dropped from rest and another particle is thrown downward simultaneously with initial speed $u$. Then:

Two skaters $P$ and $Q$ are skating towards each other. Skater $P$ throws a ball towards $Q$ every $5 \,s$ such that it always leaves her hand with speed $2 \,ms^{-1}$ with respect to the ground. Consider two cases:
$(I)$ $P$ runs with speed $1 \,ms^{-1}$ towards $Q$,while $Q$ remains stationary.
$(II)$ $Q$ runs with speed $1 \,ms^{-1}$ towards $P$,while $P$ remains stationary.
Note that irrespective of the speed of $P$,the ball always leaves $P$'s hand with speed $2 \,ms^{-1}$ with respect to the ground. Ignore gravity. At what intervals will the balls be received by $Q$?

Train $A$ is moving along two parallel rail tracks towards north with speed $72 \ km/h$ and train $B$ is moving towards south with speed $108 \ km/h$. The velocity of train $B$ with respect to $A$ and the velocity of the ground with respect to $B$ are (in $ms^{-1}$):

When is the relative velocity of two moving cars zero?