$A$ $100\,m$ long train crosses a man travelling at $5\,km/h$ in the opposite direction in $7.2\,s$. The velocity of the train is $..........\,km/h$.

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 ...........

$A$ ball is dropped from a building of height $45 \, m$. Simultaneously,another ball is thrown up with a speed of $40 \, ms^{-1}$. Calculate the relative speed of the balls as a function of time.

$A$ car $A$ is travelling on a straight level road with a uniform speed of $60 \text{ km/h}$. It is followed by another car $B$ which is moving with a speed of $70 \text{ km/h}$. When the distance between them is $2.5 \text{ km}$,the car $B$ is given a deceleration of $20 \text{ km/h}^2$. After how much time will $B$ catch up with $A$ in hours?

$A$ person sitting in a moving train with his face towards the engine,throws a coin vertically upwards. The coin falls ahead of the person. The train:

Explain the concept of relative displacement and its various cases.