$A$ 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:

$A$ man on a rectilinearly moving cart,facing the direction of motion,throws a ball straight up with respect to himself.

$A$ motor car moving at a speed of $72 \, km/h$ cannot come to a stop in less than $3.0 \, s$,while for a truck,this time interval is $5.0 \, s$. On a highway,the car is behind the truck,both moving at $72 \, km/h$. The truck gives a signal that it is going to stop in an emergency. At what distance should the car be from the truck so that it does not bump into (collide with) the truck? The human response time is $0.5 \, s$.

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 $Q$?

On a foggy day,two drivers spot each other when $80\, m$ apart. They were travelling at $70\, km/h$ and $60\, km/h$. Both apply brakes simultaneously which retard the cars at the rate of $5\, m/s^2$. Which of the following statements is correct?

$A$ ball is dropped from the top of a building $100\,m$ high. At the same instant,another ball is thrown upwards with a velocity of $40\,m/s$ from the bottom of the building. The two balls will meet after $..........\,s$.