$A$ $210 \ m$ long train is moving due North at a speed of $25 \ m/s$. $A$ small bird is flying due South a little above the train with a speed of $5 \ m/s$. The time taken by the bird to cross the train is $........ \ s$.

Two trains $A$ and $B$,initially $120\, km$ apart,start moving towards each other on the same track with a velocity of $60\, km/hr$ each. At the moment of start,train $A$ blows a whistle,which reflects off train $B$ and subsequently reflects off train $A$,and so on. If the velocity of sound waves in air is $1200\, km/hr$,what is the total distance travelled by the sound waves before the trains crash (in $km$)?

Two stones are thrown up simultaneously with initial speeds of $u_1$ and $u_2$ $(u_2 > u_1)$. They hit the ground after $6 \, s$ and $10 \, s$ respectively. Which graph in the figure correctly represents the time variation of $\Delta x = (x_2 - x_1)$,the relative position of the second stone with respect to the first up to $t = 10 \, s$? Assume that the stones do not rebound after hitting the ground.

$A$ bus is moving with a speed of $10\; m/s$ on a straight road. $A$ scooterist wishes to overtake the bus in $100\; s$. If the bus is at a distance of $1\; km$ from the scooterist,with what speed should the scooterist chase the bus? (in $m/s$)

Two trains along the same straight rails are moving with constant speeds of $60\, km/hr$ and $30\, km/hr$ respectively towards each other. If at time $t = 0$,the distance between them is $90\, km$,the time when they collide is ......... $hr$.

$A$ car travelling at $15 \,m/s$ overtakes another car travelling at $10 \,m/s$. Assuming each car is $4 \,m$ long, what is the time taken during the overtake (in $\,s$)?