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 ......... $hr$.

Two buses $A$ and $B$ are moving in opposite directions. If the first bus $A$ moves towards the east with a speed of $36 \ km/h$ and bus $B$ moves towards the west with a speed of $18 \ km/h$,then the bus $B$ appears to bus $A$ as:

Two particles are at rest on a straight track and they are separated from each other by $100 \ m$. They start their motion in the same direction,with the same acceleration $10 \ m/s^2$. Their relative displacement after $10 \ s$ will be

Two particles $A$ and $B$ are moving along a straight line with $B$ ahead of $A$. If their velocities remain constant,what will be the effect on the magnitude of their relative velocity when $A$ is ahead of $B$?

Two particles start simultaneously from the same point and move along two straight lines,one with uniform velocity $v$ and the other with a uniform acceleration $a$. If $\alpha$ is the angle between the lines of motion of the two particles,then the time at which the relative velocity is minimum is given by:

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 ...........