When is the relative velocity of two moving cars zero?

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

$A$ 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 driver signals an emergency stop. At what minimum distance should the car be from the truck so that it does not collide with the truck? The human response time is $0.5\, s$.

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?

An express train is moving with a velocity $v_1$. Its driver finds another train moving on the same track in the same direction with velocity $v_2$. To escape a collision,the driver applies a retardation $a$ to the train. The minimum time to escape a collision will be:

$A$ man is at a distance $d$ behind a bus. The bus starts from rest and moves away from the man with a constant acceleration $a$. At the same time,the man starts running towards the bus with a constant velocity $v$. Which of the following statements are correct?