The driver of a train $A$ travelling at a speed of $54\, km h^{-1}$ applies brakes and retards the train uniformly The train stops in $5\, s$. Another train $B$ is travelling on the parallel track with a speed of $36\, km h ^{-1}$. This driver also applies the brakes and the train retards uniformly. The train $B$ stops in $10\, s$. Plot speed time graph for both the trains on the same paper. Also, calculate the distance travelled by each train after the brakes were applied.

If the displacement$-$time graph for a particle is parallel to time axis, what is the velocity of the particle ?

Can the displacement of a particle be zero when the distance travelled is not zero ?

Starting from rest at the top of an inclined plane a body reaches the bottom of the inclined plane in $4$ second. In what time does the body cover one$-$fourth the distance starting from rest at the top ?

The velocity$-$time graph of a truck is plotted below

$(a)$ Calculate the magnitude of displacement of the truck in $15$ seconds.

$(b)$ During which part of the journey was the truck decelerating ?

$(c)$ Calculate the magnitude of average velocity of the truck.