Under what condition will the magnitude of the displacement be equal to the distance travelled by an object ?

Name the force which is responsible for change in position or state of an object.

Mention the nature of motion of a body if its displacement$-$time graph is a straight line parallel to time axis.

A circular cycle track has a circumference of $314\, m$ with $A B$ as one of its diameter. $A$ cyclist travels from $A$ to $B$ along the circular path with a velocity of constant magnitude $15.7\, m s ^{-1}$. Find the

$(a)$ distance moved by the cyclist.

$(b)$ displacement of the cyclist, if $A B$ represents north$-$south direction.

$(c)$ the average velocity of the cyclist.

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.