What can you say about the motion of an object whose distance time graph is

$(i)$ a straight line, parallel to the time axis ?

$(ii)$ a straight line passing through the origin making an angle with the time axis ?

A racing car has an acceleration of $4\, m s ^{-2} .$ What distance will it cover in $20$ seconds after start ?

Draw a diagram to show the motion of a body whose speed remains constant but velocity continuously changes.

Define 'average speed'. An object moves with a uniform speed of $10\, m s ^{-1}$ for $5 s$ and then with a uniform speed of $5\, m s ^{-1}$ for $10\, s$. Find its average speed.

A car manufacturer advertises that the brakes are so perfect that the car stops instantaneously. Comment.