An alpha particle enters a hollow tube of $4 \,m$ length with an initial speed of $1 \,km/s$. It is accelerated in the tube and comes out of it with a speed of $9 km/s$. The time for which it remains inside the tube is

A particle starts from rest and traverses a distance $l$ with uniform acceleration, then moves uniformly over a further distance $2 l$ and finally comes to rest after moving a further distance $3 l$ under uniform retardation. Assuming entire motion to be rectilinear motion the ratio of average speed over the journey to the maximum speed on its ways is

When the velocity of body is variable, then

A body starts from rest with uniform acceleration $a,$  its velocity after $n\,seconds$ is $v$. The displacement of the body in last $3\,s$ is : (assume total time of journey from $0$ to $n\,seconds$ )