$A$ ball is dropped on the floor from a height of $20 \ m$. It rebounds to a height of $5 \ m$. The ball remains in contact with the floor for $1 \ s$. The average acceleration during contact is (acceleration due to gravity $g = 10 \ m/s^2$): (in $m/s^2$)

$A$ block of mass $5\, kg$ is $(i)$ pushed in case $(A)$ and $(ii)$ pulled in case $(B)$,by a force $F = 20\, N$,making an angle of $30^o$ with the horizontal,as shown in the figures. The coefficient of friction between the block and floor is $\mu = 0.2$. The difference between the accelerations of the block,in case $(B)$ and case $(A)$ will be ........ $ms^{-2}$ .$(g = 10\, ms^{-2})$

An object of mass $10 \,kg$ is projected from the ground with a speed of $40 \,m/s$ at an angle of $60^{\circ}$ with the horizontal. The rate of change of momentum of the object one second after projection in $SI$ units is ........ [Take $g = 9.8 \,m/s^2$]

Match Column-$I$ with Column-$II$ correctly.

Column-$I$	Column-$II$
$(1)$ Static friction	$(a)$ Limiting friction
$(2)$ Rolling friction	$(b)$ Ball bearing
	$(c)$ Object moving on a road

$A$ parachutist with a total mass of $75 \,kg$ drops vertically onto sandy ground with a speed of $2 \,m/s$ and comes to a halt over a distance of $0.25 \,m$. The average force exerted by the ground on her is close to ............ $N$.

Assertion $(A):$ Two identical trains move in opposite senses in the equatorial plane with the same speeds relative to the Earth's surface. They have equal magnitude of normal reaction.
Reason $(R):$ The trains have different centripetal accelerations due to different speeds.