Five objects of different masses are simultaneously released vertically downwards from height $h$ (in air). Which physical quantity associated with the objects will change at the instant they strike the ground? (Neglect the air resistance.)

$A$ ball of mass $0.15 \; kg$ hits the wall with an initial speed of $12 \; m/s$ and bounces back without changing its initial speed. If the force applied by the wall on the ball during the contact is $100 \; N$,calculate the time duration of the contact of the ball with the wall in seconds.

$A$ bullet is fired from a gun. The force on the bullet is given by $F = 600 - 2 \times 10^5 t$,where $F$ is in newtons and $t$ is in seconds. The force on the bullet becomes zero as soon as it leaves the barrel. What is the impulse imparted to the bullet in $N-s$?

$A$ $140\,g$ ball,in horizontal flight with a speed of $39.0\,m/s$,is struck by a bat. After leaving the bat,the ball travels in the opposite direction with a speed of $39.0\,m/s$. If the impact time $\Delta t$ for the ball-bat collision is $1.20\,ms$,calculate the average force in $N$ acting on the ball.

$A$ machine gun fires bullets of mass $30 \text{ g}$ with a velocity of $1000 \text{ m/s}$. The man holding the gun can exert a maximum force of $300 \text{ N}$ on it. How many bullets can he fire per second at most?

$A$ body of mass $5 \,kg$ is moving with a velocity of $\vec{v}=(2 \hat{i}+6 \hat{j}) \,m/s$ at $t=0 \,s$. After time $t=2 \,s$,the velocity of the body is $(10 \hat{i}+6 \hat{j}) \,m/s$. The change in momentum of the body is ......... $kg \cdot m/s$.