$A$ 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 ......... $kg \cdot m/s$.

$A$ jet of water with cross-sectional area $A$ strikes a wall at an angle $\theta$ with the normal to the wall and rebounds elastically. If the density of water is $\rho$ and the velocity is $v$,what is the force exerted on the wall?

When the speed of a moving body is doubled:

When a $4 \,kg$ rifle is fired,the $10 \,g$ bullet receives an acceleration of $3 \times 10^6 \,cm/s^2$. The magnitude of the force acting on the rifle (in newton) is ............

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.)

In an hour-glass,approximately $100$ grains of sand fall per second (starting from rest); and it takes $2 \, s$ for each sand particle to reach the bottom of the hour-glass. If the average mass of each sand particle is $0.2 \, g$,then the average force exerted by the falling sand on the bottom of the hour-glass is close to .......... $N$.