Which force acts on a body first: action or reaction?

When a bullet is fired from a rifle, its momentum becomes $20 \,kg \cdot m/s$. If the velocity of the bullet is $1000 \,m/s$, then what is its mass?

$A$ particle of mass $m \, kg$ moving with a velocity $v \, m/s$ strikes a surface as shown in the figure and rebounds with the same speed. The magnitude of the change in momentum is:

$100$ balls each of mass $m$ moving with speed $v$ simultaneously strike a wall normally and reflect back with the same speed in time $t \ s$. The total magnitude of force exerted by the balls on the wall is

The position-time graph for a particle of mass $100 \ g$ is shown in the figure. The impulse acting on the particle at $t = 5 \ s$ is ............ $N-s$.

The unit of impulse is