Why can it be said that there is no such thing as an isolated force in nature?

If the final momentum is equal to the initial momentum of the system,then:

$A$ particle of mass $2 \ kg$ is initially at rest. $A$ force acts on it whose magnitude changes with time. The force-time graph is shown below. The velocity of the particle after $10 \ s$ is $.... \ ms^{-1}$.

$N$ number of balls,each of mass $m \ kg$,moving along the positive direction of the $x$-axis,strike a wall per second and return elastically. The velocity of each ball is $u \ m/s$. The force exerted on the wall by the balls in newtons is:

The force-time $(F-t)$ curve of a particle executing linear motion is as shown in the figure. The momentum acquired by the particle in the time interval from $0$ to $8 \ s$ will be ........... $N-s$.

Explain why the product of mass and velocity is important in producing the effect of force.