Figure shows the position-time graph of a particle of mass $4 \mathrm{kg}$. Let the force on the particle for $\mathrm{t}<0, 0<\mathrm{t}<4 \mathrm{s}, \mathrm{t}>4$ be ${\mathrm{F}}_1, {\mathrm{F}}_2$and ${\mathrm{F}}_3$ respectively. Then

For $t<0$and $t>4s$,the position of the particle is not changing i.e.the particle is at rest. So no force is acting on the particle at these intervals.

For $0<t<4s$ the position of the particle is continuously changing. As the position-time graph is a straight line, the motion of the particle is uniform, so acceleration , $a = 0$. Hence no force acts on the particle during this interval also.