A reference frame attached to the Earth

The figure shows the displacement of a particle going along the $x$-axis as a function of time. The force acting on the particle is zero in the region

The following figure shows, a heavy block kept on a frictionless surface and being pulled by two ropes of equal mass $m$. At $t=0$, the force on the left rope is withdrawn but the force on the right rope continues to act. Let $F_1$ and $F_2$ be the magnitudes of the forces by the right rope and the left rope on the block, respectively.