A box rests on a horizontal surface where the coefficients of static and kinetic friction between the box and the surface are ${}_s$ and ${}_k,$ respectively, and ${}_k=\frac{{}_s}{2}.$ A linearly increasing horizontal applied force $F$ is exerted on the box starting at time $t=0,$ as shown in the graph. The box begins to accelerate at a time between $t_2$ and $t_3.$ The force of friction exerted on the box at times $t_1,t_2$ and $t_3$ has the magnitudes $f_1,f_2$ and $f_3,$ respectively. Which of the following claims about $f_2$ is correct?