A block is placed on a rough horizontal plane. A time dependent horizontal force F = kt acts on the block. Here, k is a positive constant. The acceleration-time graph of the block is

As force starts acting, static friction opposes it and remains at rest until the limiting value. This happens until time t=μmgk. After that kinetic friction acts, and block starts accelerating linearly.Case I : (t≤μmgk)kt=fs⇒a=0Case II : (t>μmgk)kt−μmg=ma⇒a=kmt−μg