A 1.5kg box is initially at rest on a horizontal surface when at t = 0, a horizontal force $\overrightarrow{\mathrm{F}}=1.8\mathrm{t}\widehat{\mathrm{i}}\mathrm{N}$ (with t in sec), is applied to the box. The acceleration of the box as a function of time t is given by 
$\left\{\begin{array}{l}\overrightarrow{\mathrm{a}}=0,\text{ for }0\le \mathrm{t}\le 2.8\sec \\ \overrightarrow{\mathrm{a}}=(1.2\mathrm{t}-2.4)\widehat{\mathrm{i}}\text{ for }\mathrm{t}>2.8\sec \end{array}\right.$