The rear side of a truck is open, and a box of 40 kg mass is placed 5 m away from the open end as shown in Fig. On a straight road, the truck starts from rest and accelerates with 2 ms^-2. At a distance 20 m from the starting point the box falls off the truck.  Find the coefficient of friction between the box and the surface below it. 

(Ignore the size of the box)