:

Here,

Mass of the box M =40kg

Acceleration of the truck,

Distance of the box from the rear end,

Coefficient of friction between the box and the surface below it

The various forces acting on the block are as shown in the figure

As the trunk moves in forward direction with the acceleration the box experiences a forces F in

in backward direction

Under the action of this force, the box will tend to move toward the rear end of the trunk. As it does so, its motion will be opposed by the force of friction which acts in the forward direction and it is given by

The acceleration of the box relative to trunk toward the rear end is

Let t be the time taken for the box to fall off the trunk

During this time, the truck covers a distance x

Using

We get

x = 20m