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The crate is placed on the flatbed truck.

The truck accelerates and the crate remains on the position.

Explanation :

The crate is at rest with respect to the truck means there is no relative speed between them. \ \ \ \ Both the truck and crate are moving with the same acceleration.

If there is no friction at all , as the truck moves towards east, the crate slides towards west.

But there is no external matter except the truck to make the crate move with some acceleration.

It follows that the truck must exert some force on the crate pulling it along with it.

Thus the frictional force acting on the crate keeps th

Free body diagram :

Here the truck accelerates in east direction.

denotes the frictional force.

denotes the accerlerating force.

is the force due to gravity acting downward.

is the normal force.