Step-by-step explanation:
A heavy crate rests on the bed of a flatbed truck. When the truck accelerates, the crate remains where it is on the truck, so it, too, accelerates. Due to the frictional force, the crate accelerates.
The force of friction is an opposing force. The force of friction depends on the coefficient of friction and the normal force acting on the object. The frictional force is of two types i.e sliding friction, static friction.
So, the frictional force causes the crate to accelerate.