on the truck, so it, too, accelerates.
What force causes the crate to accelerate?
The friction force between the bottom of the crate and the truck surface it is resting on.
The frictional force between the crate and the flatbed causes the crate to accelerate with the truck.
If the frictional force was less than the acceleration force, the crate would not move (i.e. the truck would pull away and the crate would crash to the ground).Source(s): College physics.
The friction (resistance of movement) between the crate and the flatbed cause the crate to be pulled along by the flatbed.
This can be represented easily by rubbing your hands together quickly, as you do this friction builds up and causes heat.
Friction, unless it is bolted to the truck bed, in which case it is part of the truck and it moves due to the same cause that makes the truck move.