Why does the box slide backward when the truck accelerates?

[CursedAntagonis]

Homework Statement

A large box containing your new computer sits on the bed of your pickup truck. You are stopped at a red light. The light turns green and you stomp on the gas and the truck accelerates. To your horror, the box starts to slide toward the back of the truck. (Assume that the truck is accelerating to the right.)


Draw clearly labeled free-body diagram for the truck. (The bed of the truck is not frictionless.)

Homework Equations

N normal force
Wt Weight of the truck
fk is friction force of the box on the truck bed
Froad is the reaction force to the tires backward push
nb is the reaction to the normal force on the box from the truck

The Attempt at a Solution

I got N going upward
Wt going downward
nb going downard
fk going to the right (forward)
Froad going to the right (forward)

However, it's not the correct answer. Anyone could explain to me what I got wrong here?

Thanks.

 

[CursedAntagonis]

No one knows the answer or can lend me a hand with this?

 

[pooface]

Normal force of the box by the truck bed should be going up.