Free Body Diagram of a Truck with a Box

[Cinnamon]

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.)
Draw the force vectors with their tails at the dot. The orientation of your vectors will be graded. The exact length of your vectors will not be graded but the relative length of one to the other will be graded.

Homework Equations

N_ground: Normal force
N_b: Reaction to the normal force on the box from the truck
F_road: Reaction force to tires' backward push
F_k: The box's friction force on the truck bed
W_T: Weight of the truck

The Attempt at a Solution

N_ground points up.
W_T and N_b point down.
F_road points right.
F_k points left.

I made the length of F_k shorter than F_road.
I made the length of N_ground = length of W_T = length of N_b.

Now I'm thinking that the length of W_T and N_b should be equal but together add up to the length of N_ground so that the net force in the y direction is 0.

 

[mfb]

Can you attach a picture of your sketch? That would help a lot.

Yes you'll have to add the weight of box and truck for the force between truck and ground - in the same way you cannot simply remove the mass of the driver or any part of the truck from this force.

 

[Cinnamon]

My only options are the ones I provided in relevant equations. I assume that W_T includes the weight of the box in the truck too so I don't think that's the issue here. Here's the sketch, the two overlapping vectors at the bottom are W_T and N_b.

 

[Cinnamon]

Here's a little diagram I made too if that helps at all.

 

[haruspex]

Since you are listing all the forces on the truck, and the normal force between the truck and the box is one of these, you should not include that in W_T. So the normal force from the ground should equal their sum.
But why do you think W_T and N_b would be equal? And why have you drawn F_k about equal in length to F_road?