I'm reallizing that I'm very confused on how to make a free body diagram of forces in some situations where there is friction between two objects. I'll make an example where I cannot figure out what's the right diagram.
A $12.0 kg$ box rests on the ﬂat ﬂoor of a truck. The coefﬁcients of friction between the box and ﬂoor are $\mu_s=0.19$ and $\mu_k=0.15$. The truck stops at a stop sign and then starts to move with an acceleration of $2.20 m/s^2$. If the box is $1.80 m$ from the rear of the truck when the truck starts, how much time elapses before the box falls off the truck? How far does the truck travel in this time?
So what forces are working on the box?
(Where the square is the box and under there is the truck)
$F$ in black is the force on the truck that produces its acceleration while $f$ in green it's the force of interaction between the truck and the box. The forces in red and grey are on the $y$ axis and they are not interesting.
The main problems are about $f$ green: what is the real cause of this force?
Is it only due to static friction? I shouldn't be because the text of the problem says that the box will slide away so necessarily $f>\mu_s N$, where $N$ is the normal force on the box (the one up in grey).
Furthermore is that the correct direction for $f$? Of course the box cannot slide back but intuitvely it will fall from the left of the truck, right?
And finally should I add also the force of kinetic friction? If so what its direction would be?