Ok,now I know the answer.The point is incredibly the "equivalence principle" and the stability of an object. That seems crazy but just think of it. The horizontal aceleration of the truck(plus the gravity downward) is the same than a situation where the truck is inclined in such a way that the...
In the exact situation when the pack begins to turn over I can see those forces
gravity applied in the center of gravity
horizontal normal force(x) applied in the corner near the piece of wood
vertical normal force(y) applied in the corner near the piece of wood too.
but the point is why...
Well, in principle, this problem seems very easy but I don't know the exact explanation.Here we go. We have a truck and a pack in its back.The bed of the truck is frictionless and the pack is stopped by a small piece of wood of something like that. The size of the pack is whatever(for...