Okay, so say that I stack three boxes, one on top of another. There is friction between all surface, including the bottom box and the ground. If I apply a force to the center box only, is there any set of circumstances under which the top box will slip? I know that if the applied force is large enough, static friction between the bottom and center box will be overcome and the middle box will slip over the bottom box. Is there a way for the top box to slip out as well? I am trying to draw a free body diagram, but the only x-axis force I find on the top box is friction, which always points in the direction of the force applied to the center box. I know that the applied force can be great enough to overcome the static friction between the top and middle boxes, but it seems to me like the top box will still accelerate some in hte direction of the applied force. Is this accurate? Think about this in terms of a playing card deck- if I take a deck of cards and set it on a table, and I pull out on random card halfway, if I flick it hard enough it will go flying out of hte deck without disturbing the cards above or below it.