Firstly, I understand and apologize that this is more of a physics question than a math question.
I noticed that when I pulled a book out from under a pack of gum, the gum stayed largely in place (it did not move with the book). This confused me, so I decided to draw a diagram of the situation, assuming that my answer would be apparent once I could represent all of the forces in play. Of course (as the book was orthogonal to the ground) gravitational force and normal force could be ignored. The only other force that I see that can be in play is the friction between the book and the gum, which should make the gum move along with the book. There must, then, be some sort of force acting opposite and with a greater magnitude than friction. Is this correct, and if so, what is that force and how can it be calculated?