A librarian moonlighting as a magician wants to pull one book out of a stack of books. The stack is seven books high and all the books are of roughly equal mass M, of comparable size, and the coeffi- cients of friction between the books are u, and jux- The librarian needs the third book in the stack, counting from the bottom. The librarian gives the third book a hard, straight yank with a force of magnitude F perpendicular to the stack. In terms of M, g, and the appropriate coefficient of friction. what is the horizontal force, F, with which she must yank the book?