I have a A4 sized notebook and every time I work on something, a corner of the page gets bent up - as I roll my pen on it I notice that if I don't roll it fast enough, it won't roll off the notebook, it reaches the top part of the bent part of the page and then comes to a halt and rolls back down. Normally this would be a ball in a bowl problem, but the bowl's surface is solid and doesn't change. The bent page, however, does. What is the force that the bent page counteracts the approaching pen with? The page is bent up, but as the pen rolls on it, it starts to bend back down, but if the pen isn't rolling fast enough, the page bends itself back up and pushes the pen back. How does this work?