What causes a bent notebook page to push back against a pen?

[lendav_rott]

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?

 

[Simon Bridge]

It's a spring.

 

[jedishrfu]

The page curls because one side has absorbed more moisture than the other side (or one side has lost more moisture). The side with more expands causing the paper to curl.

The curled page then has a little springiness to it so as the pen rolls on it the paper gives and the pen slows its roll and then it pushes the pen back. If the pen rolls faster then it overcomes the springiness and rolls off and the paper springs back.