Apparently approximation plays a role in this seemingly simple issue:

Finding an approximate solution to a complex problem is an essential skill in science (and in life). Often we are faced with questions that we can’t answer exactly, but sometimes good enough is, well, good enough. Enrico Fermi, one of the greatest physicists in the 20th century, has given his name to such “Fermi Questions” – as he was famous for encouraging this skill in his students.

When it comes down to the physics it is a combination of friction and the particular layout of the books make for this challenge.

We hope you like the video!

thanks to theconversation.com for the great info