Interlacing pages of books - Need help with understanding

Hello all,

I am working on a science project based upon paper, and I have chosen the affect of friction between two interlaced books. The attachment below is something I have found on the internet, as part of my research.

I don't understand why the R_av = 0.5 * (mass₁ + mass₂) *9.81. Why is the total mass of the books halved? Could you please explain this?

Thanks in advance,

A.T.

EDIT: I am not sure if this is the correct section. If it isn't the correct section, please do let me know and I will delete this post and put it in the appropriate place.

Attached Files

Inseparable phone books, science fact or science friction.pdf (66.7 KB)