I carry all of my school supplies around in my backpack at all times, and it gets pretty heavy at times. I have a binder for all 4 APs, 2 folders, 3 various notebooks, and other odds and ends to get me through the school day. After some light research, I found the average binder weighs 3 pounds, and since my notebooks have the same amount of paper, I'll assume they'll have the same mass. 3 pounds is 1.36 kilograms, and since the other odds and ends probably are around 5 pounds, I converted it to 2.27 kg. This adds up to 11.79 kilograms, which is 115.54 N. This means my back is producing this large of a force to hold my backpack up at a constant height. The straps also have to exert a large force, so make sure you have strong straps on your backpack!