<human>:  I have a question about the physics of a bookcase in a library. Who invented the bookcase?
<bot>: I’m not sure who invented the bookcase, but I can tell you that the bookcase is a great example of a simple machine. A bookcase is a machine that uses a pulley system to move books from one shelf to another. The pulley system is made up of a rope, a pulley, and a weight. The weight is attached to the rope, and the pulley is attached to the bookcase. When you pull the rope, the weight moves the pulley, which moves the bookcase.