Jet propulsion from a closing book

When a book closes, the air between the approaching surfaces is accelerated and ejected through the open sides at large speeds. The air resists this motion, and so a reaction forces acts on the closing book. To study this force, an idealized book is placed on rollers so it is free to move horizontally. The speed gained by the book is measured for different initial opening angles and for different book lengths. The dependence on these parameters is found to be surprisingly simple, and the extrapolation to plates of infinite length agrees well with a simple model. This system is interesting because it is relatively easy to build, measure, and model, and also because colliding surfaces in a fluid are an everyday occurrence. This system can be used as either a classroom demonstration or a laboratory experiment.