Fan and Sailboat on wheels

A sailboat and a fan are both independenly mounted on wheels with the sailboat in front of the fan. If the sailboat is held stationary while the fan is on the fan will move backwards. If the fan is held stationary and the sailbot allowed to move the sailboat will move forward. The two are then linked together and the class can be asked what direction the whole unit will go.
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There is a large boat stored in 1-3 Le Conte. I've also added a sail to one of the normal Pasco carts so that the whole experiment can be done on the track for reduced friction. That cart is in Le Conte 55-2.