The craft moves to the right.
Work Step by Step
In this situation, there are two force pairs to consider that involve the craft: the fan-air force pair, and the air-sail force pair. The fan pushes a certain mass of initially stationary air to the right (action), giving it an impulse, and the reaction force pushes the fan and its attached craft to the left. The sail exerts a leftward force on the incoming air (action), large enough to stop it and to bounce it to the left, giving it an impulse. The reaction force exerts a rightward force on the sail. The air-sail impulse is approximately twice as great as the fan-air impulse because of the bouncing, and the net impulse is toward the right. If the air were all contained within a ship's closed cabin, the forces would all be internal and there would be no net propulsion.