Avner

| f=mv --> f=mv --> | ____|_______ _______|_____ f=MV <--- \___________ ____________/ ---> f=MV Consider two separate parts of a boat. Left half of the boat: Fan pushes an air particle to the right. the paricle carries a force f. As a reaction, the Left half of the boat is pushed to the left with force f. The particle, originally pushed to the right by the fan, now arrives at the vicinity of the Right half of the boat, hits the sail with force f pushing the Right half of the boat to the right. The combined boat is thus subjected to two equal and opposite forces and therefore will not move horizontally. Of course, if f is large enough, the ship would be torn apart by the stress caused by the opposing forces and as a consequence move downward