(2) isn't actually impossible with a fan and sail. The gas molecules recoil off the sail, imparting nearly 2x their original momentum (given by the fan) to the

(2) isn't actually impossible with a fan and sail. The gas molecules recoil off the sail, imparting nearly 2x their original momentum (given by the fan) to the sail. Not as efficient as just using the fan as a propeller, but it does work. One of my college professors demonstrated it in class and published a paper about it in a physics education journal decades ago [0]. [0] https://aapt.scitation.org/doi/10.1119/1.2341931 https://aapt.scitation.org/doi/10.1119/1.2341931