Treebeard's Stumper Answer
Here's another stumper about fans. We had a boat race in the pool during R&D (Research and Development) class last week when the weather was hot. Several kids tried to give their boats a boost by blowing at the sails as they swam along. It might work even better to swim along with a battery powered fan pointed at the boat. So what if some young inventor mounted a powerful fan permanently on their boat blowing into the sails. Would that work best of all?
Josh proved that a boat with a fan moves just fine. He won our R&D boat contest that way. And a fan will definitely push a boat if it is not attached. But a fan mounted on a boat blowing into the sails will NOT move the boat any more than leaning against the mast. This is a direct consequence of Newton's Third Law: the force on the fan is equal and opposite to the force on the sails, so they cancel out. The fan does push the sail, but it also acts as a propeller trying to pull the boat the other way. As James said, "The fan will try to pull the boat in half." We tried: it doesn't go.
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Marc Kummel / firstname.lastname@example.org