Mythbusters: Sail on a Boat

Back in the section on Newton’s First Law, I mentioned one of the classic examples of internal forces: a fan on a sailboat. It has since been pointed out to me a few times, usually in a somewhat smug and conclusive tone of voice, that Mythbusters busted that myth a long time ago.

mythbusters episodes
Smugly awesome.

So why did their boat go forward, as in the video below? I’m a big fan of the show, but I do think Grant’s explanation (at around 4:16) is a bit misleading.

The simplest answer is that this actually isn’t an example of an internal force as I’d claimed, because it uses the air, which is not connected to the boat. An internal force comes from something that is a part of whatever you’re trying to move – like in the case of trying to push yourself forwards with your own arm. The boat, on the other hand, doesn’t bring the air with it.

The sail in the video is curved in such a way that the air hitting it probably turns around and heads backwards. Since the boat is pushing the air backwards, the air must be pushing the boat forwards (by Newton’s Third Law). Meanwhile, the fan at the back of the boat is pushing the air fowards, which results in a backwards force on the boat. If the first force happens to be stronger than the second, the boat will go forwards. (In fact, in an earlier clip from the same episode, a little toy boat did move backwards, because that sail didn’t do as good a job at pushing the air back.)

I hate admitting mistakes as much as anyone, which is why it’s nice to so rarely be wrong. Please accept my begrudging apology and enjoy the video.