Mythbusters Blow your own sail full scale
Why Physics Stack Exchange recommends it
Physics Stack Exchange answerers and commenters point people to it as real-world proof that blowing your own sail works and does not violate Newton's Third Law.
Recommended by 1 people across 1 different months, 2014 to 2024.
What it is
A short MythBusters clip in which the team mounts a sail on a swamp boat and points the fan at it, testing whether a boat can propel itself by blowing air into its own sail. The boat does creep forward. It is an uploader's copy of the original TV segment.
What people said
Recommends D. Ennis · 2020-10 · Physics Stack Exchange · on “Can "Blowing your own sail" be shown not to violate Newton's Third Law without reference to conservation of momentum or energy?”
Here is a Mythbusters clip showing that you actually can blow your own sail. They install a sail on a swamp boat and reverse the direction of the fan. There are other examples that the ideas works, like, notably, thrust reversers on jet airplanes. [this video] This initially strikes most people (Including Mythbuster's resident engineer, Grant Imahara (may he rest in peace) as violating Newton's Third Law. Grant did not succeed in explaining away the contradiction at the end of the clip. There are some related answers to a similar question here on the Stack Exchange: Blowing your own sail? [LINK] But those answers imply that since the phenomenon can be explained in terms of the conservation laws,
Cites JRE · 2014-11 · Physics Stack Exchange · on “Will this sail-based moving system work? Why?”
It does work - poorly. The sail has to be large enough, and it takes a very strong fan. You would be be better off turning the fan and using it to propel your boat directly as they do on swamp boats in Louisiana. Proof: Mythbusters video. [this video] The boat will actually move forwards if you have a strong enough fan and a big enough sail. Imagine that instead of a sail you put your fan at one end of a pipe. With a straight pipe, you point the pipe off the back of the boat so that the fan blows air through the pipe and out the back (fan in the pipe at the fron of the boat.) This will push your boat forwards. Now put a 90 degree bend in the pipe. The fan sticks off to the side of the boat, but t
Cites Luboš Motl · 2015-08 · Physics Stack Exchange · on “If we attach a fan at the end of a boat and switch it on, will the boat move?”
The boat may obviously move forward. That's how airplanes move, too. The turbines etc. make the fluid move backwards so the boat or aircraft has to move forward. A more interesting fact is that it may also move forward by creating "wind in the forward direction" by its own fan if this wind is reflected from the sail. Mythbusters have checked this question in this real-world video [this video] and the result is that they could make it move although the speed was unremarkable. There exist many approximate pictures in which the forces are guaranteed to cancel but none of those calculations is quite exact in the real world. Ultimately, there have to be terms that don't cancel in the real world. And t
Cites Cort Ammon · 2016-08 · Physics Stack Exchange · on “Canoe riding using your boomerang as motor”
In theory he could. The trick would be that he would need to throw the boomerang in such a way that it bleeds off much of its momentum to the air before being returned. If the Olympian did so, they would be able to generate thrust. A highly related Mythbusters video: Can you blow your own sail [this video] .
Cites Emilio Pisanty · 2016-12 · Physics Stack Exchange · on “Does a light sail move forward if the light source is originated from the craft?”
It works with an actual windsail, too [this video] , and no, the Third Law isn't violated. There's plenty of 'backscattered' momentum in the reflected wind and light.
Cites Cort Ammon · 2020-09 · Physics Stack Exchange · on “Invisible Coilgun Propagation”
@iCodeSometime Mythbusters did it in Blow Your Own Sail [this video] . The physics is counterintuitive, but makes a whole lot more sense if you replace the sail with a U shaped tube that captures all of the air. This basically turns the fan into a ducted fan that's pointing backwads, and obviously will cause forward motion. As long as the net movement of the air is backwards, conservation of momentum will drive you forward.
Cites Cort Ammon · 2024-03 · Physics Stack Exchange · on “Rocket attached to a pendulum. How is energy conserved?”
And the pattern of the rocket using air to pull on the earth, and that air then colliding with the ground feels a lot like blowing your own sail [this video] , but now we're getting into really complex interacting systems that are more complicated than your thoguht experiment.