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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.

https://www.youtube.com/watch?v=uKXMTzMQWjo

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?

But those answers imply that since the phenomenon can be explained in terms of the conservation laws, Newton's Third Law isn't violated. This is not satisfying. To be satisfying, an explanation would have to state the Third Law in one or more of its forms, explain why it appears to be contradicted in terms of the statement of the law itself, and then go on to explain why it is not contradicted in terms of the statement itself.

Can the valid application of Newton's Third Law to the the phenomenon in the Mythbusters swamp boat video be explained in terms of Newton's Laws, as applied to the actual objects in the video, without reference to conservation laws?

D. Ennis
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Because the boat (the rigid structure holding the fan and sail) does not describe the entire force system, the third law does not prohibit net forces from arising that create motion. Motion of the boat is countered by (untracked) motion of the air in the vicinity of the boat.

If all the forces were internal to the boat, then we would expect the thrust from the fan and the impact on the sail would be equal and there would be no motion.

But that's not the case here. The fan doesn't have perfect directionality, it entrains nearby air, and the sail doesn't absorb the airstream but redirects it. Any of these can create a net force on vessel, causing acceleration.

The third law does't prevent motion here because the all the force couples do not act on the same object. If you instead put the fan and the sail inside a closed room on a similar boat preventing these effects, I would expect a different outcome.

BowlOfRed
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Imagine that instead of a fan and a sail inside the boat, you have a ball cannon and a wall. If the cannon fires to the wall and the ball bounces back into the cannon, the net motion after that will be zero, but if the ball hits the wall at an angle and bounces back leaving the boat, then the boat will move forward, as you basically are shooting backwards.

Thus the answer to your question is yes. once the wind bounces back it leaves the system, applying an external force to the sail.

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The easiest explanation I have heard for this study is to replace the fan and sail with a ducted fan that has a fan blowing "forwards," and a U shaped duct in front of it which redirects that air stream "backwards." For most people, this will intuitively cause the boat to move forward. They can see that the air is coming from all directions into the fan (net 0 momentum), and then being directed out backwards, with a strong backwards momentum.

As such, we have a simple Newton's third law situation: air is forced backwards, and the equal and opposite reaction drives the boat forward.

Thus the only thing which is causing the problem in our minds is the sail. We think of the sail as being a sort of "wall," so that the air coming off of it has 0 net momentum, which should mean that we don't go forward. However, we shape the curve of the sail with the expressed intent of directing air. When the air leaves the sail, it isn't leaving it in all directions. It's leaving it in a predominantly backwards direction.

Cort Ammon
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