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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?

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    $\begingroup$ You would first have to explain why you think this situation violates the third law. $\endgroup$
    – Javier
    Oct 28, 2020 at 16:50
  • $\begingroup$ The question does not say that I think the law is violated. $\endgroup$
    – D. Ennis
    Oct 28, 2020 at 18:03
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    $\begingroup$ The difficulty for me here is that I find it so immediately clear what is happening that there is no mystery to explain. It is like a magician's trick that goes wrong because you know what he did. Here you stand in the boat and blow the air; the air goes somewhere else (not in the boat); job done. $\endgroup$ Oct 28, 2020 at 18:16
  • $\begingroup$ What you say in your last sentence would be true with or without a sail, so what job is done? And I agree that there is no mystery, I just seek a vigorous description of specific forces on specific objects in this specific example, with some continuity of how they relate. $\endgroup$
    – D. Ennis
    Oct 28, 2020 at 18:34
  • $\begingroup$ Using the conservation of momentum is a valid application of Newton's Third Law. $\endgroup$ Oct 28, 2020 at 18:35

3 Answers 3

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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.

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  • $\begingroup$ I must repeat the response I used below: With all due respect, is this intended as "yes" to the question or a "no?" $\endgroup$
    – D. Ennis
    Oct 28, 2020 at 18:22
  • $\begingroup$ I thought the final paragraph was pretty clear about summarizing why the third law should not be seen as a restriction on the motion here. Do you think that the answer would be improved if I added a "yes it can", or is there something else missing? $\endgroup$
    – BowlOfRed
    Oct 28, 2020 at 19:05
  • $\begingroup$ I agree that the last paragraph is good. I do think that readers would benefit from beginning the answer to a yes/no-explain question with a yes or no statement (no sarcasm intended). The other paragraphs don't lead to clear visualizations for me. $\endgroup$
    – D. Ennis
    Oct 28, 2020 at 19:46
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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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  • $\begingroup$ it does not violate newton's law, because the ball (or the wind) are not part of the system after in bounces from the wall (sail), so there is an external force (the system is not closed) $\endgroup$
    – user65081
    Oct 28, 2020 at 17:41
  • $\begingroup$ With all due respect, would that be a "yes" or a "no" to the question? It appears to invoke conservation of momentum, but why would the cannonball bounce back? The cannonball would be moving forward and the cannon-Earth object moving backward, at a speed with inverse proportion to the proportion of the masses. When the ball hit the wall, both would stop. $\endgroup$
    – D. Ennis
    Oct 28, 2020 at 17:50
  • $\begingroup$ yes to the question, I though that was pretty clear. I added a phrase to make it more clear $\endgroup$
    – user65081
    Oct 28, 2020 at 17:50
  • $\begingroup$ Your answer doesn't address the backwards motion resulting from initially firing the ball though. $\endgroup$ Oct 29, 2020 at 13:44
  • $\begingroup$ @BioPhysicist how not? the ship moves initially backwards and it stops once the ball reaches the wall, then moves forward when the ball moves backwards, but this time will not stop because the ball leaves the ship instead of getting caught back by the cannon $\endgroup$
    – user65081
    Oct 29, 2020 at 14:36
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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.

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  • $\begingroup$ I agree with all of this, but you invoked the momentum concept. $\endgroup$
    – D. Ennis
    Oct 29, 2020 at 13:16
  • $\begingroup$ @D.Ennis You could remove the mention of momentum, although I think it requires changing the last sentence of the first paragraph to "... with a strong backwards force." I think the reason it's difficult to remove the concept of conservation of momentum is that the fundamental rationale for why you can't blow your own sail is a momentum one. The language has to seep in, even if the final physical description doesn't use momentum. Otherwise it becomes a rather unsatisfactory refutation, failing to challenge the central tenent of the problem. $\endgroup$
    – Cort Ammon
    Oct 29, 2020 at 14:56
  • $\begingroup$ Particularly in the sail case, the directions of the forces involved are constantly changing over the surface of the sail and the volume of the airspace in front of it. That is a very complex fluid dynamics problem, so we look to conservation laws to limit what could happen. The ducted fan simplifies that geometry, but the rationale for why it applies in the sail case is tricky without conservation $\endgroup$
    – Cort Ammon
    Oct 29, 2020 at 15:01

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