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I hope this is a physics topic. I thought it may be appropriate because of conservation of momentum. I was under the impression that lift in an airplane wing ( an airfoil ) was due to difference in pressure. However a physics video on Youtube said that it was conservation of momentum that generates lift , which did not make sense to me, I am now wondering about the validity of the video. The video author is a physicist so maybe I thought the physics section of stackexchange may be appropriate to ask. Is there some underlying physics I am not aware of? Thank you


marked as duplicate by sammy gerbil, stafusa, Jon Custer, John Rennie newtonian-mechanics Oct 15 '17 at 6:21

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    $\begingroup$ There is no way to avoid using conservation of momentum, because in plain mechanics conservation of momentum and Newton's 3rd Law are the same thing. $\endgroup$ – dmckee Oct 13 '17 at 5:08
  • $\begingroup$ -1. No research effort, and not clear. See related questions on this site. Why doesn't conservation of momentum make sense to you? Where is the video? You ought to provide a link, otherwise we cannot comment on what it says. $\endgroup$ – sammy gerbil Oct 14 '17 at 9:04
  • $\begingroup$ I cannot give the video or the link because I tried that once and was told that was not allowed. In any event I found a number of other sources that seemed to have an explanation of what I was looking for and as it turns out conservation of momentum plays a small part after all. A good explanation of how an airplane stays in the air is a video by sixty symbols...that is not the original video I looked at that really confused me but the latter video clarified the situation. There are many many factors involved and there is also that 3-4% unknown , a complicated process. $\endgroup$ – Sedumjoy Oct 15 '17 at 14:50
  • $\begingroup$ It is good practice for users to post links for reference in their questions or answers. The question or answer itself should be self-contained, and should explain (in summary) the relevant information in the link. What we disapprove of is "link only" questions or answers. $\endgroup$ – sammy gerbil Oct 17 '17 at 11:00

Both explanations are equivalent. The wing forces air down, so by Newton's third law (and momentum conservation), air pushes the wing up. This upward force manifests itself as pressure on the wing since air is well-modeled as a fluid and the air being forced down combines with the air below the wing to form a higher density, high-pressure region. The space the forced-down air left is now less dense and has less pressure, creating a pressure difference between the top and bottom of the wing. Because the pressure on top of the wing is less than the pressure on the bottom, the overall force on the wing is upward.

Another way to say this is that the Newtonian view is the microscopic view--imagining individual air molecules colliding with the bottom of the wing to hold it up. The pressure-based (or Bernoulli) view takes a macroscopic view, in which fluid air interacts with an obstruction and flows around with varying speeds and pressures. The Bernoulli view can be derived from the Newtonian view.

After watching the Sixty Symbols video, I'm understanding the confusion. The image that comes to mind when considering the simple Newtonian picture is incomplete in one sense because it looks like only the air under the wing makes lift. The Bernoulli aspects of the explanation gives the result that air flowing over the top of the wing ends up with a larger downward momentum, and so contributes more lift than the air flowing under the wing. The complete picture not only considers the air that interacts with the wing, but also how that wing-pushed air interacts with the surrounding air.

The simple statement is still true: airplanes stay up by moving air down. The detailed explanation gets into hairy fluid-dynamics math that tells you that wings both push and pull air down (among many other actions).

  • $\begingroup$ You might want to add that it is exactly this pressure difference which makes the air change direction. Then the equivalence becomes obvious. $\endgroup$ – Peter Kämpf Oct 17 '17 at 5:35
  • $\begingroup$ @PeterKämpf To my mind, it makes more sense to say that the movement of air causes the pressure difference, but arguing the difference is like trying to determine which force is the action and which is the reaction according to Newton's third law. I've expanded my answer. $\endgroup$ – Mark H Oct 17 '17 at 6:25
  • $\begingroup$ There is not one thing that causes another; it is all happening together. What is action and what is reaction is only up to the observer. BTW: The Newtonian explanation of flow only helps in hypersonic aerodynamics and is plain wrong in subsonic flow. $\endgroup$ – Peter Kämpf Oct 17 '17 at 14:08

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