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Isn't lift generated by Newton's Third Law? For every action there is an equal and opposite reaction

by toddh 16y ago

Confusion 16y ago
Indirectly, everything is 'described' by Newton's Third Law. However, it doesn't provide a very satisfying answer: it's a description, not an explanation[1]. Knowing the wing goes 'up' because it reacts to the action of the air pushing against it from below, does not explain why that force is larger than the reaction to the action of the air pushing it down from above. Bernoulli's principle is basically a statement about what happens when you apply Newton's laws to a macroscopic amount of fluid and allows you to determine what happens when an asymmetrical object is placed in the flow of such a fluid: when the speed goes up, the pressure goes down. Pressure is nothing but the amount of Newtonian action exerted on a surface, which brings us back to the third law. Then you find that Bernoulli's principle doesn't quite cut it, because it doesn't take into account that fluids don't actually behave like the naive application of Newton's laws would lead you to believe. However, in the end, the Coanda effect is again a description of the behavior of fluids under certain circumstances, describing the macroscopic effect of Newton's laws applied to all individual particles. [1] The exact difference between the two is subject of much philosophical debate, but most would agree that something like Bernoulli's principle has more explanatory power than Newton's law, because, for instance, it allows you to understand why a wing would lift.
- toddh 16y ago
  I'm not sure why it's not satisfying. The airplane exerts a downward force, the air exerts an equal and opposite force upwards. If you don't change the motion of the air than you can't fly. The typical tear drop wing picture doesn't change the motion of the air so it wouldn't fly. A downward deflection of air is required to provide lift in the form newton's 3rd law. Initially air moves horizontally. The wing serves to deflect air downward. So the wing exerts a downward force on the air. So by the 3rd law the air exerts and equal and opposite force upward. Just talking it out in my head. That seems way more satisfying to me.
  - khafra 16y ago
    The movement of a fluid according to Newton's laws is described by the Navier-Stokes Equations, which have no closed-form solution[1]. So, if your intuition gives you a picture of air moving downward from a wing, that's great. But if it feels like a complete explanation, that is a fact about your mind; not a fact about the world. [1] http://en.wikipedia.org/wiki/Navier-Stokes_equations http://en.wikipedia.org/wiki/Navier-Stokes_equations
    - toddh 16y ago
      An equation is a description which is surely an artifact of the mind. At least I'm using forces. So you are saying lift isn't provided as I describe?
      - khafra 16y ago
        I'm saying "lift is just Newton's third law" is approximately as descriptive as "the mind is just the interaction of weighted electrochemical links between neurons." The word "just" covers a lot of detail that can make the short explanation less satisfying--especially if you're trying to use it to build something.
      - khafra 16y ago
        Too late to edit, but I just found a good reference: http://scienceblogs.com/mixingmemory/2006/11/the_illusion_of_explanatory_de.php http://scienceblogs.com/mixingmemory/2006/11/the_illusion_of...

Isn't lift generated by Newton's Third Law? For every action there is an equal and opposite reaction

by toddh 16y ago

Isn't lift generated by Newton's Third Law? For every action there is an equal and opposite reaction

Indirectly, everything is 'described' by Newton's Third Law. However, it doesn't provide a very satisfying answer: it's a description, not an explanation[1]. Knowing the wing goes 'up' because it reacts to the action of the air pushing against it from below, does not explain why that force is larger than the reaction to the action of the air pushing it down from above. Bernoulli's principle is basically a statement about what happens when you apply Newton's laws to a macroscopic amount of fluid and allows you to determine what happens when an asymmetrical object is placed in the flow of such a fluid: when the speed goes up, the pressure goes down. Pressure is nothing but the amount of Newtonian action exerted on a surface, which brings us back to the third law. Then you find that Bernoulli's principle doesn't quite cut it, because it doesn't take into account that fluids don't actually behave like the naive application of Newton's laws would lead you to believe. However, in the end, the Coanda effect is again a description of the behavior of fluids under certain circumstances, describing the macroscopic effect of Newton's laws applied to all individual particles. [1] The exact difference between the two is subject of much philosophical debate, but most would agree that something like Bernoulli's principle has more explanatory power than Newton's law, because, for instance, it allows you to understand why a wing would lift.

toddh 16y ago
I'm not sure why it's not satisfying. The airplane exerts a downward force, the air exerts an equal and opposite force upwards. If you don't change the motion of the air than you can't fly. The typical tear drop wing picture doesn't change the motion of the air so it wouldn't fly. A downward deflection of air is required to provide lift in the form newton's 3rd law. Initially air moves horizontally. The wing serves to deflect air downward. So the wing exerts a downward force on the air. So by the 3rd law the air exerts and equal and opposite force upward. Just talking it out in my head. That seems way more satisfying to me.
- khafra 16y ago
  The movement of a fluid according to Newton's laws is described by the Navier-Stokes Equations, which have no closed-form solution[1]. So, if your intuition gives you a picture of air moving downward from a wing, that's great. But if it feels like a complete explanation, that is a fact about your mind; not a fact about the world. [1] http://en.wikipedia.org/wiki/Navier-Stokes_equations http://en.wikipedia.org/wiki/Navier-Stokes_equations
  - toddh 16y ago
    An equation is a description which is surely an artifact of the mind. At least I'm using forces. So you are saying lift isn't provided as I describe?
    - khafra 16y ago
      I'm saying "lift is just Newton's third law" is approximately as descriptive as "the mind is just the interaction of weighted electrochemical links between neurons." The word "just" covers a lot of detail that can make the short explanation less satisfying--especially if you're trying to use it to build something.
    - khafra 16y ago
      Too late to edit, but I just found a good reference: http://scienceblogs.com/mixingmemory/2006/11/the_illusion_of_explanatory_de.php http://scienceblogs.com/mixingmemory/2006/11/the_illusion_of...