0
$\begingroup$

Newton's laws state that for every force there is an equal and opposite force. I'm thinking of an airplane like a simple system: we put x force in and get y force out. An airplane has the force from thrust and the force from gravity, I understand this. The force of gravity is constant and equals the airplane's mass times gravity. It seems like airplanes cheat newton's laws. Somehow an airplane is only generating about 1/10th the force of gravity, but it is able to use this thrust force to provide the force equal to the gravity force plus a force against the air resistance (drag). I have heard lift explained in Newtons laws as the downwash of air and how the total mass x acceleration of the air in the downwash equals the mass x gravity of the airplane. In short, the airmass acceleration generated by the propeller < than airmass accelerated by the wing: how does this make sense? What am I missing?

$\endgroup$
  • $\begingroup$ See the duplicate question. Another way of thinking about this: a car doesn't have the power to climb a near vertical wall, but it can (slowly) ascend a gentle slope. $\endgroup$ – Andy May 23 '16 at 16:27
  • $\begingroup$ I read the other one. I'm asking the question in a different way $\endgroup$ – Daniel Caoili May 23 '16 at 17:59
  • 5
    $\begingroup$ It would help to explain how your question is different more than just saying that the answers on the other question aren't helpful to you. $\endgroup$ – fooot May 23 '16 at 18:14
  • $\begingroup$ You might want to read this as an introduction to the physics. There is no requirement in physics for thrust, derived from the force applied by propulsion, to equal or exceed weight. Consider one of the plans being considered to divert asteroids away from a collision with the Earth by attaching solar sails to them. The sails produce very little thrust in comparison to the weight of an asteroid but over time can accelerate them enough to alter their orbit. Newtonian physics are alive and well in all aircraft. $\endgroup$ – Simon May 23 '16 at 18:42
  • 3
    $\begingroup$ @DanielCaoili You're asking essentially the same question, though. As my answer on the other one says, the important part to notice is that thrust is countering drag, not gravity. The air hitting the aircraft is redirected downwards, which causes and equal and opposite force on the aircraft. The force is point mostly upwards and a little bit backwards (relative to the airplane.) The thrust only needs to counter the backwards part. The upwards part is equal to the weight of the aircraft (when the aircraft isn't accelerating vertically, that is.) $\endgroup$ – reirab May 23 '16 at 19:54

Browse other questions tagged or ask your own question.