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Motion of an aircraft in ground

  1. Mar 31, 2010 #1
    is the principle how an aircraft move ahead in ground similar to how a spacecraft moves ahead in empty space?

    in the ground air is propelled backward and in space since it has no air, small explosion pushes the spacecraft ahead.
    Newtons third law.

    i understand in the case of spacecraft, as there is no other object to exert force on spacecraft, it has to be Newtons third law. But not sure of aircraft movement on ground just by propelling air fastly backward, the aircraft moves ahead. As there is air to interact with the aircraft.

    So is it just the Newtons third law as in space or there is a force exerted by the air on the aircraft to moves ahead as an action-reaction due to the aircraft thrust on the air.

    Or bcoz an low pressure is created ahead of the aircraft by the propeller, the aircraft is sucked forward.

    Note: i'm not asking how lift is produced by the wing.
  2. jcsd
  3. Mar 31, 2010 #2


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    On the moelcular level there is no "sucking by under-pressure". Air molecules cannot "pull" but only "push". On this level it is simply Newtons 3rd Law or momentum conservation.

    But there are to many molecules to deal with them on the macroscopic scale, so pressures are introduced as a coarser statistical model.

    It is not about one or the other cause, but different levels of modeling the same thing.
  4. Mar 31, 2010 #3
    if the air pushes the aircraft forward, then both case cannot be similar(spacecraft in space) if i'm right as in space nothings pushes the spacecraft, there is no external force in space.
  5. Mar 31, 2010 #4


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    It's just Newton's Third law and the Conservation of Momentum Law in both cases...and it doesn't require anything to push against. It's the force from the fuel that's being burned that creates the forward motion due to the opposite reaction on the aircraft/spaceship.

    Have a read here for more basic information:

    Last edited by a moderator: Apr 24, 2017
  6. Mar 31, 2010 #5


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    An aircraft's thrust device accelerates the surrounding air backwards relative to the aircraft.

    A rocket accelerates part of it's own mass.
  7. Apr 1, 2010 #6
    is it possible to throw something away in empty space while floating Or an external force or support or explosion like a gun shot is required to throw something away.
  8. Apr 1, 2010 #7


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    In the case of a rocket, it's all internal. Ignoring gravity, once a rocket is in space, it's all internal forces. The velocity of the center of mass of the rocket and spent fuel never changes, momentum of rocket and it's spent fuel is conserved. Spent fuel is accelerated in one direction, the rocket accelerated in the opposing direction, all due to internal forces.
  9. Apr 1, 2010 #8
    i understand the above.
    but what abt "is it possible to throw something away in empty space?"

    or should i post it in a new thread.
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