Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Airplane travel questions

  1. Dec 15, 2013 #1
    Hi,

    Can someone please help me understand these two situations?

    1. When an airplane is flying from point A to point B, is Earth stationary from the point of view of the plane? While I am not sure, I think the answer to this question is yes, because a plane flies within the atmosphere of Earth and these atmospheric layers must be rotating at the same speed as Earth. Is that correct?

    2. Is it possible to take advantage of Earth's rotation (about 1000 miles per hour) to travel from point A to point B? Why can't we create a rocketship that launches from point A on Earth, goes into the outer space in a straight line, stays there until point B is underneath it (due to earth's rotation) and then comes down?

    Thanks.
     
  2. jcsd
  3. Dec 15, 2013 #2
    Yes, for the reasons you stated.
    We cannot, because the rocket will have the motion of the Earth as you launch it. In fact, this is used to advantage in getting payloads into orbit - they are usually launched to the east, so that the velocity of Earth's rotation adds to the velocity produced by the rocket.
     
  4. Dec 15, 2013 #3
    Answer may apply to older problem. Heavy planes (like the raid on Tokyo WWII) could take off from aircraft carriers because the speed of the ship was added to the speed of the plane. I think that's similar.
     
  5. Dec 15, 2013 #4

    russ_watters

    User Avatar

    Staff: Mentor

    "Stationary" is a pretty arbitrary concept; We get to decide what we want to call "stationary". For simplicity, when we are sitting on earth we call that "stationary". When we leave earth in a plane, we change our frame of reference, but the earth-centered frame we leave is still "stationary".
     
  6. Dec 15, 2013 #5

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    If you want to navigate to a specific destination point, the easiest way to do that is work relative to the earth, which is the same as pretending the earth is "stationary".

    Since the atmosphere rotates at approximately the same speed as the earth (apart from the wind speed which is usually much less than 1000 mph), and the aerodynamics of the plane is relative to the air it is flying through, again that gives no reason NOT to measure things relative to the earth. Of course that does not apply to space travel outside the atmosphere.

    That's not quite right. You sail the carrier into the wind, to maximize the wind speed over the flight deck and therefore create the lowest takeoff speed and landing speed relative to the ship, not relative to the earth (or sea).
     
  7. Dec 20, 2013 #6
    Okay. Is the direction in which the rocket goes that important? As long as we are able to get out of earth's atmosphere and hover at one point for some time and come back in when the specific point on earth is nearby, it should work, right?
     
  8. Dec 20, 2013 #7

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Yes, it's vitally important. Rockets are launched in an easterly direction, since that is the direction in which the earth rotates. As a result, the velocity the rocket requires to achieve orbit is reduced somewhat. If rockets were launched in a westerly direction, this velocity reduction would not be available, and the rocket would in fact have to achieve a greater velocity to enter orbit.
     
  9. Dec 20, 2013 #8

    adjacent

    User Avatar
    Gold Member

    No.It is not.The velocity required is same but the velocity WE give to the rocket is reduced somewhat.
     
  10. Dec 21, 2013 #9

    TumblingDice

    User Avatar
    Gold Member

    It would be, if you have a definite destination in mind. (point B)

    But OP isn't about orbit. Question was about going in a straight line and coming down at point B. Everything except the "stays there until point B" part is reasonably accomplished. Launching to the east is only beneficial for reaching latitudinal orbit. Here in the U.S., all polar orbit launches originate at Vandenberg AFB (California) and launched westerly.

    So a rocket can benefit from 'least distance' flight path just as well as airplanes benefit from flying polar routes. This may not be exactly what the OP was expecting, but keeps in the spirit of the exercise.
     
  11. Dec 21, 2013 #10

    adjacent

    User Avatar
    Gold Member

    The OP was talking about launching a rocket up at point B and stay there until earth rotates and B is below the rocket.I think this is more expensive and risky than just an aeroplane travelling.
     
  12. Dec 21, 2013 #11

    TumblingDice

    User Avatar
    Gold Member

    I think expense and risk are eclipsed by the "how" are you going to "stay there". That's why I mentioned that as the exception. However the rocket can be launched in a straight line, and it does have a 'zero moment' at the peak of its trajectory, and it can leverage the Coriolis effect to take advantage of Earth's rotational speed.

    Any advantage, of course, would depend on points A and B. E.g., you could never get rotational advantage traveling from the north pole to the south pole.
     
  13. Dec 21, 2013 #12

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Of course not. How would the plane get from A to B, if there was no relative movement between plane and Earth?


    Yes, if you travel fast enough relative to Earth, there is a apparent weight reduction, so you save some fuel. But the effect is negligible and when you fly back the same way, it is reversed. See Fig. 5 of this paper:
    http://naca.central.cranfield.ac.uk/reports/arc/rm/3680.pdf
     
    Last edited: Dec 21, 2013
  14. Dec 21, 2013 #13

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    How did you envisage doing that? To stay up there you need to be in an orbit or use an unthinkable amount of energy. To 'hover' over a particular spot (over the equator, only) you would need a geostationary orbit which is 30,000+km.
     
  15. Dec 21, 2013 #14
    Thanks folks.
    Hmm...I didn't think about this. Now it is clear to me. Thanks a lot folks.
     
  16. Dec 21, 2013 #15
    Yes, and this has been done for centuries, if not millennia. Google "trade winds".

    Today, this is seen in airline timetables: flights from North America to Europe are about one hour shorter than the other way around.
     
  17. Dec 22, 2013 #16

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Everyone who sails a boat is making use of the Earth's rotation, too. It's the Coriolis force that provides the high speed winds in the vortices we call depressions and cyclones.
     
  18. Dec 22, 2013 #17

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    It's not like there would be less wind, if the Earth didn't rotate. At high altitudes it would probably be even faster.
     
  19. Dec 22, 2013 #18
    The Coriolis doesn't produce any winds. The Coriolis force is always perpendicular to the direction of motion of the wind so it does no work. It's the pressure gradient force that does the work on the atmosphere that produces the winds.
     
  20. Dec 23, 2013 #19
    Would they still have a longitudinal component?

    Explain how the pressure gradient makes the Clipper Route possible.
     
  21. Dec 23, 2013 #20

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    You didn't read what I wrote. The coriolis force introduces vortices into what would be smoother flowing convection currents. Depressions are there because of the coriolis force. When vortices are introduced into a smooth flowing fluid, we find regions of higher (and regions of lower) speeds. That's why designers of planes, boats, cars and turbines try to suppress the formation of vortices.

    Aamof, I am pretty sure that a spinning planet with an atmosphere but no source of radiation (a nonsense scenario, I know, as any fluid would freeze) would still have circulating currents and shear movements in its upper atmosphere. These would end up affecting the whole atmosphere and the combination of centrifugal and coriolis forces would be producing winds.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Airplane travel questions
  1. Time travel question (Replies: 20)

Loading...