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B What happens if a plane nosedives perpendicular to Earth?

  1. Jan 21, 2017 #1
    Hi there. I very much enjoy this forum and have been doing so for a while.
    I have a problem and I am not sure of the explanation I came with.
    Suppose there is a plane that is nosediving perpendicular to Earth. What will the "passengers" feel? In the first seconds the acceleration will be equal with the gravitational acceleration and they will feel wheightless much like in the reduced gravity aircrafts. But what happens after that? The plane will continue to accelerate. Let's ignore the fact that it will reach his terminal velocity for a second. In that period of time in wich the plane will continue to accelerate steadily what will happent to the passengers? The constant acceleration will create the sensation of a gravity that acts towards the tail of the plane as stated by the equivalence principle or they will fall faster than the plane because of the lack of drag that the plane gets?
    Thank you very much for your time!
  2. jcsd
  3. Jan 21, 2017 #2

    jim mcnamara

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  4. Jan 21, 2017 #3
    I am thinking that without drag that the passengers would feel complete weightlessness. When drag becomes a factor the plane will reach terminal velocity. Inside the plane while it is falling with constant terminal velocity the passengers would experience normal weight and gravity. Just like we do while standing on the earth even though it moves at constant velocity through space.

    If the earth were to accelerate then we would feel a change in weight/gravity.
  5. Jan 22, 2017 #4
    Thank you. The pilots are flying in parabolas so that the acceleration will be equal to the gravitational acceleration and the weightlessness is felt for only a period of the parabola.

    So you're saying that in the time it takes to achive the terminal velocity they will feel weightless. But due too the orientation of the plane won't it accelerate faster than the gravitational acceleration very soon? Doesn't that have an effect?
  6. Jan 22, 2017 #5

    jim mcnamara

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    I am not sure if you understand that the earth as a whole is constantly "experiencing" acceleration, example: it is in orbit around the sun.

    From a physics problem set: tuhsphysics.ttsd.k12.or.us/Tutorial/NewIBPS/PS5_3/PS5_3.htm

    *the orbit is really an ellipse. The number of seconds in the example is therefore off by a small number.
  7. Jan 22, 2017 #6


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    It depends on the aircraft.

    In a glider it's as you said. You would initially feel nearly weightless. Then, if it were to approach terminal velocity you would no longer be accelerating and would experience 1g in the downward direction.

    Correct. In a powered aircraft it would depend on how long the aircraft could continue to accelerate downwards and what that rate of acceleration was. The normal acceleration due to gravity is 9.8m/s/s. If the aircraft were to accelerate downwards at around twice that, say 19.62m/s/s then you would feel a net 1g upwards. If the aircraft had a suitable room inside you could walk upside down on the ceiling. The problem is that an aircraft cannot accelerate downwards indefinitely. Typically they have a VNE (Velocity Never Exceed) limit beyond which the forces on the aircraft become too large.

    I used to fly gliders and did some 3 or 4g aerobatics in single engine aircraft back in the 1970's. Great fun if you ever get the chance.
  8. Jan 22, 2017 #7


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    There is no such time. Speed and drag will increase slowly over time. If the aircraft does not use its engines, apparent gravity felt by the passengers will slowly increase from 0 to 1g while the aircraft gets closer to its terminal velocity. In an idealized world, the aircraft never reaches this velocity, but in practice it gets so close that the difference gets negligible.

    If the aircraft uses its engines, then apparent gravity can point upwards initially.
  9. Jan 22, 2017 #8
    Negative g force is something fighter pilots routinely encounter and train to withstand.

    And here's how to use it to levitate a cell phone
  10. Jan 22, 2017 #9
    I do know that but It wouldn't have helped answer the question. It was just for the purpose of explaining that you could be traveling at a high speed and experience no change in weight.

    yes there is radial acceleration but it's small compared to the acceleration of gravity. Otherwise we would experience different weights at different times of the year and also a difference between day and night.
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