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    I understand that the equivalence principle is that the equivalence principle states that an observer in an aircraft wouldn't be able to tell whether they are accelerating upwards or in a gravitational field. However when I think of free fall it seems to be logical that the force of gravity would cancel out if the plane is accelerating upwards as gravity acts downwards but this isn't the case. Could someone explain?Name:  image.png
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    That's the 'vomit comet' parabolic trajectory flights.

    The trajectory of a projectile in a uniform gravitational field close to the earth is parabolic - it's accelerating vertically downwards at 1g even while it's still moving upwards

    The plane deliberately flies part of it's route on the same trajectory as projectile would follow - the advantage for astronaut training is that the plane can pull up before it gets too close to the ground - rather than launching trainees inside a giant hollow cannonball or similar.
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    (Original post by Joinedup)
    That's the 'vomit comet' parabolic trajectory flights.

    The trajectory of a projectile in a uniform gravitational field close to the earth is parabolic - it's accelerating vertically downwards at 1g even while it's still moving upwards

    The plane deliberately flies part of it's route on the same trajectory as projectile would follow - the advantage for astronaut training is that the plane can pull up before it gets too close to the ground - rather than launching trainees inside a giant hollow cannonball or similar.
    Thanks, though surely if it experiences 1 g when accelerating up wards if it accelerates downwards with 1 g it will experience 2g? There fore weightlessness would be achieved when it accelerates upwards as the force of g cancels out?
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    (Original post by willr77)
    Thanks, though surely if it experiences 1 g when accelerating up wards if it accelerates downwards with 1 g it will experience 2g? There fore weightlessness would be achieved when it accelerates upwards as the force of g cancels out?
    well no - the motion of the plane in this part of it's flight resolves to a constant horizontal velocity and a constant vertical acceleration (of 1g) . accelerating downwards by 1g is the same amount of acceleration as in freefall - so the trainees in the plane experience 0g.

    the diagram shows the position of the plane changing over time, note that the acceleration is downwards even while the direction of motion is upwards.
 
 
 
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