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Questions about Equivalence principle & Einstein Elevator?

  1. Feb 11, 2012 #1
    Einstein inoculated general relativity with the help of equivalence principle and space elevator as shown in the following link



    1- What is the direction of weight [force] of a person standing on the floor of aforementioned elevator? - upward means along the direction of “g” or downward means opposite to “g”

    2- Would a person feel sensation of his weight if hanging either from the ceiling of aforementioned elevator or hanging from the bottom of its floor?

    3- Shouldn't a person move towards the center of elevator due the universal law of gravitation; F = GMm/d^2?

    4- Is shell theorem applicable to this case?

    5- What would be the spacetime Geometry inside this elevatro?


    Eclectic Eccentric Khattak No.1
  2. jcsd
  3. Feb 13, 2012 #2
    The force on the person in the accelerating elevator is the same as the force on the person standing in an elevator at rest at the surface of the Earth: the floor exerts an "upwards" force on the person, the person exerts a corresponding "downwards" force on the floor.

    Yes, just like in the elevator at rest at the surface of the Earth.

    Any gravitational attraction between the person and the elevator is negligible compared to the force of the Earth's gravity on the person, or the force resulting from the acceleration of the elevator.

    See 3 above.

    Locally flat.
  4. Feb 13, 2012 #3
    Thanks for your response - Great

    As it is stated in the link

    If a ball is dropped in the elevator at rest on the Earth, it will accelerate toward the floor with an acceleration of 9.8 meters/second2. A ball released in the upward accelerating elevator far out in space will also accelerate toward the floor at 9.8 meters/second2.

    Shouldn’t the direction of a falling ball along the direction of acceleration “g” of elevator?

    I mean why a ball accelerates toward the floor at 9.8 meters/second2 if released in the upward accelerating elevator far out in space?

    It accelerates at same “g” but why in opposite direction to the acceleration "g" og elevator?
  5. Feb 13, 2012 #4
    Think of it this way: if you hold a ball inside the accelerating elevator, both you and the ball are accelerating with the elevator. As soon as you release the ball, it stops accelerating: the floor of the elevator now accelerates up towards the ball.
  6. Feb 13, 2012 #5
    :confused:My question might be not clear but

    Q:Why do things fall on earth?
    A:Due to gravity - along the direction of "g" So

    What causes the ball to fall downward toward floor when the direction of gravitational acceleration is upward?
  7. Feb 14, 2012 #6
    There may be confusion between the terms "gravitational attraction" and acceleration. When you stand on the surface of the Earth, the Earth exerts a force on you ("gravitational attraction") in a certain direction, which you call "downwards". This is directly equivalent to standing on a platform which is accelerating in the opposite direction to this force.

    This article expresses it succinctly:

  8. Feb 14, 2012 #7
    My QUESTION is why do things fall on aforementioned floor of elevator when the direction of

    1- Gravitational acceleration of elevator “ge” is upward [9.8 m/s/s]

    2- Gravitational acceleration of person “gp” is upward [attract floor of elevator]

    3- Gravitational acceleration of a ball ”gb” is upward [attract floor of elevator]

    So what causes the ball to fall downward on the floor of elevator?
  9. Feb 14, 2012 #8


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    Science Advisor
    Gold Member

    It's all relative. The elevator accelerates upwards relative to the ball, so the ball accelerates downwards relative to the elevator.
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