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  • Electromagnetic force and gravity force

    From Luigi Fortunati@21:1/5 to All on Tue May 18 14:06:37 2021
    It is correct to say that in the elevator stopped at the floor
    (1) an electromagnetic force acts between the man and the elevator
    (2) a gravitational force acts between the Earth and man
    (3) a gravitational force acts between the Earth and the elevator
    (3) no gravitational force acts between the elevator and the man?

    It is correct to say that in the elevator in free fall
    (1) a gravitational force acts between the Earth and man
    (2) a gravitational force acts between the Earth and the elevator
    (3) no gravitational force acts between the elevator and the man.
    (4) no electromagnetic force acts between the elevator and the man?

    [Moderator's note: I think that the above are meant as questions, i.e.
    "Is it correct...?" -P.H.]

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  • From Rich L.@21:1/5 to Luigi Fortunati on Thu May 20 14:56:20 2021
    On Tuesday, May 18, 2021 at 8:06:39 AM UTC-5, Luigi Fortunati wrote:
    It is correct to say that in the elevator stopped at the floor
    (1) an electromagnetic force acts between the man and the elevator
    (2) a gravitational force acts between the Earth and man
    (3) a gravitational force acts between the Earth and the elevator
    (3) no gravitational force acts between the elevator and the man?

    It is correct to say that in the elevator in free fall
    (1) a gravitational force acts between the Earth and man
    (2) a gravitational force acts between the Earth and the elevator
    (3) no gravitational force acts between the elevator and the man.
    (4) no electromagnetic force acts between the elevator and the man?

    [Moderator's note: I think that the above are meant as questions, i.e.
    "Is it correct...?" -P.H.]

    I would say "no" and "no":
    -First of all, the influence of gravity and electromagnetism do not turn
    ON or OFF based on the motion of the objects. Whether the elevator
    is stopped or moving all charges act on all other charges and all
    gravitational "forces" act on all objects.
    -Current thinking is that gravity is not a "force", but rather a distortion
    of space-time. The "force" that we feel is the force required to make
    an object deviate from its natural trajectory through space-time. This
    is entirely analogous to a centrifuge. There is no force pushing a mass outward, the outward path is the most natural path for the object to
    move in a straight line. The force is the one that pushes the object
    toward the center, which is the force that makes the object follow
    a curved path.
    -Stopped (1) and Free Fall (4): Electromagnetic forces act in both cases.
    They are what prevent the man from passing through the floor or
    walls of the elevator, whether stopped or falling.
    -Stopped (2)(3)and ("3") and falling (1)(2)(3): The gravitational distortion of
    space-time makes the man and elevator naturally follow the falling
    path. An external force is required to keep the elevator stationary in
    our "stopped" frame. Note that our "stopped" frame is an accelerating
    one, not a rest frame. When the elevator is falling freely, the man and elevator are following a "straight line" in the curved space-time around
    the earth. The only force required is when the elevator is "stopped" in
    our non-inertial frame we on earth call "at rest". This is an accelerating frame and thus a force is required to keep objects "stationary" in this
    frame.

    Rich L.

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  • From Tom Roberts@21:1/5 to Luigi Fortunati on Mon May 24 20:02:18 2021
    On 5/18/21 8:06 AM, Luigi Fortunati wrote:
    [...]

    This depends on what model you use for gravitation. Today the two most
    common models are Newtonian mechanics (NM) and General Relativity (GR)
    -- I'll answer for both.

    I presume the man is inside the elevator, nominally standing on its
    floor, in a building built on the surface of the earth (I stipulate this
    last so I can distinguiah between a floor of the building and the floor
    of the elevator).

    Here the elevator is held stopped at some floor of the building.

    It is correct to say that in the elevator stopped at the floor
    (1) an electromagnetic force acts between the man and the elevator

    NM: yes, the elevator floor pushes up on his feet.
    GR: yes, the elevator floor pushes up on his feet.

    (2) a gravitational force acts between the Earth and man

    NM: yes, this force is equal and opposite to (1)
    GR: no, there is no gravitational force, the man is diverted from his
    geodesic path by the EM force of (1).

    (3) a gravitational force acts between the Earth and the elevator

    NM: yes, it is equal and opposite to the force on the elevator exerted
    by the mechanism holding the elevator stopped at the floor of the
    building.
    GR: no, there is no gravitational force, the elevator is diverted from
    its geodesic path by the force from the mechanism holding the elevator
    stopped at the floor of the building.

    (3) no gravitational force acts between the elevator and the man?

    NM: no, there is a force, but it is unmeasurably small [#]
    GR: yes, there is no gravitational force

    [#] if the elevator is appropriately constructed, this minuscule force
    could be arranged to cancel to zero.

    Now we consider the elevator in free fall.

    It is correct to say that in the elevator in free fall
    (1) a gravitational force acts between the Earth and man

    NM: yes, this force accelerates the elevator down
    GR: no, there is no gravitational force, the man follows his geodesic
    path.

    (2) a gravitational force acts between the Earth and the elevator

    NM: yes
    GR: no, there is no gravitational force, the elevator follows its
    geodesic path.

    (3) no gravitational force acts between the elevator and the man.

    NM: no, there is a force, but it is unmeasurably small [#]
    GR: yes, there is no gravitational force, both follow their geodesic
    paths.

    (4) no electromagnetic force acts between the elevator and the man?

    NM: yes
    GR: yes

    Note in all cases, in GR the geodesic paths are accelerating downward at
    9.8 m/s^2.

    Tom Roberts

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