Electromagnetic force and gravity force

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Luigi Fortunati

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May 18, 2021, 9:06:39 AMMay 18
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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.]

Rich L.

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May 20, 2021, 10:56:23 AMMay 20
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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.

Tom Roberts

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May 24, 2021, 3:02:20 PMMay 24
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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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