[Colin2B]

Is the concept of force limited to the mathematics of Newtonian gravity and so inapplicable to areas where Newtonian gravity  does not apply?

Certainly force is defined within Newtonian laws of motion, but perhaps it helps to consider the force you and your passengers feel as your car goes around a bend.
You all agree you feel what you would describe as a force, it feels real and has a genuine effect of pushing you to one side, however, another part of you knows it is only because you are following a curved path.

[geordief]

Is the concept of force limited to the mathematics of Newtonian gravity and so inapplicable to areas where Newtonian gravity  does not apply?

Certainly force is defined within Newtonian laws of motion, but perhaps it helps to consider the force you and your passengers feel as your car goes around a bend.
You all agree you feel what you would describe as a force, it feels real and has a genuine effect of pushing you to one side, however, another part of you knows it is only because you are following a curved path.

That is not gravitational force though,is it?.Is it not simple acceleration?

I thought the main idea was that you do not feel such a force  from gravity.(in freefall). and that accelerometers read zero.

[jeffreyH]

Is the concept of force limited to the mathematics of Newtonian gravity and so inapplicable to areas where Newtonian gravity  does not apply?

Certainly force is defined within Newtonian laws of motion, but perhaps it helps to consider the force you and your passengers feel as your car goes around a bend.
You all agree you feel what you would describe as a force, it feels real and has a genuine effect of pushing you to one side, however, another part of you knows it is only because you are following a curved path.

That is not gravitational force though,is it?.Is it not simple acceleration?

I thought the main idea was that you do not feel such a force  from gravity.(in freefall). and that accelerometers read zero.

The reason behind this is that gravity acts upon every individual particle 'at once'.

When we push something we apply a force to a particular point on a surface.

Gravity applies its force throughout. It is like having a cord tied to every particle.

If they all move in unison then the whole frame does.

It can be thought of as an inertial frame being accelerated.
This appears to remove the force of gravity since the frame is keeping pace with the force.

[Colin2B]

That is not gravitational force though,is it?.Is it not simple acceleration?

No, it was only intended as an illustration of how a force can occur because of a curved path. However, remember that
Einstein used the illustration of gravity being the equivalent of an acceleration.

[geordief]

No, it was only intended as an illustration of how a force can occur because of a curved path. However, remember that
Einstein used the illustration of gravity being the equivalent of an acceleration.

Is not the acceleration in the instance adduced by Einstein    actually caused by the electro magnetic forces rather than gravity?

I think you are referring to the closed room where the occupant is unable to decide if the gravitational effects are due to gravity or an artificial acceleration.

[evan_au]

Is there  a reason (gravity) can be called a  "force"? It is not the same as Newtonian force, is it?

I was using gravity as a force the way Newton used it. But I could have used gravity as a curvature in spacetime as Einstein used it.

Gravity the familiar force we feel on our pants as we sit on a seat. And Newton's equations are not too bad an approximation (provided you are conscious of its limitations); General Relativity is a better approximation (provided you are aware of its fewer limitations).

However, when we are talking about quantum effects at the event horizon of a black hole, we are in a zone where neither Newton, Einstein or Heisenberg generate accurate predictions. So I used the more familiar model from Newton to describe why all current theories fail in this zone.

[Colin2B]

Is not the acceleration in the instance adduced by Einstein    actually caused by the electro magnetic forces rather than gravity?

Can you explain why you think that? Perhaps we are thinking of different scenarios here because I'm not aware of this example.

[geordief]

Is not the acceleration in the instance adduced by Einstein    actually caused by the electro magnetic forces rather than gravity?

Can you explain why you think that? Perhaps we are thinking of different scenarios here because I'm not aware of this example.

I am thinking of the scenario ** when an observer is in an enclosed chamber. He or she will notice objects falling in the direction of the "floor"

The question is posed  "Is the chamber artificially accelerating or is it in a gravitational field?"

He cannot give an answer as either scenario  fits the observation

I understand the only "force" involved to be that of the electro magnetism in the atoms of the floor preventing  objects escaping the chamber.


In my recent post I think I may have garbled the ideas of force and acceleration in my mind:(  Apologies if so.

**is this not a very well known example used by Einstein?

[Bill S]

The question is posed  "Is the chamber artificially accelerating or is it in a gravitational field?"

He cannot give an answer as either scenario fits the observation

Surely, there are a couple of tests you could do to distinguish, in principle, at any rate, if you have a couple of marbles and a sufficiently sensitive measuring device.

Release your marbles simultaneously from the top of the box.  They will fall to the bottom.  If you are being accelerated, their trajectories will be parallel, but if you are on the surface of a planet their trajectories will converge on the centre of the planet, so they will converge as they fall.

Alternatively, if you release one from the top of the box and one from waist height, they will maintain that separation until the first one hits the floor, under acceleration.  Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase. 

[yor_on]

What defines a planet is its spin. That will show you if you're at one or accelerating inside a rocket in 'deep space'- Reading "Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase. " lose me slightly Bill, what do you mean there?

[geordief]

The question is posed  "Is the chamber artificially accelerating or is it in a gravitational field?"

He cannot give an answer as either scenario fits the observation

Surely, there are a couple of tests you could do to distinguish, in principle, at any rate, if you have a couple of marbles and a sufficiently sensitive measuring device.

Release your marbles simultaneously from the top of the box.  They will fall to the bottom.  If you are being accelerated, their trajectories will be parallel, but if you are on the surface of a planet their trajectories will converge on the centre of the planet, so they will converge as they fall.

Alternatively, if you release one from the top of the box and one from waist height, they will maintain that separation until the first one hits the floor, under acceleration.  Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase.

True , I had not heard that explanation/resolution  before but it is valid. Nevertheless in a uniform  (ie very large) gravitational  field it would not be possible other than in principle.

By the way,Is my scenario not the one that Einstein used for his Equivalence Theory? (have I got scenarios mixed up?)


Also this link may be relevant or helpful-or just nice:

http://www.feynmanlectures.caltech.edu/II_42.html    -go down to 42–5Gravity and the principle of equivalence

[Colin2B]

I am thinking of the scenario ** when an observer is in an enclosed chamber.
**is this not a very well known example used by Einstein?

Yes, this is the famous example, but it doesn't equate gravity to electromagnetic forces. If the astronaut is at the front of the spaceship when it begins to accelerate s/he will experience freefall relative to the ship.

[geordief]

I am thinking of the scenario ** when an observer is in an enclosed chamber.
**is this not a very well known example used by Einstein?

Yes, this is the famous example, but it doesn't equate gravity to electromagnetic forces. If the astronaut is at the front of the spaceship when it begins to accelerate s/he will experience freefall relative to the ship.

I don’t think I was attempting to equate gravity to em forces.

My suggestion was that it was wrong to talk about gravity as a force –and that, in the example we are looking at  , the only forces at play are the forces stopping objects from falling through the floor.

I realize that force does play a role in Newtonian gravity but I  thought that conceptually it was wrong as an idea although it may perhaps be  correct within its limits of application(non relativistic speeds?)

I also realize that  I may be being pedantic and merely stating the obvious.

Perhaps also I have confused in my own mind the prohibition of “an instantaneous force at a distance” with a prohibition of a force  otherwise defined (f=ma)

[Colin2B]

I don’t think I was attempting to equate gravity to em forces.

OK, I misunderstood what you were implying here:

Is not the acceleration in the instance adduced by Einstein    actually caused by the electro magnetic forces rather than gravity?

[jeffreyH]

The question is posed  "Is the chamber artificially accelerating or is it in a gravitational field?"

He cannot give an answer as either scenario fits the observation

Surely, there are a couple of tests you could do to distinguish, in principle, at any rate, if you have a couple of marbles and a sufficiently sensitive measuring device.

Release your marbles simultaneously from the top of the box.  They will fall to the bottom.  If you are being accelerated, their trajectories will be parallel, but if you are on the surface of a planet their trajectories will converge on the centre of the planet, so they will converge as they fall.

Alternatively, if you release one from the top of the box and one from waist height, they will maintain that separation until the first one hits the floor, under acceleration.  Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase.

Oh that's clever. I do like that argument.

[Colin2B]

Oh that's clever. I do like that argument.

It is neat, question is would it be possible to measure it in such a small volume?

[geordief]

Oh that's clever. I do like that argument.

It is neat, question is would it be possible to measure it in such a small volume?

Feynman went on to say that the equivalence was just  applicable to a point in the chamber/spacecraft

quote "To be strictly correct, that is true only for one point inside the ship"

[jeffreyH]

So equivalence is singular. Now that is interesting and not trivial.

[yor_on]

Well, I like it too, independent thinking I would call it

I'm not sure what Bill refers to with his last argument though?  "Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase." 

1. The higher one climb the slower ones time, as described from a ground-sitter, might be one variable
2. Terminal velocity at higher altitudes is greater due to the thinner atmosphere, another

but the speed (acceleration) is the same, or is it
It may have to do with what elevation the marbles are at?

[geordief]

Well, I like it too, independent thinking I would call it

I'm not sure what Bill refers to with his last argument though?  "Under gravity, however, the lower marble will fall faster than the upper one, so the separation will increase." 

1. The higher one climb the slower ones time, as described from a ground-sitter, might be one variable
2. Terminal velocity at higher altitudes is greater due to the thinner atmosphere, another

but the speed (acceleration) is the same, or is it
It may have to do with what elevation the marbles are at?

I think it is because ,if you release two objects at a  two different distances from the Sun (as an example of a body with gravitational attraction) and both initially stationary wrt the Sun, then the object which is nearer to the Sun  will accelerate more quickly than the one that is further away -and so the distance between the two objects  will increase over time.