A simple observation regarding the equivalence of acceleration and gravity

[yogi]

. According to Newton, the man must not be passive. He must have his own force, which is why he must have his own rocket pack to accelerate him the direction of the floor of the room. This is the opposite direction the room is accelerating in..

The man has his own force - its consequent to his inertia F = ma This reactionary force has the same validity (just as real) as the primary force that produces the elevator motion

 

[girlwonder]

I'm throwing out the whole object A object B thing. i was just trying to sound smart. Really, when i did the math on it, it seemed like if you accelerated two ojects against each other, the force would increase where they touched. Apparently there is a million ways of discounting that in actual practice. i was actually investigating the possibility that gravity has more to do with spacetime and less to do with matter then we think, and that maybe gravity isn't really an essential part of mass, but rather, mass and gravity both being the rsult of the same spacetime mechanism, hence their co-relation.

Although I've gotten nowhere in my reasoning on this thread, i just got someone on another thread to admit that spacetime is "something", so i feel better now.

And i still think that if two objects perpetually accelerated against each other, the force where they touch would increase over time, if both objects were accelerating at a constant rate.

 

[DaveC426913]

And i still think that if two objects perpetually accelerated against each other, the force where they touch would increase over time, if both objects were accelerating at a constant rate.

But ... if they're touching each other, then both their acceleration and their velocity are zero.

 

[girlwonder]

If both were equal masses and perfectly centered neither would appear to move, but the force would be measurable and increasing.

i actually discussed it with an auto mechanic in terms of what would happen if you did that with cars. Pretty interesting.

 

[DaveC426913]

A couple of edits:

If both were equal masses and perfectly centered neither would <STRIKE>appear to</STRIKE> actually move, but the force would be measurable <strike>and increasing</strike>, though the acceleration and velocity would remain zero.

I was thinking along the same lines, two cars pushing against each other - but it's a terrible analogy. A car's acceleration might not have anything to do with the forces it applies. For example, how do you get a constant acceleration in a moving car? Do you hold the pedal steady and let the car pick up speed? Or do you continually apply more gas as time passes? This is also why your mechanic friend is not a good person to talk to - he's not talking about pure physics, he's talking about auto mechanics. We could digress, but let's not.

The simplest model is the use of two rockets. The force is a direct result of thrust and is quite trivial to get your head around the constant application of forces.



It also shows that your suspicion is incorrect. Two constant forces opposing each other will cancel, and the pressure between them will not increase over time. Don't misunderstand, the forces don't disappear, they still apply pressure - a constant pressure in fact - but they don't do any work.

 

[country boy]

Here's a situation that turns the problem around, and sometimes gives an alternative view of the equivalence principle.

If the man is in orbit around a planet but is in a closed compartment (say a space shuttle without windows), how does he know that he is in a gravitational field? It would seem to him that no force is acting and that there is no acceleration.

In general relativity, talk of gravitational force is removed and we are supposed to instead think in terms of following undeviating trajectories.

 

[girlwonder]

I think mechanics know a lot about mechanics. teehee. They do though, really. No physics, no car. EVERYTHING operates on physical law.

I think the car is a perfect example. The effect I'm talking about could also be reproduced by "containing" the cars with a chain back to back. The wheels would spin and the speedometer would move as if it were accelerating and the force on the chain (and the cars) would increase over time, even if the rate of acceleration of both vehicles were constant.

What I'm getting at is that the reason why the equivalence of acceleration and gravity works, is because only the floor is accelerating towards the man, and not vice versa, clearly it doesn't work then. Obviously the man was right. What's wrong is our present understanding of gravity and spacetime.

 

[girlwonder]

Oh- and country boy rocks.

 

[cesiumfrog]

Although I've gotten nowhere in my reasoning on this thread, i just got someone on another thread to admit that spacetime is "something", so i feel better now.
[...]
i actually discussed it with an auto mechanic [...] I think mechanics know a lot about mechanics. teehee. They do though, really. No physics, no car. EVERYTHING operates on physical law.
[...]
What's wrong is our present understanding of gravity and spacetime.

...

 

[girlwonder]

There, there, don't cry.

i didn't cry when you said you wanted to burn me. Seven people now do. Scientists are mean, that's for sure.

 

[JesseM]

I think the car is a perfect example. The effect I'm talking about could also be reproduced by "containing" the cars with a chain back to back. The wheels would spin and the speedometer would move as if it were accelerating and the force on the chain (and the cars) would increase over time, even if the rate of acceleration of both vehicles were constant.

But it wouldn't increase over time, this is a well-understood situation in Newtonian physics--you'd just reach an equilibrium where the force pushing the car in one direction is equal and opposite to the force pushing the car in the opposite direction, and at that equilibrium neither the car or the chain would move, and the forces would be constant.

Obviously the man was right. What's wrong is our present understanding of gravity and spacetime.

Noooo! Assuming that your intuition is right and all the physicists who say it's wrong are mistaken, before you have even studied the field in depth to know their reasons for saying so, is the path to the Dark Side! That way lies madness...people who think like that persistently tend to end up becoming crackpots, hardened in their beliefs that they know better than the whole physics community. Don't take those first steps down that road! Remember that all the physicists who did come up with genuine revolutions in physics, like Einstein, were people who had studied existing physics front-to-back and understood it in great depth, and moreover the area where they came up with a breakthrough was almost always one that was widely acknowledged to be a "problem area" by the physics community, like reconciling the "aether theory" of electromagnetism with the observation that light was always measured to move at the same speed in Einstein's day, or the problem of reconciling quantum physics with general relativity today.

i didn't cry when you said you wanted to burn me. Seven people now do. Scientists are mean, that's for sure.

Hey, don't take the "burn her" votes too personally, it's just that you set up the poll so that was the only logical option to pick for someone who didn't think you had found any sort of genuine problem with the equivalence principle.

 

[girlwonder]

OKay, maybe you're right, but i trust my mechanic and he said that eventually the force would get so strong as to break the cars, so i wouldn't do it with my car.

Anyway, because i continue to learn and be open minded i will not be a crackpot. Can't you see that i must ellicit arguements by digging my heels in. And the simplicity of what I've said has restrained the responses. And maybe I'm not showing all my cards either.

Anyway, i was curious to know how many people would burn someone else for suggesting something blasphemous. They could have just not voted, like most people, when confronted with no honest choice. They got a kick out of voting to burn me, probably cause they thought it would hurt my feelings. Just like i got a kick out of getting them to show their true colors. i guess I'm twisted too. teehee.

 

[girlwonder]

And there is no problem with the equivalence of acceleration and gravity. The problem is what we think we know about the relationship between gravity and mass. And you have to admit, our understanding of gravity is a bit loose, non?

 

[DaveC426913]

I think mechanics know a lot about mechanics. teehee. They do though, really. No physics, no car. EVERYTHING operates on physical law.

Yes, not auto mechanical law, as you'll see here:

I think the car is a perfect example. The effect I'm talking about could also be reproduced by "containing" the cars with a chain back to back. The wheels would spin and the speedometer would move as if it were accelerating and the force on the chain (and the cars) would increase over time, even if the rate of acceleration of both vehicles were constant.

"...the speedometer would move as if it were accelerating..."

Yes. it would. And it would be LYING. Because the car is NOT accelerating.


"...force on the chain (and the cars) would increase over time..."

Prove it.


I think the force on the chain would increase over time because you would be applying an INCREASING force, not a constant force.

And this is why it is a lousy example, and this is why a mechanic is the wrong person to ask.

If you want to underatand the problem, you must remove the extraneous variables. Simplify the experiment.

 

[country boy]

Hey, don't take the "burn her" votes too personally, it's just that you set up the poll so that was the only logical option to pick for someone who didn't think you had found any sort of genuine problem with the equivalence principle.

I wanted to vote, but there was no category for "send flowers."

 

[girlwonder]

okay I'm going to prove this to you all, and I'm going to do it real simple like, real low-tech and cheap just to be a brat, a little experiment.

 

[DaveC426913]

okay I'm going to prove this to you all, and I'm going to do it real simple like, real low-tech and cheap just to be a brat, a little experiment.

Before you do the experiment, will you do us the courtesy of critiquing your set up so we can be satisfied it's valid? eg. If you decided to do this with two cars, I would (continue to) raise serious objections about how you're going to achieve "constant acceleration".

 

[gabee]

I think mechanics know a lot about mechanics. teehee. They do though, really. No physics, no car. EVERYTHING operates on physical law.

lol

What I'm getting at is that the reason why the equivalence of acceleration and gravity works, is because only the floor is accelerating towards the man, and not vice versa, clearly it doesn't work then. Obviously the man was right. What's wrong is our present understanding of gravity and spacetime.

It WOULD work if the man were to accelerate toward the floor. Imagine the man wearing a belt, to which is attached a chain that passes through a small hole in the floor. Then the same "unseen force" pulls on this chain instead. The man pushes on the floor, which accelerates the room, and in his frame of reference the forces are exactly the same as before. The point is that you WON'T have two forces--one pulling on the man and one pulling on the room in the opposite direction. Newtonian gravitational force is a function of the two masses: GMm/r^2. To simulate the gravitational pull of the Earth in the man's frame of reference, you can EITHER pull on the man OR pull on the room. Not both, or the man would feel twice as heavy!

By the way, even if two forces were pulling on the man in opposite directions (as you describe with the two cars) there would be no net acceleration in either direction and the forces on the man would be constant. Instead of complicating the matter by trying to describe lying speedometers and burning rubber and friction against the road, just think of two forces. A constant applied force has a constant acceleration...not an increasing acceleration...so the net force would be zero and he would not move. Only if you increased the forces for some reason would the forces on the man increase (they won't do it naturally over time!).

 

[country boy]

okay I'm going to prove this to you all, and I'm going to do it real simple like, real low-tech and cheap just to be a brat, a little experiment.

Please don't hurt yourself. Some demonstrations are better left as thought experiments.

It is true that gravity is not like other forces we are familiar with. When you are being accelerated in your car, you feel the force on your back. However, when you are being accelerated by gravity (in free fall) you don't feel any force. The reason for this difference is that the force of gravity, as described by Newton, is proportional to each mass that it acts on, so that in F = ma the mass cancels. This means that the acceleration does not depend on the mass. The gravitational force is being applied to all parts of your body and all those parts are accelerating at the same rate. There is no transfer of force through your body, no pushing at your back, to give you a sensation of applied force.

When you are stationary in a room in a gravitational field you are being pulled down against the floor, but you don't move down because the floor is applying an equal force up against you. You feel that force pushing you up, but you don't move up because gravity is causing you to apply an equal force down. It's easy to see that the little man in the accelerating room feels these same forces. The case of your two cars pathetically tugging against each other with their wheels spinning is analogous to the room being pulled up by a chain while your ankles are being pulled down by another chain. (Gravity is nicer than a chain because it distributes the pull instead of concentrating it at your feet.)

The special property of the gravitational force that it acts on all masses equally, i.e., that it accelerates all bodies at the same rate, is what led Einstein to the equivalence principle. There is no apparent difference between being in free fall in a gravitational field (in a falling elevator) or floating freely with no gravity (in a room in deep space). Likewise, the accelerating room in zero gravity feels the same as the stationary room in a gravitational field. It's all because the inertial mass 'm' in F = ma is the same as the gravitational mass 'm' in Newton's gravitational force.

Note, I have now cast my vote.

Incidentally, are you related to Little Stevie?

 

[girlwonder]

Okay, I've really thought about it. You're right. i get it now. It was a misunderstanding. The force would still be constant, but what fun I've had.