What is ment by the pharse "Accelerated by Gravity

[beammeupscot]

I teach classes in basic thermodynamics to technicians the power production industry. In a class that I teach we define the following:

 

Mass as: Amount of matter an object contains.

 

And weight as: Force exerted by the mass when it is accelerated by gravity.

 

A student of mine questioned that statement: How is mass resting on the earth being accelerated?

 

So then we looked up the definition of Accelerate and found the following:

 

"1. To cause to move faster; to quicken the motion of; to add to the speed of;" and it didn't seem to aid me in my explanation.

 

I did find the basic description of gravity to be an attraction caused by mass. But then he asked how does that attraction work? I didn't know and stated that I would try and find out and get back to him with the results. Any help that I could have no this would be appreciated.

 

Scott

[Skye]

Weight can be defined as the force required to prevent an object from falling, which neatly sidesteps the whole issue:)

[JaKiri]

I don't see why we have to have the 'force required to stop it accelerating' business.

 

It's just the force applied by the presence of another mass.

 

That statement in the book is just wrong though.

[beammeupscot]

Skye replied:

 

Weight can be defined as the force required to prevent an object from falling, which neatly sidesteps the whole issue.

 

 

My reply......

 

But the definition of weight being mass accelerated by gravity is the standard given in most reference texts.

 

An object truly "accelerated by gravity" would be increasing it's relative velocity toward the center of the mass producing the gravity field. And even more to the point an observer traveling with the object would experience the sensation of free fall or weightlessness.

 

I think the origin of this statement comes from Albert Einstein and his gedanken experiment that he used to explain GTR.

 

From the book "Relativity, The Special and the General Theory" by Albert Einstein: Section 20

 

(chapter title)

The Equality of Inertial and Gravitational Mass as an Argument for the General Postulate of Relativity.

 

"We imagine a large portion of empty space, so far removed from stars and other appreciable masses, that we have before us approximately the conditions required by the fundamental law of Galilei. It is then possible to choose a Galileian reference-body for this part of space (world), relative to which points at rest remain at rest and points in motion remain in motion continue permanently in uniform rectilinear motion. As a reference-body let us imagine a spacious chest resembling a room with an observer inside is equipped with apparatus. Gravitation naturally does not exist for this observer. He must fasten himself with strings to the floor, otherwise the slightest impact against the floor will cause him to rise slowly toward the ceiling of the room."

 

Break- It seems to me that this observer even at this point has no idea if he is in a gravity free environment or in a gravity field being accelerated at constant rate by that gravity or even which way is up or down. When in freefall in a gravity field (in a vacuum) an observer would have the same sensations as being in a gravity free environment. The observer would be experiencing weightlessness. Again more confusion about the phase weight is mass accelerated by gravity.

 

Back to the book....

 

"To the middle of the lid of the chest is fixed externally a hook with rope attached, now a "being" (what kind of being is immaterial to us) begins pulling at this with a constant at this with a constant force. The chest together with begains to move "upwards" with a uniformly accelerated motion. In course of time their velocity will reach unheard-of values provided that we are viewing all this from another reference which is being pulled with a rope.

 

But how does the man in the chest regard the process? The acceleration of the chest will be transmitted to him by the reaction of the floor of the chest. He will therefore take up the this pressure by means of his legs if he does not wish to be laid out full length on the floor. He is then standing in the chest in exactly the same way as anyone stands in a room of a house on our earth. If he releases a body which he previously had in his hand, the acceleration of the chest will no longer be transmitted to this body , and for this reason the body will approach the floor with an accelerated relative motion. The observer will further convince himself that acceleration of the body towards the floor of the chest is always the same magnitude, whatever kind of body he may happen to use for the experiment.

 

Later in the chapter Einstein states: "A gravitational field exist for the man (was observer and is now a man) in the chest despite the fact that there was no such field for the co-ordinate system first chosen"

 

I say.... What gravitational field? The one we tricked the observer into believing existed in the chest? In doing this Albert Einstein falsely convinced everyone that gravity is the same as acceleration. It is not. The effects may be very similar but the cause is no where near the same. In doing this Einstein mistakenly focused modern physics on the effect of phenomena and not the cause. We still do not understand time, gravity, inertia or many other forces nearly as well as we need to. As result much of modern physics is "built on a house of cards" that will someday crumble. One modern physicist stated that Einstein came before his time and that we would have come to SPR and GTR (in time) through string theory or Quantum mechanics. In my opinion, if it had worked that way the response (at that point) would be "so what, of course it looks that way".

 

Scott

[Radical Edward]

most reference texts are wrong, there does not have to be acceleration for there to be weight, just a force. this is simple to see because the units for force and weight are the same (Newtons)

 

Incidentally, Maxwell was on the verge of finding the Special Theory of Relativity, but he died.

 

Furthermore, Einstein's argument revolves around the fact that there is no way to tell the difference between inerial and gravitational mass (so they are the same) and also if you are merely being accelerated by a rope or in a gravitational field. there is no experiment that can distinguish the two, so the two are effectively the same. From this you then shift to curved space, and get all the correct results, absolutely the correct results so far.

[beammeupscot]

Radical Edward Replied:

 

There is no experiment that can distinguish the two, so the two are effectively the same.

_____

 

My Reply...

 

In Einstein's time this was true but now with today's instruments an observer could tell the difference. One being that by dropping two objects and carefully measuring the trajectories the weights would stay parallel in the inertial frame and both would track a path toward the center of the massive object producing the gravity in the gravitational frame.

 

Now all that being said I do agree with what STR and GTR explain for the most part. The only problem I have with GTR is where he says gravity and inertial acceleration are the same. They are not the same, all though the effects are very similar, the causes are completely different.

 

Scott

[superchump]

Try telling me that if you not an outside observer, you'd know the difference between acceleration and graviation. Think of it this way. You're standing in a windowless elevator that sits in a very tall shaft out in space somewhere far from a gravity source. The elevator is accelerating in a way that provides approx. one gee as earth does. A friend who is on earth, in a windowless box that is stationary relative to the earth, is feeling one gee too. Neither of you know what setup you're in. How could either of you tell in what setup your in until the box either slows down or stops accelerating? Ignore the fact that the box had to start accelerating so you were weightless at the time. Pretend you wake up without knowing where you are or something hehe.

[beammeupscot]

Superchump said:

 

Try telling me that if you not an outside observer, you'd know the difference between acceleration and graviation. Think of it this way. You're standing in a windowless elevator that sits in a very tall shaft out in space somewhere far from a gravity source. The elevator is accelerating in a way that provides approx. one gee as earth does. A friend who is on earth, in a windowless box that is stationary relative to the earth, is feeling one gee too. Neither of you know what setup you're in. How could either of you tell in what setup your in until the box either slows down or stops accelerating? Ignore the fact that the box had to start accelerating so you were weightless at the time. Pretend you wake up without knowing where you are or something hehe.

__________

 

Einstein allowed tests to be run inside of the test chamber. Here is HIS quote, "As a reference-body let us imagine a spacious chest resembling a room with an observer inside is equipped with APPARATUS (emphasis added by me)." These were the conditions HE set..... Not me, not you, Einstein set them to explain his reasoning of GTR. What I am saying is THERE ARE TESTS that can be run. To tell if you are on the earth is a slam dunk due to tidal forces etc. Now I would admit that a non-rotating massive body that produces gravity oh in the range of 9.8m/s/s would be tougher. But the fact still remains that the gravitational force would be directed to the CENTER of the mass and that WOULD be detectable. Now we are trying (haven't succeeded yet) to detect gravity waves and or gravitons. If either of these technologies become fact, they would invalidate EEP. Ironic isn't it, the theoretical knowledge that would lead us to state that gravity waves and gravitons exist and could be detected would invalidate an important part of that theory?

 

Also my problem with this is the fact that by equating acceleration and gravity, true understanding of how gravity works has been delayed.

 

Scott

[JaKiri]

I'm afraid you're wrong.

 

If there is such widespread evidence of General Relativity being incorrect, why haven't we heard more about it?

[Radical Edward]

yes you can tell if you are in earth's gravitational field or in an accelerating lift out in the middle of nowhere, but that is because the acceleration in earth's field is towards a point (lets assume you have some really neat gear and can tell the change in trajectory because of this), and the acceleration in a lift is unidirectional, however this has nothing to do with einstein's point. all you are detecting is the difference between two systems (one is spherical and one is effectively one dimensional). His point is that there is no difference between a gravitational acceleration, and any other acceleration, and nothing you have said demonstrates otherwise.

[Radical Edward]

Originally posted by Radical Edward

yes you can tell if you are in earth's gravitational field or in an accelerating lift out in the middle of nowhere, but that is because the acceleration in earth's field is towards a point (lets assume you have some really neat gear and can tell the change in trajectory because of this), and the acceleration in a lift is is in one direction, however this has nothing to do with einstein's point. all you are detecting is the difference between two systems (one is spherical and one is effectively one dimensional). His point is that there is no difference between a gravitational acceleration, and any other acceleration, and nothing you have said demonstrates otherwise.

[Skye]

I don't see why we have to have the 'force required to stop it accelerating' business.

Because as far as I know weight includes centripetal acceleration as well as the gravitational force.

[JaKiri]

Originally posted by Skye

Because as far as I know weight includes centripetal acceleration as well as the gravitational force.

 

Centripetal force is entirely made up (under normal circmumstances) from gravitational force.

 

Most people don't 'get' that it's not a force in it's own right, it's just something that is mathematically required to have rotation around a point at constant radius.

 

Centripetal acceleration + gravitational force =/= weight.

 

Gravitational force = weight.

 

It's just that you'll get a slightly lesser value for the normal reaction because some of the force is 'used' mantaining rotational movement.

[beammeupscot]

yes you can tell if you are in earth's gravitational field or in an accelerating lift out in the middle of nowhere, but that is because the acceleration in earth's field is towards a point (lets assume you have some really neat gear and can tell the change in trajectory because of this), and the acceleration in a lift is unidirectional, however this has nothing to do with einstein's point. all you are detecting is the difference between two systems (one is spherical and one is effectively one dimensional). His point is that there is no difference between a gravitational acceleration, and any other acceleration, and nothing you have said demonstrates otherwise.

[beammeupscot]

Radical Edward said:

His point is that there is no difference between a gravitational acceleration, and any other acceleration, and nothing you have said demonstrates otherwise.

 

If an object is accelerated by gravity with nothing resisting it, it is weightless. But if a object is accelerated by an increase in velocity, (by some other force than gravity) it has weight. The two are not the same. The only time gravity results in weight is when the force of gravity is resisted. Now acceleration does produce an effect that mimics the effect when gravity is resisted by some other force. But to say they are the same thing is stepping over the line.

 

TBH My view on this subject has oscillated back and forth a number of times. I can see both sides of the argument. But one of the reasons I currently on the anti-Einstein side is what I have read that he wrote to explain GTR and his dismissal of the existence something like Tesla's Either. It seems the modern name for this is Quantum foam (flux) or Zero Point Energy.

 

To say that acceleration and gravity are the same only makes sense if there is some Either like material (spacetime?) that moves (is accelerated?) in a gravity field and is stationary in a gravity free situation.

 

THANK YOU all for your responses on this subject as my overall goal is a deeper, truer understanding of gravity and you are contributing to this.

 

Scott

[JaKiri]

Zero point energy isn't the same as an ether. The relative nature of space is independent of it; an ether implies absolute positioning.

 

Someone else can deal with the rest, I have to pack.

[s pepperchin]

The quick reply to ths thread is that weight is the force a palnet exerts on a mass. it has the same units as force because they are the same, however not all forces are weights. If I spin a mass on the end of a string the string has a certain tension this is not the weight of the mass.

[swansont]

The quick reply to ths thread

 

Quick? Two years, eight months is quick?

[Klaynos]

Quick? Two years, eight months is quick?

 

That all depends on your reference