Split Thread The validity of classical physics (split from: DWFTTW)

[semper]

Humb is far, far superior to Humber. Humber occasionally and inadvertently mixes facts into his stories, thus causing irrevocable internal inconsistency and therefore horrible damage to his arguments. Humb on the other hand has a clarity of nonsense, which is pure enough to stand unbreached.

 

[semper]

Dear Humber, please stop being wrong about the relationship between the drag force of a parachute wrt to its airspeed. You are in contempt of any number of valid experiments and theory.

 

[spork]

...Humb on the other hand has a clarity of nonsense, which is pure enough to stand unbreached.

But is it not part of humber's true brilliance that he mistakenly includes a fact here and there just to keep you guessing?

humb and humber - the whole is greater than the sum of the parts. It is not exactly the yin and the yang in that one could exist without the other - but neither alone could offer the rich and complex experience that is humb & humber.

 

[spork]

Dear Humber, please stop being wrong about...

Dear gravity, please stop acting in proportion to the mass of the two bodies and inversely as the square of the distance between them.

 

[semper]

But is it not part of humber's true brilliance that he mistakenly includes a fact here and there just to keep you guessing?

humb and humber - the whole is greater than the sum of the parts. It is not exactly the yin and the yang in that one could exist without the other - but neither alone could offer the rich and complex experience that is humb & humber.

And only now do I see the truth, the equivalence of the letter H and the letter D.

 

[recursive prophet]

What spork said, semper. And I'll let you in on a little secret. You ain't seen nothin yet! Wait till humbest arrives and reveals the 'Completeness Theory.'

 

[Dan O.]

And this one, Dan. Not that it has any bearing on the problem at hand, but another example of humber "correcting" a correct statement.

I like it. The statement is actually true for a certain interpretation with humber's chosen frame of reference^*but in general it is not.
^* Brought to you by the letter L

• "For a such harmonic motion dv/dt is greatest at zero crossing." #3081

 

[John Freestone]

Hello Michael, and thanks for that. I still rather feel like there's some doublethink going on here, though. I accept that from a certain theoretical point of view (or a more true one in the fullness of time, perhaps) acceleration and gravity are for some purposes "equivalent". However, I feel that from the classical perspective, as wikipedia agrees, there is no acceleration for a body stationary on the earth, but when it falls due to gravity it accelerates with g, which is even called "the acceleration due to gravity". From a classical perspective and a chosen (Earth) frame, the elevator can be seen and measured moving faster over time by that amount.

Once again, this isn't a question of calibration: an accelerometer measures the force due to acceleration. This is an absolute, not a relative value (Humber has spread some confusion with the idea of "absolute accelerometers": in fact all accelerometers produce absolute measurements). The accelerometer cannot tell if the force is due to gravitational pull, rocket engine or some other source. An accelerometer will read zero when it experiences zero acceleration.

Yet the link to instructions on using one discusses adding -9.8 m/s/s to get the "true acceleration".

In passing I will also note that the "confusion" that humber was spreading was in this case to do with the existence of "absolute accelerometers", not "relative ones", and the original response to that was that all accelerometers were relative and to disbelieve his assertion.

As Sol said, acceleration and gravity are equivalent: just by looking at the accelerometer, you can't tell if you're standing in a room at the surface of the earth or standing in a room that is being accelerated through space with a rocket engine.

And I did not dispute mender's 1,0,0,0 replies, merely the assertion that when you take the ground away from a stationary object resting on it, and it falls through a vacuum, "of course" there's no acceleration, and I posted links to places that explained that you have to adjust for the gravitational field to get the correct acceleration (for instance, in level flight) which we know (from a classical perspective) is not accelerating vertically. The accelerometer cannot discern acceleration from gravity, and responds only to the forces generated. It is up to human beings to decide which forces are produced by what.

Again, from the classical perspective, I find it slightly irritating that I have seen many descriptions of systems dealing with lateral motions and forces, where it has been said in passing "there is also a force of gravity, and a normal force, but these are equal and opposite, so there is no net force vertically, and no acceleration". Now, suddenly, when we take the normal force away, the answer to the question is quite different, and verges on Relativity, which again I have to remind everyone humber has been criticised for introducing.

Another point on this: if acceleration and gravity are "equivalent", that means that they are equivalent in certain situations. If they were no different from each other, there would not be two names for them and we would not be having this discussion. The fact that the gradient of a g-field discerns one from another is telling, surely? The point I was trying to make was that an accelerometer is not sophisticated enough to tell the difference, but there may indeed be a difference. I am not educated enough on this to say whether they are in actual fact different, and I'm not sure whether Einstein or anyone who came after could be said to know for sure, but my point stands: if they're "equivalent" they're not "the same".

If you want to make a simple accelerometer, just attach a known mass to the end of a spring scale. If the mass is 1 kg and the scale reads a weight of 1 kg (we need to be careful with "mass"' and 'weight", since we use the same units for both), the accelerometer is experiencing an acceleration of 1 g.

Or as I would put it, it's experiencing a gravitational field, and is stationary w.r.t. the body causing that field.

If the scale is in a free fall situation (for instance if you hold it while jumping off a diving board) it will read zero weight, corresponding to zero acceleration.

Or as I would call it, accelerating at 1 g towards the ground, which is why it is not advisable to do so without water in the pool.

Here again the equivalence of gravitational force and force due to acceleration comes into play. When the elevator is stationary with respect to the earth, it has to be supported by something that exerts an upward force on it exactly equivalent to its weight, to counteract the acceleration due to gravity. The accelerometer in the elevator reads 1 g in an upwards direction. As soon as this force is removed, the elevator is in a free fall situation. We see it start to accelerate at 1 g towards the centre of the earth: now the accelerometer inside reads zero.

My point entirely.

For us surface-dwellers, who are used to the acceleration of 1 g we always feel through our feet, the elevator was stationary and is now accelerating.

Or from a classical perspective, we don't feel an acceleration through our feet, we feel a normal force balancing the equal force of gravity, which is why we are stationary in the vertical axis.

For the accelerometer, which cannot differentiate between gravitational acceleration force and other acceleration, it was accelerating but is now no longer doing so.

Stupid accelerometer.

As the elevator falls through the earth, here's what we on earth see: its speed increases but its acceleration decreases. When it reaches the centre of the earth, its speed is at a maximum but its acceleration is zero: at the centre of the earth gravitational force is zero. As soon as it goes past the centre of the earth, it starts being accelerated in the opposite direction, which causes it to slow down: it will continue to slow down (while its acceleration, in the opposite direction to that in which it is travelling, increases!), until it reaches the other end of the tunnel, at which point its acceleration is once more at 1 g but its speed is zero. Now its starts going back the other way.

It's ok, Michael, I understand how acceleration can be in the opposite direction to a body's velocity, which slows it down. I think you'll find I introduced the whole of this footnote, and described it inaccurately only because I did so as if the earth were a point mass. I was corrected and educated on that, posted the correct view, with a link to Kepler's Shell Theorem at wikipedia. Had I been right about the point mass equivalence, the elevator's acceleration would increase towards the centre, theoretically reach the infinite, and suddenly reverse vector direction as it passed.

The weird thing is that we see all these changes in acceleration, but the accelerometer inside the elevator always shows a value of zero. If you're using Newtonian mechanics to analyse this, you might say that at any point along the voyage, the "actual" acceleration of the elevator is producing a force in a direction away from the centre of the earth, that exactly counteracts the force of gravity towards the centre. This isn't surprising, since the "actual" acceleration is being directly produced by the force of gravity experienced at that point.

I'm sorry, but this doesn't seem to be anything like what I have been reading for months now about Newtonian mechanics, and seems like a bit of doublethink. Newton would say that the normal force was removed, which had balanced the forces, and now there is a net force towards the centre of the earth (and an equal one on the earth upwards), and that now there is acceleration occuring. The argument that the acceleration towards the earth creates an equal and opposite force to balance gravity and hence give a magical zero acceleration is just nonsense from a Newtonian perspective!

If you're using general relativity to analyse the movement of the elevator, you'll say that it is following the shortest path in a region of space-time that is curved due to the mass of the earth.

I'm happy to concede that point. I don't really know much detail about GR. However, Newton, classical mechanics, the title in the thread, does not include warped spacetime.

 

[John Freestone]

John, if we model the Earth as a true sphere we can think of it as an infinite number of infinitely thin shells. ...

Thanks, that's come home to roost now. I will remember Kepler's Shell Theorem to my dying day.

Also, what Sol said about gravity and acceleration is on the money (of course).

Of course. I'm very trusting of sol's physics. I just feel that the line between classical mechanics and relativity has been crossed when we start to describe falling elevators as not acclerating because they're in a g field.

Basically, you can consider yourself to be in an inertial frame if:

A) You're free-falling in a gravitational field (and therefore accelerating!) - in which case you will ignore the effect of gravity entirely.

I'm not sure why you can't consider yourself in an inertial frame, as we have been doing all through these cart discussions, when you are not acclerating. Please note the bits I've bolded. Why do people keep posting to correct me and writing facts that recapitulate my view?! When gravity is balanced by the normal force, you can ignore it too, and there is no accleration. When you are in freefall you can ignore it because you're in freefall, accelerating!

OR

B) if you are not accelerating - and then you must consider the force of gravity as an external force.

Again, my point entirely. The accelerometer, however, measures [ETA: actually, it READS] 1g, because it measures forces, and perhaps for traditional reasons, I'm not sure. Whatever the reasons, if the thing was in a zero gravitational field, and with no acceleration, I think it would read zero. If suddenly a massive body materialized next to it to give 1 g worth of gravity, it would measure 1 g. It has not suddenly accelerated, however. It measures forces. It has stayed completely still, and will begin to accelerate with the gravity that just came into being, if not resisted by another force.

Gravity and acceleration are wierd that way. Of course as Sol points out, the only distinction that can be made between gravity and linear acceleration are the gradient effects (lines of gravity are not parallel, but converge toward a point - and gravity is stronger toward the source).

Yet one is gravity and the other is linear acceleration. ETA: Linear acceleration is an acceleration, gravity a force, for one thing. One has a 'source' the other not. The lines of gravity are not parallel - how are the lines of linear acceleration arranged?

 

[RossFW]

John,

Have a look at my zero-g training video. These people are REALLY in true freefall. The aircraft can be flown to exactley counteract forces due to air resistence, so that all that is left is gravity, exactley as if in a vacum.

I've done this many times in Aerobatic aircraft. What's on the panel is an accelerometer and the whole point is to go fron +1 to 0 'G'.It's FUN!

As I said (and Humber couldn't understand) it is indistinguishable from being in zero G in space. As someone else said, you are ALWAYS being accelerated by gravity anywhere in the universe, but as everything in YOUR frame of reference is in uniform acceleration, it is exactley the same as a complete absence of gravity.

If suddenly a massive body materialized next to it to give 1 g worth of gravity, it would measure 1 g.

ONLY if something stopped it moving towards that body, and so produced a reactive force. If the accelerometer was in freefall, it would simply start a uniform acceleration towards the body, and still read zero. In it's simplest form, an accelerometer is a suspended weight that registers acceleration due to it's own inertia relative to the rest of the instrument. If the WHOLE INSTRUMENT, INCLUDING the weight, is subjected to a uniform acceleration, the weight stays where it is relative to the rest of the instrument, and thus registers zero G

 

[RossFW]

On of hundreds of types and accelerometer design. Your ears and stomach work also.

And in the video I linked, the passengers ears and stomachs (as well as any other type of accelerometer they had with them) would be telling them they are at Zero G, just as if they were in space.

 

[John Freestone]

John,

Have a look at my zero-g training video. These people are REALLY in true freefall. The aircraft can be flown to exactley counteract forces due to air resistence, so that all that is left is gravity, exactley as if in a vacum.

I've done this many times in Aerobatic aircraft. What's on the panel is an accelerometer and the whole point is to go fron +1 to 0 'G'.It's FUN!

As I said (and Humber couldn't understand) it is indistinguishable from being in zero G in space. As someone else said, you are ALWAYS being accelerated by gravity anywhere in the universe, but as everything in YOUR frame of reference is in uniform acceleration, it is exactley the same as a complete absence of gravity.

I don't dispute that. I understand. However, from a classical perspective, you are feeling no effects of gravity, no net force, because you are accelerating towards the earth - I have a feeling you take an orbital arc, don't you, and if you did so too long you've either got to get into orbit or you squish.

ONLY if something stopped it moving towards that body, and so produced a reactive force. If the accelerometer was in freefall, it would simply start a uniform acceleration towards the body, and still read zero. In it's simplest form, an accelerometer is a suspended weight that registers acceleration due to it's own inertia relative to the rest of the instrument. If the WHOLE INSTRUMENT, INCLUDING the weight, is subjected to a uniform acceleration, the weight stays where it is relative to the rest of the instrument, and thus registers zero G

Actually, I agree with that now. I am imagining the impossible situation where the accelerometer registers the gravitational field without being resisted by another force, when of course all its parts will begin to accelerate simultaneously, so it will continue to be in freefall and register zero g. However, now it is accelerating. It has non-zero velocity. Increasing velocity. There were no forces of gravity on it. Now there are. Does it now just stay still, or accelerate?

ETA: Turn this round, a planet materialises, but a normal force resists the fall. Now it reads 1 g. Has it ACCELERATED, or is it registering gravity, a force?

Again, you seem to be arguing with the wrong bit. You can equate these effects all you like from the relativity position. Are people actually saying that Newton would agree that a falling elevator is not accelerating towards the earth? Why do people keep describing it as "accelerating" due to "gravity" at g m/s/s? Why was it all right to discuss objects accelerating towards terminal velocity before, and no-one said "No, they're not accelerating."

 

[RossFW]

Has it ACCELERATED, or is it registering gravity, a force?

...And this is where it gets curly, because I believe the answer is both!

 

[John Freestone]

...And this is where it gets curly, because I believe the answer is both!

But again, this is a sophisticated answer post-Einstein. Newton's answer, please.

 

[John Freestone]

And in the video I linked, the passengers ears and stomachs (as well as any other type of accelerometer they had with them) would be telling them they are at Zero G, just as if they were in space.

Again, you're assuming that Zero G means zero acceleration. You feel zero force of gravity and your stomach objects because you are accelerating towards the earth. The 'normal force' usually transfered through the wings from the air would like to balance the force of your weight, but you keep making the floor recede. Hence unbalanced forces, hence acceleration. AFAIK, you can't do that by moving downwards even at a fixed velocity. You accelerate downwards.

When in "space" (by which I presume you mean actual zero gravitational field), there's no gravitational force. There's no need for a normal force to balance it. Hence you feel no normal forces in your ears or gut, which they don't like. I agree that to them and the accelerometer, no difference can be detected between these scenarios, but their owner can look out the window. If they use the boosters, they'll feel an acceleration, and they'll see it. If someone is secretly piling mass into their vicinity, and their accelerometer does not register any change, that's because they're falling towards the mass, and they'll see that out the window too.

 

[spork]

Why do people keep posting to correct me and writing facts that recapitulate my view?!

My apologies John. In point of fact I wasn't trying to correct you. As I do often with this thread, I use my dandy new humber fast scroller. I saw some back and forth on the gravity vs. acceleration issue, and I added my 1.2 cents (hard economic times) because I've found it a subtle and fascinating issue for some time. I was not specifically trying to correct anyone, but rather clarify a subtle issue (which I've probably failed at).

When gravity is balanced by the normal force, you can ignore it too...

Ah yes - but only because you view it as a 2-dimensional problem. You don't just ignore gravity and vertical acceleration; you ignore the vertical degree of freedom altogether.

If suddenly a massive body materialized next to it to give 1 g worth of gravity, it would measure 1 g.

Actually, if a massive body suddenly materialized next to our free floating accelerometer, the accelerometer would in fact start to accelerate toward that body, but continue to read zero acceleration.

...It has not suddenly accelerated, however.

"suddenly" is trickier still. The "knowledge" that this massive body just appeared travels at the speed of light. The gravitational effect travels at the same speed. So it would not accelerate toward it instantly, but pretty darn soon after it materialized. And in any case the accelerometer would read exactly zero throughout.

Linear acceleration is an acceleration, gravity a force

Gravity is not a force. Gravity times mass is (or produces?) a force. Gravity is more accurately considered an acceleration field.

One has a 'source' the other not.

Correct. This is the root of the two gradient effects seen with gravity, and not seen with linear acceleration.

The lines of gravity are not parallel - how are the lines of linear acceleration arranged?

Linear acceleration is seen as parallel. In other words, two accelerometers in the accelerating cart would show acceleration in the exact same direction. Two accelerometers in a cart in a gravitational field (if prevented from accelerating) would show an acceleration in slightly different directions - both pointing toward the center of mass of the nearby massive body.

 

[spork]

If someone is secretly piling mass into their vicinity, and their accelerometer does not register any change, that's because they're falling towards the mass, and they'll see that out the window too.

Yup.

 

[spork]

I just made a couple of drawings to try and show the distinction between what accelerometers see if the cart is sitting on the ground and experiencing 1g vs. a cart being accelerated in the absence of a gravitational field. I imagine everyone already gets this, but I was in the mood to make the diagrams.

 

[humb]

Ummmmm... check your PMs, spork.

 

[John Freestone]

My apologies John. In point of fact I wasn't trying to correct you. As I do often with this thread, I use my dandy new humber fast scroller. I saw some back and forth on the gravity vs. acceleration issue, and I added my 1.2 cents (hard economic times) because I've found it a subtle and fascinating issue for some time. I was not specifically trying to correct anyone, but rather clarify a subtle issue (which I've probably failed at).

Thanks, spork. I don't feel that any apology is necessary. I just wanted to clarify it too, and I wanted in particular to clarify it in relation to the heading of the thread. It began because Michael C made a very clear and "obvious" statement that of course an accelerometer in a falling lift would read zero, because it is not accelerating. There is no drag, he said, so it's in free fall, hence it's not accelerating. It subsequently appears that it is not accelerating because as it accelerates the acceleration produces a force which opposes gravity or something. Hence, despite the walls of the building going past at a velocity that increases at 9.8 m/s/s, you're not accelerating at all. I will now spend the next three months correcting all previous posts about kites and carts and balloons. When you're not accelerating in a gravitational field, you're accelerating. Fine. It's an accelerating frame of reference, and the walls are accelerating at the same speed. When you freefall, you stop accelerating, and the walls continue to accelerate upwards at 9.8 m/s/s. Everything on the earth's surface is accelerating outward at that rate while it's motionless with respect to everything else except things that are falling, which are {ETA: stationary not accelerating } because they're falling. Have I got it now?

Ah yes - but only because you view it as a 2-dimensional problem. You don't just ignore gravity and vertical acceleration; you ignore the vertical degree of freedom altogether.

Newton seems to say that there's a force down on you and a force up on you, and they equal out, so you remain at the velocity you were at before. We've been discussing classical mechanics and bashing humber over the head with F = ma long enough. Suddenly we're in Einstein country, where spacetime is curved and mass causes an acceleration field.

Actually, if a massive body suddenly materialized next to our free floating accelerometer, the accelerometer would in fact start to accelerate toward that body, but continue to read zero acceleration.

You must have speed scrolled through the next bit. RossFW (I think) put me right about that. However, as I said, if a mass suddenly materialised next to you with a physical resistance, a normal force, the accelerometer would now measure 1 g. You haven't physically moved in classical terms, you haven't accelerated in classical terms. Your accelerometer is measuring a force of gravity on its moving part. It measures 0g. Then it measures 1g. It didn't move. Great these things arent they?

"suddenly" is trickier still. The "knowledge" that this massive body just appeared travels at the speed of light. The gravitational effect travels at the same speed. So it would not accelerate toward it instantly, but pretty darn soon after it materialized. And in any case the accelerometer would read exactly zero throughout.

Sure. "Suddenly" was just rhetoric. Again, Newton didn't deal with the speed of light as far as I'm aware, at least if he did, I don't think it comes into his laws of motion. Classical mechanics - have we actually defined it in this thread at all? I thought it was pretty much Newton's three laws, and the conservation of energy and conservation of momentum. I didn't think warped spacetime and the speed of light came into it.

Gravity is not a force. Gravity times mass is (or produces?) a force. Gravity is more accurately considered an acceleration field.

Hmmm. I'm not sure about that. Once again, from a Newtonian perspective, I thought gravity was considered a force, F = G m1 m2/d2, which produced an "acceleration due to gravity", g. if not resisted.

Correct. This is the root of the two gradient effects seen with gravity, and not seen with linear acceleration.

So gravity is not the same as acceleration? How do you explain the way we call something "equivalent" without there being two different things to equate? Doesn't it mean that there are equivalences, rather than that they are the same thing?

Linear acceleration is seen as parallel. In other words, two accelerometers in the accelerating cart would show acceleration in the exact same direction. Two accelerometers in a cart in a gravitational field (if prevented from accelerating) would show an acceleration in slightly different directions - both pointing toward the center of mass of the nearby massive body.

And again, in classical mechanics, a spring with a load on the end would be described as having a force of gravity upon it. An accelerometer isn't much more than a spring measurer.

I think that these are academic points to some extent. If we take F = ma, the classical explanation suggests that there is a real force, acting on a real mass, and acceleration is a sort of measured change of position over time over time, like it is the result and response of a mass to force. I realise that in reality it is not like that, and that these are all interlinked. I realise that there is something very difficult with action at a distance, as Newton worried about, and that Einstein's view is more encompassing. But really we're getting into the realms of cosmological philosophy. If Michael and you are happy to consider a falling elevator as at rest and the ground accelerating up towards it, fine. We should all get out of the way in either case.

We could just answer the implied question of the thread title, "The validity of classical physics: nope, superceded", and move on. Let's describe the cart on the treadmill from GR theory instead, stop lying about kites and planes maintaining level flight due to balanced forces, and not experiencing acceleration vertically. Let's trawl through this classical mechanics thread making it all agree with Einstein.

I realise that you weren't arguing much about this with me. I just feel that Michael's statement about a falling elevator not accelerating didn't take any account of the context of the conversation, presented this as obvious because he knows what an accelerometer would read (implying that zero G = no acceleration), that this was completely counter to everyday or 'classical' understanding, and he failed to - and still has failed to - say why the building is changing its relative velocity over time wrt the elevator. He hasn't clarified that this statement is only true from the perspective of all stationary objects in a grav field accelerating upwards or whatever it is that they're supposed to be doing.

I'd like to potter about in Newtonian space a bit longer, find my land legs, you know, before I take on Relativity. Call me old. Besides, we don't need any of the last 100 years of advanced physics to make sense of a falling lift, just as was chucked at humber repeatedly when he mentioned anything general-relativistic. And furthermore, people keep using my supposedly wrong terminology, talking about accelerating elevators not feeling any acceleration, to refute my argument. We just need to stick to the same terms, or it's going to get mental.