## accelerated solid bar

Discussions on classical and modern physics, quantum mechanics, particle physics, thermodynamics, general and special relativity, etc.

### accelerated solid bar

I wanted to respond to the following assertions about the nature of acceleration of a solid rod. The following quotation was buried so deep in a thread made by Andrex, that I have to extract it here.

Dave_Oblad » July 1st, 2017, 11:05 pm wrote:Hi Andrex,

Good thread. Now if one treats Reality as granular at the Planck length
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Imagine a Rod, which is a column of Atoms. Imagine we focus on the first two ranks of atoms at the front and the last two ranks of atoms at the back. The spacing between all ranks is even when inertial in flat space. But accelerate the rod from the front (pull it) and the rank spacing between the first two ranks and the back two ranks becomes uneven (non-linear), basically because the last two ranks is the last to know that acceleration is being applied (pulled from the front).

If you reverse this and push from the rear, the same non-linear ranking occurs because the rear is compressed and the front is last to know. Thus any acceleration being applied always has the same effect on the rank spacing along the rod. Rank Spacing is greater at the front than at the rear in the direction being accelerated.

This is not physically correct. The original poster held up these claims as "fact" in support of a personal theory of cosmology.

The only way that type of compression would happen to an iron bar, would be if you shot something at it that moves faster than the speed of sound in iron. (which is far greater than the speed of sound in air). That kind of collision would likely also destroy the bar by breaking it into pieces. If the projectile was heavy enough, the bar would disintegrate into a hot dust.

At any lower velocity, the physics of momentum transfer determine what happens. You either have 1.) a recoil called a perfect elastic collision or if any sticking takes place you have 2.) an inelastic collision.

In many cases for iron, there is a moment where collision begins, and that change will create a "ding", an audible sound. That ding sound is literally the compression racing from one end of the bar and back. The racing back and forth is a compression of the iron atoms, and it happens so quickly that the frequency is in the audible range. The ringing of a compression wave is racing through the bar at the speed of sound (in iron). That wave motion is independent of the acceleration of the bar as a whole. (ie. the actual acceleration is determined by momentum transfer)

Coming back to the original poster's claim, this compression wave (creating the ding sound) would be the same in all materials, since all materials are bound in the same spacetime lattice. (this is his reasoning). That is clearly not physically correct : different materials create very different sounds.

hyksos
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### Re: accelerated solid bar

hyksos » 01 Nov 2017, 21:56 wrote:I wanted to respond to the following assertions about the nature of acceleration of a solid rod. The following quotation was buried so deep in a thread made by Andrex, that I have to extract it here.

{quote snipped}

This is not physically correct. The original poster held up these claims as "fact" in support of a personal theory of cosmology.

This is a somewhat of an invitation for the 'personal theorists' to defend their ideas on a thread in the physics forum. It is fair to discuss the mainstream position of a lengthwise accelerated rod here, but if it degrades to personal theory defenses, it must go back there.

That said, I think Dave_O's position is correct for a rod subjected to a non-destructive jerk in the classical, non-relativistic limit. It is not correct for a uniformly accelerated rod though, but I don't know if that was he meant it to be.

BurtJordaan
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### Re: accelerated solid bar

BurtJordaan,

I was going to ask Dave O if he gave the accelerated rod example as a euphemistic analogy of the function of gravity, or if it he meant this literally happens. (Andrex would have scrolled my post off the screen)

hyksos
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### Re: accelerated solid bar

Momentum transfer is a fundamental phenomenon. It goes all the way down to the particles. While sound and resonant frequency is amenable to a model based around "balls and springs", I don't believe that transferring momentum ever reduces to a similar model.

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### Re: accelerated solid bar

hyksos » 03 Nov 2017, 05:38 wrote:Momentum transfer is a fundamental phenomenon. It goes all the way down to the particles. While sound and resonant frequency is amenable to a model based around "balls and springs", I don't believe that transferring momentum ever reduces to a similar model.

Correct, but I think Dave's example is not so much about momentum transfer, but rather on what happens inside an inertial rod when it gets a jerk, being hit lengthwise at one end by a much heavier object, but not so hard as to permanently deform the rod. That end will be compressed and a compression (sound) wave will travel along the rod, with the far end compressing last. Then a decompression wave will be returning and setting up a resonance that will lose energy and eventually die away. "Ringing down", loosely speaking.

After that the whole rod will be moving inertially again, having gained some momentum relative to the original inertial frame. If however the jerk was caused by the sudden onset of a constant push (or pull from the other end), the rod will then (after the ringing down) accelerate uniformly in its own (accelerating) frame and keep a constant length, being slightly compressed if being pushed or slightly stretched if being pulled. It will however relativistically contract continuously as seen in the original inertial frame.

This is analogues to a rod either standing up or hanging down in a gravitational field, while the original inertial frame accelerates past it (observer in free-fall).

BurtJordaan
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### Re: accelerated solid bar

We are obviously dealing with the phenomenon described by F=ma in nature. But worse, we are dangerously flirting with the question "Why is F=ma true?" Like we are asking what more fundamental thing in the universe causes F=ma to be a good theory of large bodies?

Most sane people would stop somewhere here and let fundamentals be fundamentals. (F=ma is true because the universe is like that, end of story.) On the other hand, you can start asking questions about why is there inertia at all to begin with? This question reduces to "What is mass, really?"

Two avenues of thought split off here.

1.) Inertia happens because of the Higgs Mechanism. Space is occupied by a scalar field that trades off ziggs with particles that have "mass", causing a drag effect from acceleration.

2.) What is mass? One answer is that m = E/c2 But why is that true? The answer is that if you had a tiny cube with perfectly reflecting walls, (whose mass is defined to be zero) and there was light bouncing around inside it. That cube would act as if it had mass to an outside observer. The amount of mass that an outside of observer would measure would be

m= E/c2

Where E is the total energy of the light reflecting around inside.

This sounds totally wacko at first glance. However, this exact reasoning is used to calculate the mass of larger nuclei (earth metals). If you just add up all the masses of the nucleons in many elements, you get a number. But the measured mass is always lower than that number This is called the "Mass defect" in physics textbooks. The answer as to where the extra mass went, is that some of the mass was converted into the binding energy of the nucleons.

This is not a joke or a personal theory. See http://physics.bu.edu/~duffy/sc546_notes10/mass_defect.html

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### Re: accelerated solid bar

Anyways.. getting distracted. Back to the iron bar.

Uniform acceleration is not due to a ball-and-spring microscopic model. I admit that such a model might work out well in a computer simulation or a video game. But in the real world, the molecules of the bar are sitting in a "field" determined by electron binding between adjacent lattice points. The initial collision sets up a "thwack" noise or a "ting", and those sounds will have some wavelength related to the vibrations of the colliding bodies. Afterwards more fundamental effects take over regarding momentum transfer into "colliding pusher" and "metal bar". The colliding pusher will lose momentum as the bar pushes back against it with its inertia. But why inertia? Why momentum transfer? And why momentum conservation? Those questions are only answerable with the above post and its digressions.

hyksos
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### Re: accelerated solid bar

hyksos » 20 Nov 2017, 23:38 wrote:The answer as to where the extra mass went, is that some of the mass was converted into the binding energy of the nucleons.

This is not a joke or a personal theory. See http://physics.bu.edu/~duffy/sc546_notes10/mass_defect.html

This principle is similar to the gravitational binding energy of planets. The individual pieces of rubble that have coalesced into the planet had a larger total gravitational mass than what the resulting planet has. In this respect, binding energy is negative. In simpler terms, the rubble have lost potential energy as they came together and hence when summed, "weighs less". This is understood reasonably well.

Inertia, on the other hand is not that well understood. AFAIK, the Higgs field gives the particles mass, but not inertia (directly), because that may depend on the present Hubble scale of the universe. Very confusing, though.

BurtJordaan
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