Newton's laws in variable mass systems

[Dickfore]

DH's position is just silly.
It is unsurprising that he employs the logical fallacy ad authoritam

To take a case where it is utterly nonsensical to describe the momentum flux as a force, we can look at a fluid moving at constant (horizontal) velocity U, and choosing as our control volume that at t=0 starts out as a line segment of length 1, broadening into a rectangle, where one side remains stationary at the initial position of the line segment, the other vertical side moving with (horizontal) velocity V.
Letting d be the density of the fluid, the momentum containe in our control volume is simply U*V*d*t, with a rate of change U*V*d

In this case, the rate of change of momentum within our control volume cannot be ascribed as the effect of an acting force.
Forces act solely upon material particles, not upon arbitrarily chosen spatial regions*. The rate of change of moementum is solely due to flux of momentum, i.e, a quantity that has the same units as force, but is still wholly distinct from force.


Note:
Some use "momentum flux" to designate what I'd call "momentum flux density", i.e, the rate of momentum transfer per unit area(by means of momentum-carrying particles leaving the control volume).


*Remember that in the classical world time&space are dynamically inactive quantities, merely the empty box within which dynamics and the play of forces occur.

I don't have the faintest idea what you are talking about.

 

[arildno]

My view might be silly, but it is the one most widely supported in literature.

Again, just meaningless appeal to authority

 

[stewartcs]

Again, just meaningless appeal to authority

That's actually not a meaningless appeal to authority.

If DH had used himself as the authority, then made a claim that his position was true because he said it was, then it would be a logical fallacy. However, since DH is referring to separate reputable sources (i.e. peer reviewed literature) it is not a logical fallacy.

If using reputable literature as an authoritative reference is a logical fallacy then no one on this site should be posting unless they have performed original research (implying they would be committing a logical fallacy).

However, with that being said, if DH is a "rocket scientist" then he may very well qualify as an expert on this subject and may indeed make authoritative claims without committing a logical fallacy.

CS

 

[Dickfore]

That's actually not a meaningless appeal to authority.

If DH had used himself as the authority, then made a claim that his position was true because he said it was, then it would be a logical fallacy. However, since DH is referring to separate reputable sources (i.e. peer reviewed literature) it is not a logical fallacy.

If using reputable literature as an authoritative reference is a logical fallacy then no one on this site should be posting unless they have performed original research (implying they would be committing a logical fallacy).

However, with that being said, if DH is a "rocket scientist" then he may very well qualify as an expert on this subject and may indeed make authoritative claims without committing a logical fallacy.

I have yet to see him provide a peer-reviewed Journal or a redacted textbook in which the equation:

<br /> \mathbf{F} = \mathbf{F}&#039; + \dot{m} \, \mathbf{V}<br />

is derived (see the beginning of post #14).

 

[D H]

I have yet to see him provide a peer-reviewed Journal or a redacted textbook in which the equation:

<br /> \mathbf{F} = \mathbf{F}&#039; + \dot{m} \, \mathbf{V}<br />

is derived (see the beginning of post #14).

You have been given a reference to exactly such a derivation twice in this thread. Here they are (both link to the same article):

Plastino & Muzzio, Celestial Mechanics and Dynamical Astronomy, 53:3 (1992) http://articles.adsabs.harvard.edu//full/1992CeMDA..53..227P/0000227.000.html.

This may be of use here:
http://articles.adsabs.harvard.edu//full/1992CeMDA..53..227P/0000227.000.html

 

[Dickfore]

You have been given a reference to exactly such a derivation twice in this thread. Here they are (both link to the same article):

What equation exactly in the first reference has the derivation?

 

[D H]

Equation (2). Technical papers don't need to spell out the blatantly obvious; they just need to say that some result is blatantly obvious (e.g., the stock phrase "the reader can readily see"). From the paper (emphasis mine):

If we consider the simple case of a variable mass, and we write Newton's second law as:

\vec F = m\frac{d\vec v}{dt} + \vec v\frac{dm}{dt}

we can easily see that it violates the relativity principle under Galilean transformations.​

Be honest: Do technical papers need to spell out the obvious?

 

[Dickfore]

Exactly! And, if you understood this as an argument in favor to your claim, you are illiterate when it comes to scientific literature.

 

[D H]

No. I placed this paper in camp 3, Newton's laws don't apply to variable mass systems, which in my mind is a stick one's head in the sand position. Aerospace engineering is plain old Newtonian mechanics. We've been flying for over a century now, and flying modern rockets for nearly that long. Descriptions of flying machinery is almost always done in a Newtonian (read: not Lagrangian) sense. So to say that Newtonian mechanics doesn't apply to such devices is just silly.

 

[Dickfore]

No. I placed this paper in camp 3, Newton's laws don't apply to variable mass systems, which in my mind is a stick one's head in the sand position. Aerospace engineering is plain old Newtonian mechanics. We've been flying for over a century now, and flying modern rockets for nearly that long. Descriptions of flying machinery is almost always done in a Newtonian (read: not Lagrangian) sense. So to say that Newtonian mechanics doesn't apply to such devices is just silly.

Your position has clearly failed. Do not change the thesis. It would be the most mature thing you can do if you admitted your error about the fallacious Transformation Law that you presented in post #14.

 

[D H]

Please. I have not changed my thesis and there is no flaw in post #14 given the definition F=dp/dt.

 

[D H]

There is some flawed mathematics in this thread. Post #61 implicitly assumes there is no momentum flux inside the control volume -- i.e., it assumes that all particles are moving at the same velocity.

 

[Dickfore]

lol, keep striking.

 

[D H]

The only swingin' and missin' is yours. This is a standard problem in graduate level aerospace engineering classes. A rocket is not a point mass. The center of mass of the fuel is not the center of mass of the vehicle as a whole. As fuel is consumed the overall vehicle's center of mass will move inside the vehicle. This motion of the center of mass has an effect on vehicle dynamics. Challenge to you: What are the equations of motion for such a rocket?

 

[Dickfore]

Everything in that post is well defined. You didn't read it.