Physics in troubles: the real equation of force is F = ma and not F = dp/dt

[martillo]

45 minutes ago, studiot said:

At last.

We are getting somewhere.

No just air resistance of course.

 

The conclusion is that

By itself in free space the rocket cannot 'run the rocket engine' and travel at constant velocity.

Alternatively the a working rocket muct be under acceleration unless it is also being acted upon by an external force for example gravity or air resistance.

If it did not do so it would be breaking all 3 of Newton's laws.

Right for me too.

By the way, I'm waiting for your treatment of the problem as we begun with your post:

20 hours ago, studiot said:

Well I was just following Dr Cowan from London University all those decades ago, when I learned this stuff.

Here is his introduction to the subject.
I was just trying to flesh out the explanations further and show where the mathematical tricks occur.

Note very carfully what he says about f = ma   v  f =mdp/dt

and his use of r as distance moved by centre of mass.

The subject for me is to analyze for if the Force Equation F = ma would hold or not for variable mass according to that your presented book, do you remember?

I'm rereading it and seems the professor actually agrees with me! Is that right or am I taking it wrongly? Please explain with which equation the book agrees: with F = ma or with F = dp/dt for variable mass in the rocket. I'm a little confused...

 

Now I see it clearly:

DR. Cowan agrees with me that the equation used in the motion of rockets is F = ma = mdv/dt for variable mass!

Excellent. I was worried about that. The claim of the thread still stands!

I don't know what should we discuss then. Can you tell me now, I'm a little confused...

Edited February 24, 2023 by martillo

[joigus]

On 2/23/2023 at 1:28 PM, studiot said:

dV = -v d(lnM)     where lnM is the natural log of M

I'm checking up on everything I can. I get the same as you. If motion of respective CoM of both rocket and exhaust are collinear and in the absence of external fields.

Of course, forces must be identified with dp/dt. What's tricky is the momentum of what and what force on what 'object.'

I'm considering the exhaust as one big indistinct thing, although its CoM must move in a predictable way.

[martillo]

1 hour ago, martillo said:

Now I see it clearly:

DR. Cowan agrees with me that the equation used in the motion of rockets is F = ma = mdv/dt for variable mass!

Excellent. I was worried about that. The claim of the thread still stands!

I don't know what should we discuss then.

So, even for rockets with variable mass the equation holds and it actually works in practice. Just a question remains: if it must be generalized or not. "Just" one big problem: Relativity Theory falls. It cannot handle that. There's no other case where it would not hold, I think.

So, what's next? To discuss other possible implications of the Equation of Force be F = ma and not F = dp/dt or to look for an alternative to Relativity Theory? Or both?

These are the subjects now. May be new threads must rise, I don't know...

Edited February 24, 2023 by martillo

[Lorentz Jr]

27 minutes ago, joigus said:

Of course, forces must be identified with dp/dt. What's tricky is the momentum of what and what force on what 'object.'

I'm considering the exhaust as one big indistinct thing, although its CoM must move in a predictable way.

If the rocket ejects a mass dm at speed v_e in a time dt, the acceleration of the mass is a = v_e/dt. So the force acting on it is F = ma = dm(v_e/dt) = v_e(dm/dt). In the rocket's reference frame, that's also dp_gas/dt.

Edited February 24, 2023 by Lorentz Jr

[joigus]

2 minutes ago, Lorentz Jr said:

If the rocket ejects a mass dm at speed v_e in a time dt, the acceleration of the mass is a = v_e/dt. So the force acting on it is F = ma = dm(v_e/dt) = v_e(dm/dt). For the exhaust gas in the rocket's reference frame, that's also dp/dt.

Absolutely. I'm looking at it from every perspective I know, and I can't see any reason why this shouldn't hold true. I can't see any way in which dp/dt is not valid here either.

I think we agree on that. Am I right? 

[Lorentz Jr]

2 hours ago, joigus said:

F_ext stands for 'total external force' for both the rocket --with unburnt fuel-- and the exhaust.

F_ext is the external force on the rocket, including unburnt fuel. Exhaust gasses aren't part of the system.

48 minutes ago, joigus said:

I think we agree on that. Am I right?

Yep.

[math]\displaystyle{ \Delta v = -v_e \ln\left(1 - \frac{r_m}{m_0}\Delta t\right) = v_e \ln\left( \frac{m_0}{m_0 - r_m\Delta t}\right) }[/math]

For constant [math]r_m \equiv -dm/dt[/math] in a vacuum.

Edited February 24, 2023 by Lorentz Jr

[joigus]

1 hour ago, Lorentz Jr said:

F_ext is the external force on the rocket, including unburnt fuel. Exhaust gasses aren't part of the system.

Ok.

In the notes that I'm taking to in order to understand the problem better, I'm distinguishing F_e,ext, F_r,ext, and F_ext=F_e,ext+ F_r,ext, which might seem a little bit overthinking it, but is not too bad an approach if one wants to make sure we're not missing anything.

[studiot]

2 hours ago, martillo said:

DR. Cowan agrees with me that the equation used in the motion of rockets is F = ma = mdv/dt for variable mass!

 

1 hour ago, Lorentz Jr said:

If the rocket ejects a mass dm at speed v_e in a time dt, the acceleration of the mass is a = v_e/dt. So the force acting on it is F = ma = dm(v_e/dt) = v_e(dm/dt).

 

3 hours ago, martillo said:

v_e = velocity of the expelled fuel in relation to the rocket assumed constant

 

3 hours ago, martillo said:

v = absolute velocity of the rocket

 

3 hours ago, martillo said:

u = absolute velocity of the expelled fuel

 

5 hours ago, martillo said:

I agree although I don't think there's a problem with the x notation.

Herein lies the hidden difficulty of analysis.

These velocities are not all in the same coordinate system.

So if considering them as dx/dt the x refers to different coordinate axes

So you cannot use the second derivative to get acceleration directly.

 

Further at least one of v, u , and ve must be negative.

 

The same coordinate sytem and sign conventions must be used for all derivational calculations.

 

[Lorentz Jr]

34 minutes ago, joigus said:

I'm distinguishing F_e,ext, F_r,ext, and F_ext=F_e,ext+ F_r,ext

Atmospheric forces on the exhaust gasses can get complicated.

https://www.brainkart.com/article/Back-pressure-and-optimal-expansion-at-the-end-of-the-Rocket-nozzle_5114/

Edited February 24, 2023 by Lorentz Jr

[joigus]

2 minutes ago, Lorentz Jr said:

Atmospheric forces on the exhaust gasses can get complicated.

Agreed. I was thinking more about a rocket orbiting a planet. So external forces would be conservative.

[martillo]

I would like to move to which would be my next step and I have some questions to the moderators of the forum.

As I said, if the actual Force Law would be F = ma generalized for variable mass, Relativity Theory would not handle that and an alternative would be needed. I worked hard on a possible new theory and developed a manuscript of 112 pages to present what would be just the start point of it. The description is not suitable to be presented in just an article of a journal with peer review. I have already sent the manuscript to the secretary of the most prestigious universities in the area of Physics for possible evaluation but I would like to also share my approach here in the forum. My intention is to post a thread to discuss just the beginning of the theory, a couple of short sections but also uploading the manuscript (pdf of 1Mb) for those who would like to have at this time a complete picture of where the things could go now.

A big problem the theory has is that it would be the worst possible theory. As I say in the manuscript:

"The proposed new theory is consistent with Classical Physics, Photon’s Physics, the Einstein E=mc² formula, Planck E=hυ formula and the De Broglie relation, although some corrections must be made.

It disagrees with Einstein’s Relativity Theory, the “Quantum Physics” based on the “Wave Mechanics Theory”, the Electromagnetic Wave Theory, the Rutherford-Bohr model of the atom and today’s subatomic “Standard Model” based on the Quarks Theory."

To give a brief description of the theory I have a quote:

"Classical Physics is coming back reloaded! With a new definition for the electric and magnetic fields and the right structure for the elementary particles it really works now."

The manuscript could be updated times to times and so I would like to also provide the link of a site of my own for any one be able to stay up to date.

I think I could do this here in the Speculations Forum without violating any rule of the forum but I would need the approbation of the moderators of the forum for if something is not as I think it is.

Any advice would be welcome.

 

Edited February 24, 2023 by martillo

[swansont]

6 minutes ago, martillo said:

Relativity Theory would not handle that and an alternative would be needed.

Since relativity is well-tested and agrees with nature, isn’t this an admission that your proposal is wrong?

[martillo]

2 minutes ago, swansont said:

Since relativity is well-tested and agrees with nature, isn’t this an admission that your proposal is wrong?

I think that the premise is questionable since, just for instance, RT requires dark matter to exist while is something that haven't been demonstrated to exist. So is possible that a new theory could surge. I would like to present my approach but if you think is not appropriated I will not do it.

[Genady]

11 minutes ago, martillo said:

RT requires dark matter to exist

No, it does not.

[martillo]

13 minutes ago, Genady said:

No, it does not.

Why is dark matter being looking for so hard then? As far as I know the galaxies' dynamics observations don't match with the relativistic predictions.

I'm asking for the possibility to present a new possible theory in the Speculations forum. If something goes wrong the thread could be moved to trash or just deleted. 

Edited February 24, 2023 by martillo

[Genady]

Just now, martillo said:

Why is dark matter being looking for so hard then?

For other reasons, but not because RT requires it to exist. The reasons can be easily found in many sources.

3 minutes ago, martillo said:

As far as I know the galaxies' dynamics observations don't match with the relativistic predictions.

They don't match with non-relativistic predictions as well.

9 minutes ago, martillo said:

I'm asking for the possibility to present a new possible theory in the Speculations forum.

Present the first paragraph.

[joigus]

44 minutes ago, martillo said:

I would like to move to which would be my next step and I have some questions to the moderators of the forum.

As I said, if the actual Force Law would be F = ma generalized for variable mass,

AAMOF, relativity makes your intent to supersede F=dp/dt with the 'bold new physics' F=ma even more implausible, as p has an extra dependence on v which makes the connection between F (in relativity the 4-force) one step more logically --and calculationally-- removed from acceleration. It has terms that do not involve second-order time derivatives. One term is proportional to velocity, and the other is proportional to acceleration.

So no.

 

[martillo]

10 minutes ago, Genady said:

Present the first paragraph.

To present a theory in just a paragraph? I don't understand...

[Genady]

Just now, martillo said:

To present a theory in just a paragraph? I don't understand...

No. Just present the first paragraph of your manuscript and ask members for their response.

[martillo]

1 minute ago, Genady said:

No. Just present the first paragraph of your manuscript and ask members for their response.

Not necessary here. I already gave a good initial description of it while asking moderators' approbation and I'm already getting your responses here.

[Genady]

Just now, martillo said:

Not necessary here. I already gave a good initial description of it while asking moderators' approbation and I'm already getting your responses here.

If you are satisfied with these responses, then there is no need to present the rest of your theory.

[martillo]

19 minutes ago, Genady said:

If you are satisfied with these responses, then there is no need to present the rest of your theory.

No I'm not. The question is if the moderators are satisfied with your responses.

 

Thinking better.

No one expresses to be good to analyze even just the beginning of a new possible theory. I think is not a good idea then. I think you would prefer to wait how Physics Science evolves to discuss the things.

I also think now that certain members will make the discussion too hard to follow. Waste of time.

Thanks for your comments. Is not my intention to just stay bothering other ones and I don't want to get bored too.

May be on other opportunity in a far future, who knows...

Edited February 24, 2023 by martillo

[Genady]

1 minute ago, martillo said:

The question is if the moderators are satisfied with your responses.

This is irrelevant.

[martillo]

16 minutes ago, Genady said:

This is irrelevant.

I edited my previous post. Just take a look. You will get satisfied now.

[joigus]

44 minutes ago, martillo said:

Why is dark matter being looking for so hard then? As far as I know the galaxies' dynamics observations don't match with the relativistic predictions.

Both DM and DE are consistent with GR. Sigh.