studiot

So as you have been told several times f = ma is used but only in the appropriate place and is not the beginning of the dynamical analysis of a rocket. Only in a simple analysis.

At least we can agree on this Wry smile. What do you make of Physical Similarity and Dimensional Analysis Duncan or Dimension Theory Hurewicz and Wallman ?

But I didn't say that and it isn't true all the time. I said At the start the momentum (note not the rate of change of momentum) is exactly zero. That is not the same thing. dp/dt comes later in the analysis when we can say that it starts off at zero and is conserved so the total momentum of the whole system is always zero. That fact lets us form an equation between parts of the system that do not have zero momentum by themselves. So we can then say that the upward momentum - the downward momentum must equal zero Which is a very useful condition. This equality leads us to consider the third law forces between the parts by way of the second law. The logic of all this is inexorable if you follow it correctly. But it is easy to leave a step or two out when you are so familiar with it you become bored with it.

+1 But we do not deal in unsupported assumptions. We (like to think) we have sound reasons for these assumptions It does and it compresses. The compression of rocks at depth is enormous.

But Sears started, where I and swansont suggested you start - with conservation of mementum. Sears also derives the conditions where f = ma is applicable first, before he uses it also as I indicated in my last post. Scienceforums offers you the facility to view enlarged vrsions of the attachments, and I specifically uploaded large enough versions for this. At full enlargement it seems just fine to me. Note yet again I am saying ( as are others) you need both versions for the rocket. You still haven't acknowledged what I said about simultaneous equations. As an electrical engineer try this. V = IR and P = IV Are two different equations about different physical quantities. Because they have some common simultaneous variables you can substitute to find an equation for say power in terms of either voltage or current and resistance. P = V2/r = I2R These can be extended further to find energy and other properties. The rocket is an equivalent situation in mechanics.

This is not maths or physics. The constants can't be zero. E = e2/r is not dimensionally consistent. It may be of interest to you as a new member to discuss ways of presenting mathematics to best suit yourself. Scienceforums offers particularly wide ranging possibilities - better than any other I know of. Although the Tex / markup is not as good as some. There is the sandbox https://www.scienceforums.net/forum/99-the-sandbox/ for testing and also there are several tutorials specific to this site . Don't hesisitate to ask how to do some of this when you need it.

always ?? I'm fed up with members that don't bother to read what's offered, just argue apparantly for the sake of it. Here is a simple treatment which explains why using f = ma makes life more difficult. Because m is not necessarily constant. Professor Sears had a particularly clear way of putting matters.

I am neither fond nor not fond of you. However you specifically offered So I asked to take you up on your offer. So far you have expended several posts avoiding doing so.

Which is exactly why I asked @martillo to start at the beginning. When the rocket is standing ready to launch it has exactly zero momentum. So we can write a conservation of momentum equation right there. This is usually taken for granted and also what I am referring to when I asked about what are they hiding ? Of course but what I am saying is that in order to solve or analyse this problem we start with a complete rocket and fuel as 'the system' and then add further equations coming from the application of mechanics to parts of that system. I further point out that all of the equations are simultaneous so that one can substitute from one into another Again a tried and tested technique. I am also say that there is nothing revolutionary in this, indeed the technique is well used in many simpler systems.

So no mathematics then ? Just like your hand wavy references.

I'm willing to be educated, but the rules here are quite clear. I should not need to go offsite to read the material fundamental to the issue. So fire away with your mathematics please.

I asked you what you know. You haven't quoted a single number. So tell me just how you know what v is ? And you are telling me that i should use f = ma instead of some other equation. What exactly will this give me and where will I get the numbers to put into this equation ? And where exactly does your f = ma fit into your claim that you know v and -ve ? Why do you think I specified a rocket ready to launch ? If you don't want to calculate anything why are we going through all this rigmarole ?

Start at the beginning and try to see where all these websites you refer to have hidden the analysis. You have a complete rocket standing there on the ground ready for launch. What do you know and what do you want to calculate ? In other words what are you variables ?

So what's your response to what I actually said please ? The calculus of variations is totally unnecessary for this. It barely reaches into ordinary differential equations and can even be done without them.

I'm sorry but this glib oversimplification of several important points in Physics gives entirely the wrong impression. You have made several outrageous statements, with not a shred of support. Do you understand, for instance the difference between the angular momentum and spin quantum numbers ?

Er...? Variational calculus ? That's second year university stuff. I said simultaneous equations. In the UK you start learning about these around 12 or 13, actually before you learn any mechanics. But it isn't one simple equation. You can't calculate anything with one simple equation as you have more an one variable involved. How many equations do you require to solve for 3 variables ?

I am trying to help you, not criticise so try not to react in haste. I didn't say you didn't study Mechanics. Quite the opposite in fact. I said you missed something important, when you did study Mechanics. No, I agree a rocket is not static, but it can be quasi static otherwise known as steady state. But that was not the point. Static systems are in general simpler than dynamic ones, indeed one way to solve synamic ones is to reduce them to static or quasi staic ones. Please read carefully my point about simultaneous equations before you reply. Even the simpler static systems may need simultaneous equations for solution.

The problem with this is that it is in opposition to one expression of a fundamental point of relativity. Notably that the is no preferred point of view. Discussing only from the POV of the own frame of a subject, is a preferred point of view.

Then consider the possibility that you have a fundamental misunderstanding of Mechanics, that goes back nearly to your first studies of the subject. Your basic misunderstanding is this: A rocket is a mechanical system. There is no 'the equation' for this system ie one single solitary equation. A very common situation in Mechanics is that you need to consider more than one equation to solve a system. You need a set of simultaneous equations. This is the case with the rocket system (rocket for short). All that is happening is that you have an equation for the whole system and another one for part of the system. When studying Mechanics you learn this very early on when calculating loads on a static system and reactions via forces and moments.

OOps! I inadvertently gave g the wrong units. It should of course be, m/s2 - not m/s Many thanks to @Sensei for pointing this out.

You are correct. It is a coincidence. But Genady is also correct that the units don't match. The numbers match in the case of Earth, but not in the case of say, Jupiter. Let us look at some figures. For Earth Dearth = 1.47 x 1011 metres - distance to sun For Jupiter Djupiter = 7.41 x 1011 metres - distance to sun For Earth gearth = 9.81 m/s For Jupiter gjupiter = 24.79 m/s For both G = 6.7 x 10-11 N-metre2/ kg2 So for Earth gearth / G = 9.41/ 6.7 x 10-11 = 1.47 x 1011 kg/metre2 which is the same number of kg/metre2 as the Earth is metres away from the sin. BUT For Jupiter gjupiter / G = 24.79 / 6.7 x 10-11 = 3.7 x 1011 kg/metre2 which is quite different numerically than 7.41 x 1011

I'm not interested to your (other) argument. But I would ask if this is not because of the sentiments put so fully in the words of the Hymn "Immortal, Invisible, God only wise" Definitely implying that God is central to that religion.

Very clever, John, +1 I would like to point out that not only is Nature v Nurture not balck and white, but the issue of belief or disbelief is also non binary. I didn't catch on to Joh's argument until he potently either. For myself as a youg child I just accepted the idea more or less as I was taught. About 12 I worked out for myself the logical inconsistency of an omnipotent being and for a few years I followed the debate. In doing so I never came across any conclusive pros, but the cons began to build up. Then I decided on my third way. I just don't care. I don't need a man in a long black or white garment and a pointy hat to tell me what's what. I can work that out for myself. So I asked myself "would it make any difference to how I go about thing ?". Since my self answer was no I came to the conclusion that the isue was irrelevant to me.

Thanks for a more detailed explanation. +1

What do you mean gas, minerals ? You can put solids or liquids into an 'ignition tube' and heat in a bunsen burner flame. Or you can put some on a carbon block or in a crucible and heat with a blowpipe. As others have said to heat gas you normally pass an electric discharge through to make it glow. Using an ignition tube is a chemical test or tests. Does it give off a gas?, does it burn? does it decompose totally? If so what colour changes are there and so on. Glow discharge methods require some sort of spectroscopic analysis of the light of the glow (which may only be visual).