studiot

You specified construction costs. No it did not. It is even debatable whether central government ever had the right to cause them to be sold, since they did not pay for them in the first place. Where did I say any of these were new ? All houses need to be constructed at some point in their history. As a matter of interest some blocks of flats were sold in the north of England at £1 (I think the first was in Liverpool) for a whole blockbecause the local authority which had built them and actually owned them could not afford to upkeep them any more. I do think, however that your comment about the cost/worth of things is extremely poignant however. It just doesn't cover all bases. Also it is worth pointing out that the value of a property - as reflected in its current sale price - is hoped by the owners to be greater than their original purchase price, whether they just bought it or actually built it, but may end up being less. And that is called a free market. Yes I totally agree we have the means, but I would add not the will. +1

Well you would be wrong. We have already discussed the €1 houses in Italy. But Mrs Thatcher did exactly that with "The right to buy scheme" , which was only fairly recently ended in the UK. I don't know enough about other European countries to comment further in detail on them. And you still haven't confirmed whether you are including land costs and planning costs in your costings. It's the attitude and high anti-objectivity of the smart assed quips I am objecting to. Well I most certainly call this distortion of what I actually said cherry picking with the implication very clearly being that I want to claim it is " Their fault". Yes, that's a phenomenon - not the biggest problem, but one that's easy to cite by people who want to prove it's 'their choice; their fault'. Here's an article on the subject: I think it is such a shame that this thread has become simply a vehicle for those who want to hurl all embracing mud at anyone who want to objectively, and dare I say it - scientifically - unpick what is a very complex and in some ways sad situation. What is a homeless person ? I came across someone recently whos is technically homeless, (Of no fixed abode) but actually lives in a quarter of a million £ boat and runs a fleet of vehicles. I know that is an extreme case and plenty of truly homeless folk would jump at the chance to have a consistent roof over their heads. But I don't see how solutions can sensibly be proposed or enacted without starting with a proper analysis of who the homeless are, where they come from and why they are homeless. I know they are not one group to be lumped together but a hugely disparate lot. Nor am I holding UK experience out as any sort of paragons. I have already cited the grossly outdated and unfair vagrancy acts. But there are also many good goings on here and they should also be put forward. For example the Big Issue scheme has been said by many users to provide that sense of personal dignity and worth that allows or has allowed sellers to climb out of their difficulty by their own efforts. We had a seller in the South West who was well known and respected for a decade, who raised tens of thousands of £ for charity, yet chose to live in hostels for the homeless, moving round the SW. I know he finally quit and bought a modest property of his own. I repeat the situation is vary very complicated.

I think cherry picking from someone else's post should be beneath both of you. iNow I was responding to a point purely about resources, including cost. Peterkin how many UK homeless have you actually met to discuss their situation with ? And how familiar are you with UK vagrancy laws ? Both of you I have not in any way said nothing should be done. Quite the reverse if you bother to read my entire posts on this subject. There are, after all, precious few of them.

I didn't see an explanation from @Alex_Krycek to questions about the distinction between homeless people and homeless addicts. Nor a response to my own questions. Different problems require different solutions. Yes I agree in principle with CharonY but it may be worth noting that in the UK the vagrancy laws have not been repealed as our elected representatives have far more important squabbling and finger pointing to do than tidy up centuries old archaic legislation. You can in principle in the UK be prosecuted it you go out with no money in your pocket. These same laws require 'vagrants' to move on after a certain number of nights in any one place. These regulations are more rigidly enforced. So how is a homeless person supposed to return to normal, get a job and settle back into the community ? No one here has yet raised the issue of whether the homeless would actually take up social housing af any description. Again UK experience is that a significant % will actually prefer to remain homeless and 'free', because the cost to them of conforming in any type of available accomodation is too high. Finally I highlighted the difference between the US and Europe, in that the US land prices and palnning regulations play different roles in the 'cost of construction' - I didn't get a response from swansont on this.

The slip was too good to miss. Especially as the original proposal doesn't seem to have many (any?) backers for a variety of paractical, moral, social and legal reasons. Sadly nor has it had many admissions that there is a serious problem which I thought was clearly laid out in the OP, although I do not agree with the proposed solution. It is a pity that this thread has become rather entrenched hard line.

I didn't know it was random ?

Actually I wasn't suggesting that. Europe also suffers from this problem, though I have not seen any statistics to show the crime rate of homeless addicts is worse than many other sectors of the community. I am merely challenging swansont's assertion about construction costs. What do these include ? Interestingly there are quite a few do-it-up programmes on UK TV at the moment. One is just running a series on these €1 properties. They bought one (two actually) and have been doing them up.

In the US maybe. But I don't agree that it applies in more heavily populated Europe.

Interesting, but you didn'r answer my first question. Do you consider electric field line can touch, although they nmay not cross - Is there a difference ?

@Alex_Krycek If a heroin addict is someone who is addicted to heroin, What is a homeless addict ?

Looks green to me, thanks. Yes it is a good workbook so +1 to @joigus for remembering it.

Interesting, thanks. +1

No I had simply forgotten all about it. David Kay has done a good job as a specialist in line with the Schaum philosophy. There is many wrinkles in that book and of course quite a few exercises. But these are scattered throughout the book, there is no specific section or chapter explicitly on your desired topic, you will find bits and pieces all over the place.

Thank you for your replies. Sorry I spelled field incorrectly. 1. Yes I agree but not four your reason. All they have to do is for all four lines to touch (not cross) at P. 2. I'm sorry the diagram was not clear I was in a hurry The there is only one part of a field line shown the thicker continuous one. Yes It need to have started somewhere on a charge, but that is not relevent to my question. More fully my question was if you have a field line coming from somewhere is it possible to have what I called electrical effects on one side but not the other ? You could ask the same question of a charge. Can you have electric field lines in the space on side of a charge but not the other ?

Two questions for you. 1) Do you regard the diagram 1 as showing 2 field lines (BD and CA) or 4 fields lines AP.BP,CP DP) ? 2) Is the situation in diagram 2 possible with an electrical effect on one side of a filed line but not effect on the other side ?

OK to answer this I would like to start with a bit of history. The first person to use the word tensor was Hamilton, I think in 1841, though he actually meant something different when he introduced dyadics. The first person to use the word tensor in the modern sense was Voight in 1898 for studies in crystallography. Inbetween those times the pillars of maths were changed substantially by many workers, Ricci and Levi Cevita being at the forefront of developing tensor maths, although they did not call it that. In those days and into the early part of the 20th century it was called 'the absolute differential calculus'. Then Einstein introduced his summation convention (1916). Again at first there was no standard but by the 1930s the convention of contravarient upstairs only , covarient downstairs only and mixed both upstairs and downstairs had emerged. But with no regard to index order or spacing. This convention carried on to the end of the 20th century and beyond. Some still use it today. But by the end of the 20th century the advantage of proper index ordering had become apparent so today newer authors are beginning to leave spaces where some indexes are not used for some of the terms, as in this extract from Fleisch Vectors and Tensors - Cambridge. OOps pressed the go button before I was finished so I am carrying on editing. So the moral of this history is that you need to have your date calendar about your person when reading the literature. Your second question about exercises is much more difficult as you presumably want exercise with answers and perhaps worked examples ? The idea of drill exercises is a very good one though. Here is a series of videos, this one in the middle focused on your question. https://cosmolearning.org/video-lectures/few-tensor-notation-exercises/ As to books, Fleisch is already mentioned, Sokolnikoff - Tensor Analysis - Wiley - is a good and clear mid 20th cent text with exercies and some answers Zafar Ahsan - Tensors - PHI learning Delhi is modern (2018) again with some exercises and some answers. Bickley and Gibson - Via Vector to Tensor - EUP again offers some exercises and answers. Lawden - Introduction to Tensor Calculus, Relativity and Cosmology Wiley or Dover - offers exercises but no answers. Sadly your Synge and Schild offers neither exercises nor answers it is a treatise. Finally there is the issue of associated mathematics - The Hodge star, wedge and V products the geometric v the algebraic viewpoint and so on. I hope some of this helps.

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.

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.

To do what exactly ? I'm not being funny, this is a fundamental property of a rocket drive we are talking about. You have not directly answered my question which started - Is it possible ...? You have made nearly 10 pages of complaint about conventional analysis of the rocket drive. So exactly what value of ve would be required to maintain the condition V = a constant ? What is your mathematical relationship between ve and V that would make this happen ?

I didn't get the point... What exactly did you not understand ? If the rocket is moving forwards whilst burning and expelling exhaust is it possible to adjust the rocket drive so that its velocity, V, is constant ? If it is possible how would you achieve this ? Also please please read the rest of my post properly before making wrong guesses about coordinate systems.

Thank you for bringing that up, you have reminded me that I wanted to refer to your use of opn / closed systems. By a closed system you should actually mean an isolated system. Open systems allow mass and energy exchange with the outside world. Closed systems allow energy but not mass exchange with the outside world. Isolated systems do not allow either energy or mass exchange with the outside world. https://mechaengineerings.wordpress.com/tag/closed-system/ This is really as used in Physics and Thermodynamics, ansd does not take forces into account. Mechanics adds some additional concepts for Forces The free body diagram Force isolation Neither of which mean that no forces act on the body of interest.

There is much to be learned from the study of the rocket system including just how much is hidden in assumptions and definitions. For instance what is the coordinate system we are placing our equations in ? The centres of mass of both the rocket and the exhaust are not only moving, relative to a some fixed 'ground' observer, but also moving within the rocket and exhaust subsystems respectively. This makes the application of calculus more difficult, particularly if we stick to the traditional d/dx notation for space. Ther is not such issue with d/dt however. But where is the origin of x located ? Then the question of definitions By 'rocket' I mean the rocket plus any unburnt fuel, at all times. By exhaust or firing or burning I mean burnt and expelled fuel. There is no delay between burning and expulsion that needs to be considered. But a further interesting question that comes out of my earlier one. Can the rocket system drive forward under power at constant velocity? If so under what conditions ? I feel sure that overhasty appreciation (or not) of these fundamentals lies behind all the misunderstands shown in this thread.

No, pr is not constant, is just pr = p0 - pe . Both increasing in time with the energy of the combustion of the fuel. I'm glad you understant that. But what do you mean by 'both' ? I only asked about pr But since you mention it, what about po and pe ? It is necessary to make some assumption about pe ; it is usual to assume a constant burn so pe is then constant. But po is more tricky since po = psystem = procket at the start of the burn. But as the burn proceeds the rocket not only gains velocity, it looses mass.

Think carefully about what you just proposed. is pr constant ? If so why? If not why not ?

I said Edit correctly attribute quotation end edit So why oh why did you respond I even explained in some detail why I did not start with a stationary rocket. I also said that the analysis must nevertheless cope with the case of starting with a stationary rocket as with any other possibility. A further tip. Rocket engineers are less concerned with acceleration v momentum as they are with questions like How to maximise final velocity ? How to maximise acceleration ? How to maximise burn time ? These are not all compatible so they need a pretty flexible analysis for all that and more.