DrD

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Everything posted by DrD

  1. JAG: Only a government could keep a straight face and insist this made any sense. Amen, amen, amen!! DrD
  2. The word you want here is "follow," not fallow. To follow is to go with someone or thing, to go where they go. "Fallow" is an adjective that describes a field left for a season to rest without a crop.
  3. I cannot speak for everyone else, but I never discuss perpetual motion! JAG, I just wanted to put some realism into your Ponzi scheme. DrD
  4. To my mind, this quiz focused on mostly the wrong things. These are, for the most part, things that can easily be looked up in a design manual or a handbook. A much more interesting quiz would focus on the understanding and application of idea, the ability to correctly model systems, and the ability to understand and correctly interpret data. DrD
  5. I just completed this quiz. My Score 80/100 My Time 161 seconds  
  6. It is certainly possible that links 2 and 4 have the same length. However, it is not necessary for this to be true. If L2 = L4, that is a special case. Whatever sort of project can you make from this super simple mechanism? DrD
  7. Mechanics Corner A Journal of Applied Mechanics and Mathematics by DrD, #36 Base Acceleration Problem Introduction In a recent post (#35) I mentioned that I often participate in another forum called Physics Forums (PF). The problem that I want to discuss here is an elaboration on a problem that recently appeared at PF. I'm going to add a little bit of complexity to the problem (the problme as stated at PF was extremely simple) in order to make a particular point. The system of interest is shown in Figure 1, a body with a single wing attached to one side. You might consider this to be one side of an airplane, or perhaps a stirring paddle used to mix paint. The mass of the wing is M, and the center of mass for the wing is at the point marked CM, a known distance u from the main body. We are told that the main body has an acceleration a sub z in the z-direction, and that the whole system is immersed in a viscous liquid such that the drag force is proportional to the square of the velocity in the z-direction. Our concern is with the connection between the wing and the main body. We need to determine the shear and bending moment on that connection due to z-direction motion. BaseAccelerationProblem-36.pdf
  8. Suresh, I'm glad that you found it interesting. DrD
  9. Hey, JAG!! Let's have a little realism here. You give me a dollar, and I will give the next guy 1/2 dollar. He in turn gives the next person 1/4 dollar, etc. until you get nothing at all back. That's the way the world works. There is no free lunch!! DrD
  10. Sure would be nice if this was in English. I have no idea what it is about. DrD
  11. Quite a few interesting comments on reasons for design failure. Thank you JAG in particular. I would like to tell a story about a failure that I saw once long ago that was somewhat different. It involved a design that had been developed in a government laboratory, manufactured in very small quantities with tight controls for testing, and then put out to industry for mass production. In order to bring the unit price down, the government arbitrarily specified rather loose tolerances, far more loose than anything that had been allowed in the development phase. But then, the government added a performance specification, that the product must function according to design. The result was that the mass production companies were bidding, based on nothing more than the drawings and specifications. It was implicit in the drawing package that a product made according to the drawings was expected to meet the performance specifications, and this was the way the bids were developed. My company was unfortunate enough to win the bid. We found through bitter experience that it was entirely possible to build the product according to the drawings but still fail the performance test at the end. This resulted in massive amounts of rejected products. My job was to show mathematically that this was entirely possible, that the loosened tolerances allowed for performance failure. This was a failure driven by a desire to reduce costs to the purchaser. The result was the destruction of my employer; a company with over 100 years experience in the field was driven to bankruptcy. DrD
  12. Absolutely amazing!! So many people who freely admit that they do not know what they are talking about, but in the next breath, they say they are just sure that this idea will work!! Absolutely stunning!! DrD
  13. For a journal bearing surface, it is obvious that a circular surface is essential; a square section in a square hole would not turn. A square section in a large round hole would have contact at 4 points at most, very little load bearing area. If we are going to use a circular section at a bearing, then there is no point at all to using a square section between the bearings. A square section would be heavier, and only a slight bit more stiff. The square section would require more material for no real benefit. Square or other polygon shapes are used for special situations, but only where the is an evident advantage in doing so. DrD
  14. Your diagram is almost self-explanatory for any one who will think it through. PID stands for Proportional, Integral and Derivative, the three modes of feed back used in the system. Look at the diagram! each one operates on the error signal, and contributes to the eventual control action to minimize the error. DrD
  15. These two concepts are not related the way you evidently think that they are. An aircraft is in accelerated flight when it flies a horizontal circle. That does not cause any increase in lift at all. If you think in terms of straight line flight, then yes, flying faster will produce more lift. If you want to go faster but no higher, you adjust the trim tabs, modifying the airfoil to reduce the increased lift. DrD
  16. Which book he needs depends in part on the purpose for which he is learning this material. Does he want to be a boiler designer? Or does he want to drive a steam locomotive? Or perhaps he wants to be an operating engineer in a steam power plant? Each of these needs to know different aspects of boiler operation, but it is unlikely that any of them know it all. He should go to the library and start researching this topic. DrD
  17. I have no idea where this is going. DrD
  18. Mechanics Corner A Journal of Applied Mechanics and Mathematics by DrD, #20 © Machinery Dynamics Research, 2015 A Question of Stability Introduction The word stability in its several forms is widely used in nontechnical communication. A person whose life it highly consistent from day to day is said to have a stable life. When the political situation in a particular area appears to be unlikely to change, it is said to be stable. A person who is well balanced and unlikely to be easily provoked to anger is said to be a stable person. When the medical condition of a sick or injured person ceases to get worse, the person is said to be stabilized. A company on the verge of bankruptcy is said to be an unstable company. But what does the word stability mean in a technical context? Each of the foregoing examples hints at the technical meaning without really being explicit about it. A factor g = accel of gravity was missing in the potential energy expression. That is now corrected. Stability.pdf
  19. Mechanics Corner A Journal of Applied Mechanics and Mathematics by DrD, # 35 Machinery Dynamics Research, 2017 Good News --- Bad News Rolling Disk in a Rolling Ring Introduction Well, it looks like Mechanics Corner is back, at least in terms of an occasional post. It will probably be less frequent than previously, but there are just too many interesting things to talk about to remain entirely silent! The title for this post may leave you wondering what is the Good News, and what is the Bad News? Why is there both? Well, let me tell you about it ... GoodNews-BadNews-DiskInRing.pdf
  20. Thanks, JAG, for a most useful, real life, lesson. I've seen it myself, and I know the problem is very real. I think it is partly intellectual laziness, and partly a desire to be a part of the "in group,' that is, those "in the know." If I speak in acronyms, it looks like I know what I'm talking about, even if the actual content of my speech is foolishness. No one can be quite sure what I said, and if I speak with authority, who will challenge me? Great note!! DrD
  21. Yup, its a picture all right! Is there a question, comment, or anything else to go with it? DrD
  22. How on earth is heat applied to boil the water in this system? This certainly does not look like any boiler that I have seen. Perhaps this is how they are built in India? DrD
  23. Dear Marcus, It looks to me like you have five pulleys, not three, but that will not change things very much. Let R = D/2 be the large pulley radius (looks like this applies 3 times) and r = small idler pulley radius (two of these) Assume no belt slip or stretch, so the relations between belt and pulley speeds are v = R*Omega = r*omega where v = belt speed Omega = large pulley speed (rad/s) omega = small pulley speed (rad/s) Then the kinetic energy is T = (1/2)*[(I1 + I2 + I3)*(v/R)^2+(I4+I5)*(v/r)^2+M*v^2] where I1, I2, I3 are the large pulley Mass Moments of Inertia I4, I5 are the small pulley MMOI M = mass of belt and load on the belt Factoring out a v^2, the effective inertia is Meff = (I1+I2+I3)/R^2+(I4+I5)/r^2+M Be sure to understand that these are MASS values, not WEIGHTS.
  24. This relates to the post above just how? I fail to see the connection. That said, a kill switch is usually a device to shut off an engine. There may be different ways to do this, depending upon the engine type. For a spark ignited engine, it is often done by shorting the ignition, either at the magneto or coil, or sometimes directly at the spark plug. For a diesel engine, it must be done by shutting off the fuel supply.
  25. In some senses, this question is like asking the difference between dogs and cats. They are simply different animals. The operate differently. Yes, both require an electrical input, but even that is not the same for both. It is not possible to give a simple, one paragraph explanation of how each works. Most folks accept that they are different, and if you really want to know how one or the other works, you have to get in and dig through the analysis. DrD