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  • V. T. Toth added an answer in Gravitational Field:
    Do gravitons identity change?

    The Nobel Prize in Physics 2015 recognises Takaaki Kajita in Japan and Arthur B. McDonald in Canada, for their key contributions to the experiments which demonstrated that neutrinos change identities.


    Also, new experimental data, which show that neutrinos have mass, are forcing theorists to revise the Standard Model of particle physics.


    The Standard Model of elementary particles and interactions is one of the best tested theories in physics. It has been found to be in remarkable agreement with experiment, and its validity at the quantum level has been successfully probed in the electroweak sector. In spite of its experimental successes, though, the Standard Model suffers from a number of limitations, and is likely to be an incomplete theory. It contains many arbitrary parameters; it does not include gravity, the fourth elementary interaction; it does not provide an explanation for the hierarchy between the scale of electroweak interactions and the Planck scale, characteristic of gravitational interactions.


    In standard model, the graviton mediates the force of gravitation in the framework of quantum field theory. If it exists, the graviton is expected to be massless (because the gravitational force appears to have unlimited range) and must be a spin-2 boson. The spin follows from the fact that the source of gravitation is the stress–energy tensor, a second-rank tensor.

    This old view on graviton is not able to solve the gravity problem in standard model, so seems we need a new definition of graviton.

    To define graviton, let’s consider to a photon that is falling in the gravitational field, and revert back to the behavior of a photon in the gravitational field. When a photon is falling in the gravitational field, it goes from a low layer to a higher layer density of gravitons. We should assume that the graviton is not a solid sphere without any considerable effect. Graviton carries gravity force, so it is absorbable by other gravitons; in general; gravitons absorb each other and combine.

    When some gravitons are around a photon (or other particles) they convert to color charges and enter into the structure of photon. Color charges around particles/objects interact with each other. There exists so much graviton around any particle. There are many layers of gravitons around a photon. The first layer is closed with photon, so that its gravitons interact with charge and magnetic fields in the structure of photon. The second layer interacts with the first layer and third layer and so on. Therefore; when a stone is falling in the gravitational field of the Earth, two layers of gravitons are applied to it, first layer up (at high h) and second down (at high h-dh). In down layer, the density of graviton is greater than up, so the stone falls and its kinetic energy increases.

    This new view on graviton shows, identities of graviton changes, in fact it has mass with changeable spin.



    • Source
      [Show abstract] [Hide abstract]
      ABSTRACT: To date, there is no way to explain the process that describes how particles as photon absorb gravitons. According to the results of our years-long research we can definitely say that the best way for explain this process is using color charge concept from photon properties. This model might be called "Creative Particles of Higgs" or CPH theory. Gravitons behave like charge particles and in interaction between gravity and photons, gravitons convert to negative and positive color charges and also magnetic color. These color charges and magnetic color form the electromagnetic energy.

    + 1 more attachment

    V. T. Toth

    Dear Hossein: Energy is energy. That's the whole point about gravity. Once you have a stress-energy tensor, it doesn't matter what its origin was. There is only one kind of energy. Whether its origin is the electromagnetic field, the weak interaction the strong interaction, rest mass, motion (kinetic energy)... doesn't matter. If it mattered, gravity would violate the equivalence principle, which is known to be true experimentally. As to the energy of an infalling photon, it depends on the observer. A freefalling observer would see a lot less change in the photon's energy than, say, an accelerating observer maintaining a constant position relative to the black hole. So there is no intrinsic change to the photon; everything is relative to observers. And no, the photon's mass doesn't increase; it doesn't have any rest mass to begin with, only relativistic kinetic energy and momentum. Furthermore, a photon does not "generate electromagnetic fields". Rather, photons are the excitation quanta of the electromagnetic field.

  • Sudhakar Singh added an answer in Hadoop:
    Running MapReduce application on hadoop using eclipse IDE takes less time than running as JAR file on a Terminal, why is this the case?

    I am using Hadoop-2.6.0 and Eclipse Kepler/Juno. I have executed my MapReduce Job in both way and I found that running jar file on Terminal takes more time than direct execution for Eclipse IDE. I could not understand the reason behind it. Please help, I could not decide which way I should follow ?

    Sudhakar Singh

    @Adnan I am agree with you.

    But there are two cases:

    First-- Initially I was running code in Eclipse using Run as. I had also run the same code via terminal after exporting it as jar file. In both cases I was using one node and input file contains some lines.

    Second-- 10 MB data is not small if our algorithm is very computation intensive (e.g. I am running Apriori algorithm). I have checked on one node cluster and by adding more nodes. Execution time greatly reduces after adding more nodes up to a limit (as per Amdahl's law).

    For such cpu intensive task I want to use Spark which runs computation in memory. Can you help in Spark also.

  • Emmanuel Baranger added an answer in Buckling Analysis:
    How do I include geometric imperfection in a model?

    hello every one 

    i m working on delamination buckling analysis in abaqus 6.14...

    now i want to add geometric imperfection in my model while i m using general static step for buckling analysis ,,,

    can any one help me how i can  add geometric imperfection in my model

    i don't perform eigen value analysis because i dont need different modes and their buckling load..

    i just apply axial displacemnt on two  parts joined by cohesive element with delamination between them....but now i want to add imperfection to it too..i have studied in the abaqus documentation how to add imperfection but that require eigen value analysis,,which i dont want

    i just need to know if there is any other way to add imperfection to my model since i m using general static step..

    thank u

    best regards

    Emmanuel Baranger

    the simplest way to do is to generate the inp file and then to modify the table of the points coordinates using a script (python, matlab...).


  • Benjamin Grena added an answer in Fluorescent Dye:
    Can you suggest a fluorescent dye soluble in polar aprotic solvents?


    I would like to find a fluorescent dye with absorption in near to mid UV range (≈200-400nm) and maximum emission in the visible, which could be dissolved in organic solvents such as propylene carbonate or other carbonate esters.

    I would also be interested in non-fluorescent dyes with similar solubility.

    Thank you in advance for your suggestions!

    Benjamin Grena

    Thanks for your replies! @Friedrich and @Peter, great resources, thanks! It sounds like coumarins are an interesting family.

  • Elsayed Ahmed Elnashar added an answer in Legal Research:
    How law research is evaluated in your country - by national research agencies and/or universities?

    In Slovenia metrics is used (impact factor of journals, citation etc.). However, when compared to science, law is not doing so well and is in inferior position. I will need concrete data for arguing that law and science cannot be measured in the same way. E.g. Harvard Law Review only publishes 1 or 2 articles per issues, that's around 10 articles per year. Physical Chemistry Chemical Physics, however, publish 80 articles a week, 48 issues a year. That's a substantive difference. One journal publishes the same number of journals as all SSCI journals in the field of law together. I think no legal scholar can publish 80 SSCI articles a year, yet chemists do.

    Elsayed Ahmed Elnashar

    For position research is evaluated in your country … by specialization and professional committees of the supreme of the universities council  

  • Roger M. Pearlman CTA asked a question in Cosmology:
    Distant star light, most distant stars in the various directions to help determine if a disk or a round visible universe?

    how many directions do we detect stars from 12+/- billion light year distance?

    Based on our being the approximate center of the visible universe. (Per The Pearlman Spiral we are the approximate center of the entire physical universe)

    If round:
    If the visible universe is round and we are the center of the visible universe
    and we see light from one star / or cluster 12 billion light year distance there should be visible star light from that approx distance in each direction.

    If a disk:
    If the universe is a disk the approximate distance we see from one direction we should see from other directions along the plain, but the up and down The Pearlman Spiral cosmological redshift hypothesis and cosmology model would predict not as distant.

    Also please advise if you know where the data/measurements is/are available,

    TY, rm

  • Stephen Crothers added an answer in Gravitation:
    In GR, can we always choose the local speed of light to be everywhere smaller that the coordinate speed of light? Can this be used in a theory?

    It seems that many, if not all, solutions of Einstein's equations, such as black holes and grav. waves, can be given coordinates x\mu in such a way that the local speed of light is always slower than the coordinate speed of light. Think of gravitational lensing: the index of refraction of a gravitational potential always seems to be >1, in practical examples, so a gravitational potential slows light down, and never speeds it up (if coordinates are chosen carefully). This wouldn't be true for a negative-mass Schwarzschild solution, but that seems to be outlawed in nature.

     Now this was only a conjecture, I have not attempted to prove it. How would a rigorous mathematical theorem be formulated? And did anybody - and here I mean a wise person, not the average blogger - ever try to do something interesting with this observation? Like constructing a “hidden medium” for curved space-time?

    Stephen Crothers


  • Cynthia George added an answer in Academic Writing:
    Citing Wikipedia?
    While Wikipedia is more and more popular with students, professors discourage them from using it and bar them from citing it. What are reasons (to cite or) not to cite Wikipedia?
    Jul 31, 2012
    Cynthia George

    Here's some scholarly work about Wikipedia from the late, great Aaron Swartz: http://www.aaronsw.com/weblog/whowriteswikipedia

  • Bruce Weaver added an answer in Normality Test:
    How can I run a normality test in SPSS with blanks?

    Hi there.. I am working with SPSS on a wide correlation study. I figured out that while I ask for a normality test, SPSS analyzes the values in the string of each variable, when every single variable for that entry, for that line, has a value. If any variable is missing a measurement in a specific line, all the line is omitted.Thus, many values from the other variables are omitted as well from the study and reported as missing. Hence, the normality study is only based on those lines, that are completely full. I would need to run once a normality test, but keeping the variables separated, analyzing all their entries.

    Thank you very much for any help.

    Bruce Weaver

    As Andrew has noted, the default option for missing values (for the EXAMINE procedure) is LISTWISE deletion.  If you change it to PAIRWISE, you should get the results you expect.  If you paste the syntax, change /MISSING=LISTWISE to /MISSING=PAIRWISE.  In using the GUI, click on OPTIONS, then select "Exclude cases pairwise".  (Pairwise doesn't really make sense here, as it's really one variable at a time.  But that's the keyword in SPSS, presumably because that's what it is for other procedures, like CORRELATIONS.) 

    Here's an example of the command syntax:

    EXAMINE VARIABLES=v1 v2 v3 v4
      /PLOT NONE
       /CINTERVAL 95


  • Patrik Johansson added an answer in Amyloid:
    What are the emission peaks like in ThT assay for amyloid beta aggregation?

    I'm using ThT assay to observe aggregation states of amyloid protein 25-35, there seemed to be rather large fluctuations in emission intensities for replicate tests and many noise peaks were recorded.

    I incubated A-beta (50µM) in PBS and diluted A-beta with glycine buffer (pH8.5) to a final concentration of ~10µM for ThT assay in a  black 96-well plate. Excitation and emission wavelengths were 450 and 485nm, respectively.

    Here are 2 problems:

    1. I suppose glycine buffer should help stabilize A-beta so it remains in the same aggregation state during ThT assay, but I got quite different emission intensities at 485nm for replicate samples. I understand its better to look at relative values within one test, but what might be the common fluctuations in absolute values for the same samples in different wells?

    2. Also I scanned excitation wavelengths from 250nm to 465nm and recorded emission intensity at 485nm, there was a emission peak at ~280nm, I wonder if anyone has seem this peak in their ThT assays, and where this peak is from. 

    Thanks in advance! Lin

    Patrik Johansson

     Hi Lin,

    Have you also done excitation spectra for your fibrils without the ThT dye? It has been shown that amyloid fibrils can exhibit intrinsic fluorescence, even without dyes or ring-structures in the protein. This fluorescence is thought to be related to the rigid peptide backbone-structure and I think the mechanism for it is still somewhat debated. My guess is that the resonance structure of the peptide bond, in combination with long-range non-disturbed H-bonding in the backbone of fibers give rise to electron delocalization responsible for this observed fluorescence. If you still have not been able to sort out the origin of the excitation peak that you observe at 280nm, this may be something to consider. 

    Here is a link to a chapter about intrinsic fluorescence in amyloid structures: 


  • Ciprian Popa added an answer in Sediments:
    Does anyone have experience with QEMSCAN analysis of uranium ore samples with a lot of organic matter?

    The investigated samples come from the sandstone-type uranium deposit. Low grade, fine-grained mineralization has a complex character. Ore horizons A (freshwater sediments) and B (washout sediments) contain a lot of organic matter (up to 10 wt%), which makes the problem in focusing and mapping of uranium and uranium-bearing phases. Thanks for any help!

    Ciprian Popa

    Hi Petr,

    do you have a backscattered electron detector? Try to use it. The differences should be instant due to Z of the elements involved.

  • Tea Romih added an answer in UV-Visible Spectroscopy:
    Is there any method to determine concentrations of Cu, Pb and Cd in solution using UV-Visible Spectrometer?

    Is there any method to determine concentrations of Cu2+, Pb2+ and Cd2+ in solution using UV-Visible Spectrometer?

    Tea Romih

    The compendium of Standard Methods for the Examination of Water and Wastewater (http://www.standardmethods.org/) features a section on colorimetric methods (e.g. based on forming UV-VIS-detectable colored metal complexes) for the detection of a number of dissolved cations and anions, including the ones you are searching for. It includes validated protocols. Unfortunately it is not open-access, but since it is a fundamental literature for institutions involved in water research, I would suggest you to try to find it at your university library or some specialized technical library.

  • Stanley Turner Paxton added an answer in Reservoir Characterization:
    Does anyone have a good research paper related to well logging and formation evaluation?


    I badly need a research paper related to well logging and formation evaluation and reservoir characterization. If you have please, send to me

    Advance thanks 

    Stanley Turner Paxton

    Shamim Ahammod: So much depends on the nature of your need: 1) for teaching? (what level of student, a big consideration) or for your own research/interpretation needs?, and 2) type of rocks...are you concerned with conventional reservoirs, or unconventional reservoirs?  I have a bundle of articles that were referred to me by Schlumberger personnel...however, this bundle is for unconventional reservoirs only. In terms of a text book, I find "Well Logging for Earth Scientists", 2nd Edition, by Darwin Ellis and Julian Singer, published by Springer, 2006, is an excellent reference and source book.

  • Stephen Cheung added an answer in Brief Psychotherapy:
    Is anyone undertaking a systematic review of solution focused brief therapy?

    We are currently in discussion with a local health service, social work department, around the potential use of a single session therapy approach for engaging with family members for consumers in mental health. 

    A brief search for contemporary literature has provided a limited response is anyone currently undertaking work in this area?

    Stephen Cheung

    Dear Mark,

    You can also try the the following websites and book:

    Franklin, C., Trepper, T. S., Gingerich, W. J., & McCullom, E. E. (Eds.). (2012). Solution-focused brief therapy: A handbook of evidence-based practice. New York: Oxford University Press.

    + 2 more attachments

  • Javier Amores asked a question in Plating:
    Can someone help me interpret a resutl in a 3M Staph Express Petrifilm?

    I analized two chocolate samples for S. aureus using 3M Staph Express. After 24 hours my plates were filled with blue-green and black colonies, so I used the Express Disk. After 1-3 hours my plates had these large yellowish zones. For what I've read in the 3M interpretation guide, the zones have to be pink to be considered S. aureus, so I don't know what to make out of said result. I have linked a picture of the plates, hope it's clear enough.

    Thanks in advance.

  • M J Mahesh added an answer in ANSYS Workbench:
    Is it possible to plot bending moment for a component in ansys workbench?


    Does anybody know how to plot bending moment for a component in ansys workbench? Does it need any settings change while firing run??

    M J Mahesh


    Which element are u using for modelling ur bracket?? 

  • James Renwick Beattie added an answer in Artificial Datasets:
    What is the implication of sampling a complete data set instead of only training set?

    I want to ask about the consequences or things to keep in mind when sampling the complete data set to overcome class-imbalance issue. I have come across numerous examples where only the training set was sampled using an appropriate technique/s but the test was left as it is. I have balanced the whole data set prior to classification and want to ask if this creates any issue that I should be aware of? Plus when I sampled the whole data set prior to classification, it gave better results in comparison to when I only applied sampling on the training set.

    An elaboration on the above matter will be much appreciated along with any references in favor or against such approach


    James Renwick Beattie

    If the test set is sampled along with the training set (common in many exploratory studies) then the test set is biased by the same selection methods used to sample the training set. This means it is not fully independent and only validates your model for use in the same subsample of the population, you cannot extend the claims for your method. Training sets are subsampled for balance so that detection of rare groups are not biased due to outliers within those groups. You need to esitmate the population characteristics of each group with an equivalent amount of accuracy.The model is equally exposed to case and control (treatment/no treatment, group a/group b) so the model is unbiased towards either.

    As Goran suggests it is preferable that the test set is more representative of the general population, not the subset sampled for training. A true independent validation set will not impose the same sampling restrictions and will allow you to evaluate the performance of your test in a realistic real world setting.

  • Miltiadis Lytras asked a question in FP7:
    What is happening with the EU funded projects on E-Learning Knowledge Management. Why is there so poor exploitation of final deliverables?

    Why we can not exploit the technologies and prototypes delivered in BIG projects in FP7? Will be the same story for HORIZON 2020 program. It is tragic!!!

    And what about the "reviewers" in EU committees/ evaluators of proposals?

    Why there is so tendency same groups to promote same consortiums?

  • Tomáš Hluska added an answer in Protein Estimation:
    Why is my ammonium sulphate protein precipitation not working properly?

    I started with partial purification of amylase using ammonium precipitation technique.While going for step by step saturation the recovery of protein was less than 10% of the crude amylase.So i started with 20ml crude enzyme direct 100% saturation.The precipitates were dissolved in 500ul of phosphate buffer and poured into dialysis bag,dialysis bag was dipped in phosphate buffer.After 24 hours there were  crystals in the beaker contain buffer. I changed the buffer and kept for another 24 hours yet  another day i see the crystals in the beaker This process continued for a week. After a week also some crystals were seen in the beaker but i startet with the protein estimation. I was expecting atleast 80% recovery but again i got only 12% recovery.....the quantity of protein had also increased from 500ul to 1ml

    Tomáš Hluska

    What was the source of your protein? Did you check the supernatant after salting out for remaining activity? It may be that your protein stayed in the solution. That happened to me as well.

  • Elahe Heydari asked a question in Fourier Transform:
    If we want to Compute the 2D Fourier transform of the image, how to Make sure the zero frequency is at the center of the Fourier transform plane?

    in image Processing , I compute the 2D Fourier transform of the image , Is the zero frequency  at the center of the Fourier transform ? how to make sure?

  • Avinash Oojorah added an answer in Social Media in Education:
    Which types of Social Media Social Networks do you use for your teaching in a university/college classroom?

    If you are interested we are also running a survey. If you want to participate drop me a message

    Avinash Oojorah

    My colleague and I currently use Facebook to support teachers in their usage of technology in classrooms. At elementary school level we have a following of 1600 teachers. The page is a nice tool for support, interaction and sharing. For the secondary school page, we have around 700 users. However the dynamics are markedly different. There are more lurkers than participant in this one. I would be interested in your survey.

  • Mahdieh Nazari asked a question in Griess Assay:
    Nitric oxide measurement in spinal cord tissue?

    Hi everyone, I try to measure total nitric oxide metabolites (NO2 and NO3) in spinal cord tissue using colorimetric griess assay. To do this, I use griess reagent and vanadium chloride. I precipitate proteins with alkaline solution (NaOH) and ZnSo4. The problem is while the standards have clear pink colors and the standard curve is linear, the color of samples is cloudy violet. So the absorbance is falsely higher than the color observed. I mean the color may look like 2 or 3 uM standards but from the standard curve the concentration calculated 15 or 20 uM. Does it related to proteins which may not precipitated perfectly or other components in the sample which may react. Any suggestions? Should I use sample blank as well? thanks

  • William Von Witt added an answer in Brain Imaging:
    What programs are you using for brain segmentation?

    I am needing to create a brain mask from T1 and T2 images to export to MATLAB.

    I use MRTrix3 and FSL for tractography, I have FreeSurfer but don't know how to use it properly yet, also 3DSlicer, but I have minimal experience in doing anything except looking at images in those two.

    William Von Witt

    Ah, thanks Avery, I did try BET but it didn't work and I assumed that was because I was using a T1 scan. The other thing is that I am using mouse imaging data, that shouldn't make a difference though, should it? 

  • Alison KM Hadwin added an answer in Genomic DNA Isolation:
    How do I remove small fragments of RNA from genomic DNA extraction?

    I did a genomic DNA extraction of fungal tissue using CTAB extraction. Samples were treated with RNAse during the the final stages of the protocol but the gel shot below was what I got (genomic DNA + bands below 100 bp). I am assuming they are small fragments of RNA though I am not ruling out DNA.

    WIth the assumption it was RNA I did an RNAse treatment with 1 ul of 100 mg/ml RNAse A in 100 ul of sample for 2 hours at 37°C and ran a gel. 

    But as seen in the second gel shot, the small bands still remained. I am assuming it looks more abundant because it's a smaller gel thus it is a close up shot.

    So the main question is, is it really RNA and how do I get rid of it? I need to have pure high concentration, high molecular weight DNA.


    + 1 more attachment

    Alison KM Hadwin

    If you need to have only the high molecular weight DNA you could try doing a gel purification: cut out only the area at the size you want and extract the DNA (I usually use a kit for this, but there are various protocols to do it from scratch if you don't have the right kit). This will get rid of the fragments regardless of whether they are DNA or RNA.

    Did you grind the mycelia under liquid nitrogen before using CTAB?  As Aasim Majeed mentioned the fragments could be sheared DNA;  reducing the amount of grinding might help reduce the amount of shearing.

  • Nathaniel F. Shoobs asked a question in Descriptive:
    If a species is described from a whole specimen that is split into two or more museum lots, is the holotype both of the lots, or just one?

    Here's a question for the ICZN buffs out there:

    If a species is described from a whole specimen that is split into separate museum lots, is the holotype both of the lots, or just one?

    For example: I describe a novel species of snail from a specimen, and the description includes a description of the shell, the soft parts, and the radular anatomy. When depositing the specimen, I separate the shell and it gets a dry catalogue number. I separate the radula as well and it gets a separate catalogue number. The soft tissues get their own number and go into the wet collection. Are all of these lots still the holotype? Are they syntypes? They are all parts of the same organism but are obviously separate museum objects if they are prepared this way. 



  • Kritika Subramanian added an answer in Medical & Health Profession Education:
    Mental Health of recent, young CALD refugees and asylum seekers in Australia.
    Most refugees and asylum seekers report to be generally physically and mentally healthy compared to local Australians (health advantage). However, after living in Australia for several years some of these refugees and asylum seekers self-report less healthy physical and mental health outcomes whereas others self-report continued better mental and physical health outcomes.
    1.What social determinants of health factors would be influencing these differences in health outcomes of these CALD clients?
    Kritika Subramanian

    Hi, I was wondering if you found any answers to the question you posed? I'm working on a paper addressing the use of healthcare by refugees and the cultural impact on their health beliefs. I realized from my research that there is a wide discrepancy in health stats as you mentioned and I would love to know your insight. 

  • Razak Mohammed Gyasi added an answer in Questionnaire Design:
    How do we confirm the validity of a questionnaire?

    I have to present a seminar on Types of Validity -

    In that perspective 

    1. How do you pretest and pre-design a questionnaire? Is it same as Pilot study?

    2. In context to medical parlance - how would we explain criterion , content , face , concurrent and translational validity and other types?

    Appreciate your cooperation 

    Kindly please attach any relevant pdf and please if possible give suitable examples

    Thanking you

    Razak Mohammed Gyasi

    Dear Ashish,

    1. The validity of a research instrument such as questionnaire is generally determined by conducting a reconnaissance or scouting survey on a small sample with similar baseline characteristics (but not the same study sample). This will provide you the opportunity to make adjustments and modifications to the questionnaire items so that the items will appropriately measure issues they are meant to measure.

    2. These list are the various forms of validity ranging from the lowest to the highest level. First understand validity as the extent and or the ability of a scale/measure to measure what it is intended to measure.

    Face validity represents the simplest or the least scientific form of validity and it is based on the normative judgement of the researcher to see whether the scale measures what it claims to measure.

    Content validity is more scientific because the researcher goes beyond mere value judgements and makes sure all measurement devices provide adequate coverage of the investigative questions.

    Criterion validity is concerned with how the measure or the scale agrees or correlates with other standard measures of the same construct. For example how does the teacher's assignment of a student's intelligence agrees with the student's IQ test results?

    Concurrent validity is a form of criterion validity which demonstrates when scores obtained from a new measure are directly related to scores from a more established measure of the same variables. See also predictive validity.

    Wishing you the very best in your research endeavours.

  • Kristian Lyng asked a question in Pain:
    In which complete knee-flexion position(eg. Squat) will the pressure on the meniscus be the highest? In a narrow or wider stance?

    Hello everybody!

    How can it be possible to feel no pain in a full-squat, but feel pain in the knee when deadlifting with a wider stance(Sumo-deadlift) ?  Is the pressure on the meniscus greater when greater abduction in hip occur? 

  • Bachir Achour added an answer in Fluid Flow:
    What do they mean by baroclinic vorticity?

    Baroclinic vorticity is related to fluid flow past cylinders, a file in support of it is attached.

    Bachir Achour

    Dear Reyaz,

    In a barotropic fluid temperature does not vary on a pressure surface and therefore through thermal wind the geostrophic wind does not vary with height. In contrast a fluid is Baroclinic if the term ∇ρ × ∇p different from 0, for example if temperature varies on a pressure surface then ρ = ρ(p, T) and the fluid is baroclinic. In a baroclinic fluid the geostrophic wind will vary with height and there will be baroclinic generation of vorticity.

    "The geostrophic wind is the theoretical wind that would result from an exact balance between the Coriolis effect and the pressure gradient force. This condition is called geostrophic balance. The geostrophic wind is directed parallel to isobars (lines of constant pressure at a given height). This balance seldom holds exactly in nature. The true wind almost always differs from the geostrophic wind due to other forces such as friction from the ground. Thus, the actual wind would equal the geostrophic wind only if there were no friction and the isobars were perfectly straight. Despite this, much of the atmosphere outside the tropics is close to geostrophic flow much of the time and it is a valuable first approximation. Geostrophic flow in air or water is a zero-frequency inertial wave".

    With my best regards

    Prof. Bachir ACHOUR

  • Al-Motasem I Aldaoudeyeh added an answer in Electrical Power Systems:
    Why, as we increase the switching frequency, must the power rating be sacrificed?

    How we can relate switching frequency and power rating of power semiconducting devices? i know its inverse. As we increase the switching frequency power rating must be sacrificed. But i want to know why??

    Al-Motasem I Aldaoudeyeh


    There are three types of losses that take place in power electronics. On-state losses, off-state losses, and switching losses. Your question is about the last.

    When we increase the switching frequency, there will be more losses in the switching period itself. The details of how this happen will heavily depend on what type of switch we are talking about (IGBT, MOSFET, Transistor, etc), but you may think about it like this:

    Consider a transistor that is completely off, when we turn it on, it will not be on instantly as we usually assume it in simulations, but rather, there will be a short time in which the current will raise from zero up to specific value. Simultaneously, the voltage will decrease from certain value (because the switch was like open-circuit) to zero. Waveforms that explain this are shown below

    Now if you looked carefully at the figure, you will see that during switching transient period, both the current and voltage are not zero, thereby we have losses. Now increase the switching frequency, and these losses will be more relevant! Remember that the switching characteristics above are simplified to give intuitive understanding. Again, the switching transients vary depending on the switch itself.   Regards, Al-Motasem