What is Cross: Definition and 1000 Discussions

A cross is a geometrical figure consisting of two intersecting lines or bars, usually perpendicular to each other. The lines usually run vertically and horizontally.
A cross of oblique lines, in the shape of the Latin letter X, is also termed a saltire in heraldic terminology.
Throughout centuries the cross in its various shapes and forms was a symbol of various beliefs. In pre-Christian times it was a pagan religious symbol throughout Europe and western Asia. In ancient times, the effigy of a man hanging on a cross was set up in the fields to protect the crops. The cross was even considered a male symbol of the phallic Tree of Life; thus it often appeared in conjunction with the female-genital circle or oval, to signify the sacred marriage, as in Egyptian amulet Nefer with male cross and female orb, considered as an amulet of blessedness, a charm of sexual harmony.

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  1. D

    Deflection of Tapered Beam with Elliptic Cross Section

    I am working on deriving expression for deflection of a tapered beam with an elliptic cross-section. Hence, area moment of inertia is a linear function of the beam length. The beam is fixed at one end, and a concentrated force F is applied on its tip at the free end. I am using the known...
  2. A

    Skew bending in a circular cross section (proof)

    Good day all I'm looking for the proof of stress generated in case of skew bending applied in acircular cross section ( I browsed internet the whole day without finding anything convincing) we use with many thanks in advance!
  3. S

    How to calculate total cross section from differential cross section

    I was doing the calculations for this: http://fermi.la.asu.edu/PHY531/cylinder/index.html But I can't figure out how to go from \frac{d\sigma}{d\phi} to the total cross section. My guess was that you did the integral from \phi=0 to \phi=2\pi, but that's not helping since I can't tell either how...
  4. V

    A Photoabsorption cross sections

    If we look into the UV photoabsorption cross sections of different gases, a kink in the cross section is seen at wavelength between 10-100 Angstrom. Can some one explain why this happens ?
  5. B

    Resistance of a semicircular conductor with a rectangular cross section

    Homework Statement There is a conductor with the square-shaped area. the Radii are r1 , r2 with width b and resistivity ## \rho_R##. Find the resistance R between A and B 2. Homework Equations ##I = \iint_A\vec J \cdot d \vec A## ## \vec J = \kappa \vec E ## ## \vec E = \rho \vec J## ## V =...
  6. SanPhysics

    Calculate the required Cross sectional area of toroidal core

    For example how to calculate required cross-section area of the toroidal core for 100VA transformer.Is that any simple formula.
  7. ChrisVer

    A What is a Fiducial Cross Section?

    I have come across the term quiet often, but I find it difficult to explain it to myself (or others) and even interpret it... So what is a fiducial cross section? Is it the measured cross section once you factor out the detector acceptance and the different cut-efficiencies (mainly motivated by...
  8. Philosophaie

    I Is this 4-Vector cross product calculation correct?

    Could someone tell me if this 4-Vector cross product is correct: i j k t dx dy dz 1/c*dt Ex Ey Ez Et =[(dy(Ez)-dz(Ey))-(dy(Et)-1/c*dt(Ey))+(dz(Et)-1/c*dt(Ez))]*i -[(d(E)-d(E))-(d(E)-d(E))+(d(E)-d(E))]*j...
  9. S

    Evanescent waves at change of cross section

    Hey guys, if I consider a Bernoulli-Beam with a constant jump in cross- section as shown below Then I get the two differential equations for the bending mentioned above. Let us now assume that a traveling wave from the left w_incident = C*cos(k_1*x - ωt) arrives at t = 0 at the jump of...
  10. Vectronix

    B Derivative with the double cross product

    Is there a spatial derivative that uses the del operator and the double cross product? If so, is it used in physics?
  11. P

    Hardy Cross method with open loops

    Hi. I am using the Hardy Cross method to simulate the gas flow inside a multiple hearth furnace (as shown in figure A). Figure B is a representation of the furnace's body as a pipe network. The segment A-AH represents the actual hEarth's, while segments B-AI and C-AJ represent the exhaust ducts...
  12. D

    Energy to Cross section conversion

    Homework Statement [/B] Using n-Pb collision experimental data, find sigma_0 in units of 1/keV^2 and convert to cm^2 (I'm copy-pasta'ing from Mathematica, sorry for mess) \[Alpha]n = 7*10^-49m^3; mn = 939565; <--- Neutron mass in keV/c2 \[HBar] = 1*10^-3; <--- Natural units and again keV*s b...
  13. T

    I Can a cosmologist cross an event horizon?

    Suppose I am orbiting a black hole (BH) at some distance d outside its event horizon (EH), and with orbital velocity v. I do not like where I am, so I try to increase d by using an amount E of energy to increase my orbital velocity to v', where v' is whatever is necessary to escape the BH and...
  14. Kirk Truax

    What is the macroscopic cross section for natural uranium to thermal neutrons?

    Hey all, Does anyone happen to know the value of ν∑f for natural uranium? Here ν is the average number of neutron released from fission and ∑f is the macroscopic fission cross section of uranium. Kirk
  15. Wrichik Basu

    B What is the real life utility of the dot product and cross product

    Today, my teacher asked us what is the real life utility of the dot product and cross product of vectors. Many of us said that one gives a scalar product, and one gives a vector product. But he said that, that was not the real life utility of the dot and cross product. He asked us, "Students...
  16. ltkach2015

    A LeviCivita in Orthogonal Curvilinear Coordinate System: "Cross Product Matrix

    If given a position vector defined for a orthogonal curvilinear coordinate system HOW would the matrices that make up the Levi Civita 3x3x3 matrix remain the same? "Levi Civita 3x3x3 is said to be independent of any coordinate system or metric...
  17. JTC

    The Cross Product and Angular Momentum

    Hello I need help to explain the affect of the cross product without the its current symbolism, but for angular momentum. I can explain angular momentum in terms of the cross product of 3D space formulated like this: |r| |v| * sin(angler.v) e-perp to r and v Eq.1 (I can explain this to...
  18. P

    I Deriving the E Cross B Drift Velocity: A Puzzling Explanation

    I am looking at this brief derivation of the E cross B drift for a plasma, starting at the top of page 3: http://www.physics.sfsu.edu/~lea/courses/grad/motion.PDF. I am a bit confused as to the justification that the drift velocity is perpendicular to the magnetic field, as stated at the top of...
  19. C

    Calculating the Cross Product Vector: Is It Possible to Find VxW?

    Okay so, I am wondering if it is possible to find the actual cross product (not the magnitude of the cross product) from this information 1. magnitude of both vectors 2.angle between vectors 3.plane the vectors lie in Is there any way to calculate that cross product vector? Thank you very much...
  20. P

    I Gamma ray cross sections from classical plane wave

    I am aware that in classical electrodynamics a metal exponentially attenuates TEM waves. I am wondering if this fact is sufficient to obtain estimates of the cross section of low energy gamma rays in metals. Obviously it would fail for higher energy gamma rays where Compton scattering and pair...
  21. S

    Charge passing through a cross section for a time varying current

    Homework Statement The current in a wire varies with time according to the equation I(t) = 6.00 A + (4.80 A/s)t, where t is in seconds. How many coulombs of charge pass a cross section of the wire in the time period between t = 0.00 s and 3.00 s Homework Equations I=dq/dt The Attempt at a...
  22. P

    Why does 135Xe have a high thermal neutron cross section?

    Title says it all. Made a few google searches and couldn't find any information. Is there a reason why it is so large, or is it just a coincidence of nature?
  23. andylatham82

    I How to find angle between vectors from dot and cross product

    Hi, hopefully a quick question here...how do you calculate the angle between two vectors if the only information you have is the value of their scalar product and the magnitude of their cross product? Thanks! Andy
  24. E

    I Atmospheric Photolysis and its relation to Absorption Cross Section

    Dear Forum: I have a question about atmospheric photodissociation. I use methane as an example, but any atmospheric gas molecule would suffice. Methane, CH4, has a photodissociation energy of 439 kJ/mole at 298oK, meaning that CH4 + hv --> CH3 + H , hv<274nm. It also has an “absorption cross...
  25. R

    What is the difference between monohybrid and dihybrid cross

    and if I have two plants the first one with a genotype of YyTT and the second one with a genotype of yyTT fertilise them this would be a monohybrid or dihybrid cross.
  26. Muthumanimaran

    Finding the scattering cross section

    Homework Statement In an experiment carried out with a beam of thermal neutrons it is found that on traversing a 2mm thick foil of 197Au, some 70% of the neutrons are removed. What is the total thermal neutron cross-section for this isotope of gold? Comment on the result of the cross-section...
  27. chandrahas

    I Calculating Collision Cross Section in Fusion Reactors

    Most Fusion reactors, and the leading ones like JET, use high temp. plasma and confine it. So, the plasma would approximate the Maxwell- Boltzmann distribution. This means that only a small portion of the plasma has enough energy to fuse. But, collisions are much more often, right? Since not all...
  28. B

    Find beam stress in a beam with varying cross section

    Homework Statement Hi, I'm trying to figure out how to calculate the stresses in my beam. The cross section varies (shown in the picture), and I don't know how I'm supposed to think to find the maximum stress. I appreciate tips or links to learn how to do it. Thanks. Sorry if the picture is...
  29. S

    K X-Ray Production in PIXE: 2 MeV vs. 4 MeV

    Homework Statement In PIXE (Proton-Induced X ray Emission), if you increase the beam energy from 2 MeV to 4 MeV, do you expect that the factor of increase in the cross section for the K X ray production from Ni is higher than that from S, or vice-versa? Homework EquationsThe Attempt at a...
  30. F

    I Can anybody give me a list of all cross products in physics.

    Can anybody give me a list of all cross products in physics. I have the following in my list: Torque $$\vec{\tau}=\vec{F}\times\vec{r}$$ Angular momentum $$\vec{L}=\vec{r}\times\vec{p}$$ Velocity $$\vec{v}=\vec{\omega}\times\vec{r}$$ Biot-Savart law $$\vec{dB}=\dfrac{\vec{i...
  31. Jovy

    What is the sine of the angle between two given vectors?

    Hello, I apologize in advance for the way this post looks. I am new to this forum and I've never used LaTeX Primer. I noticed that someone has prevoiusly asked the same question, but I still do not understand how to get to the answer. Also, I tried posting an image but I could not; and this...
  32. bwest121

    I Question about Cross Product and Derivative

    Hi everyone, Given a vector-valued function ##\vec{A}##, how do I show that: $$\vec{\nabla} \times \left(\frac{\partial \vec{A}}{\partial x}\right) = \frac{\partial}{\partial x}(\vec{\nabla} \times \vec{A})$$ In other words, are the cross product and derivative commutative w/ each other? I...
  33. P

    Energy from the Sun received at the Earth cross section

    Homework Statement My homework involves calculating the amount of energy the Earth receives from the Sun in a given year. I actually have the answer, but what I'm confused with is that the calculation says that the Sun "sees the cross section of the Earth, a circle". So in the calculation, we...
  34. M

    I Difference between absorption and scattering cross sections

    Hi. I'm trying to model the attenuation (extinction) of a weak laser through a gas sample using the Beer-Lambert law. I've found that the attenuation cross section can be written as the sum of both a scattering and absorption cross section, however I'm having difficulty finding a source that...
  35. charlesmartin14

    Help with derivation of two-body relativistic cross section

    Homework Statement I am trying to derive the 2 body cross section, as given in https://web2.ph.utexas.edu/~schwitte/PHY362L/QMnote.pdf dσ/dΩ Homework Equations I am stuck on d√s/dp=v where √s=(Ea+Eb)=E The Attempt at a Solution p=Ev (relativisitic energy-momentum relationship) √s=E...
  36. I

    Nuclear reactions, cross section, reaction rate

    Homework Statement A beam of ##^{50}##Ti nuclei impinged on a ##0.5 mg/cm^2## thick ##^{208}##Pb target in an experiment lasting 176 hours. During that time a total of ##3260 \; ^{257}##Rf nuclei were detected. The beam intensity was throughout the experiment constant at ##6.6\mu A##. Each...
  37. BenAS

    B Confused about how long it takes to cross an event horizon

    If there were 2 observers, 1 at rest with CMB, and the other falling into a black hole, how much time passes for the observer at rest with CMB while the other falls past the event horizon? I ask because I keep reading that time dilation (due to gravity) will cause the object falling into appear...
  38. B

    What is the Direction of A X B Using the Right Hand Rule?

    Homework Statement The direction of vectors A and B are given below for several cases. For each case, state the direction of A X B. a) A points east, B points south. b) A points east, B points straight down. c) A points straight up, B points north. d) A points straight up, B points straight...
  39. C

    LaTeX Latex Cross References, citations etc not working

    Hi, not sure if this is the correct section so feel free to move if needed. First time using latex to create a pdf here, having trouble getting cross-references and citations to work. They just appear in the text as question marks. I got my bibliography to work, using Bibtex, but when using...
  40. A

    Can you explain this cross cut

    Homework Statement I don't understand the cuts or the views. For example if we make a horisontal cut, which view we should show? http://1.1m.yt/UJasa7v.jpg Homework EquationsThe Attempt at a Solution I have in the book an example that if we make a vertical cut we show the upper view (view...
  41. A

    Engineering Free Exercises on Right Angle Projections & Cross Section Views

    I am looking for free solved exercises about: right angle projections, (cross) section views Here are some pictures of my book so you can understand what I am looking for http://1.1m.yt/3cNNDl-.jpg http://1.1m.yt/FekMn6x.jpg http://1.1m.yt/1t3KEer.jpg http://1.1m.yt/urA5V2_.jpg...
  42. F

    Variation of shear stress at the rectangle cross section

    Homework Statement In the notes , I don't understand why the shear stress is maximum at the edge ( circle part) . Homework EquationsThe Attempt at a Solution I think it's wrong . Refer to another diagram attached , i found that the shear stress varies parabolically across the vertical length...
  43. kiwaho

    A Equivalence equation between cross section & half life?

    Is there equivalence equation between cross section & half life? For beta decay, we usually use half life to describe how fast or slow the decay undergo. For nuclear reaction, we use cross section to describe the possibility of reaction. In a sense, they reflect the same root physics spirit. Is...
  44. kiwaho

    A Why so big of a difference in cross section by different sources?

    if you look up the cross section in Janis books, usually for same reaction, different cross sections are quoted by different laboratories. For example, the values of thermal neutron capture reaction 9Be(n, r)10Be: EAF-2010, 0.0075b ENDF/B-VII.1, 0.01b JEFF-3.1.2, 0.0088b JENDL-4.0, 0.0085b...
  45. B

    A Spinor Lorentz Transform via Vectors - Cross Product Issue

    The Lorentz transformation operator acting on an undotted, i.e. right-handed, spinor can be expressed as $$e^{-\frac{1}{2} \sigma \cdot \mathbf{\phi} + i\frac{1}{2} \sigma \cdot \mathbf{\theta}}.$$ There is a very cool, almost childlike, derivation of this expression in Landau Vol. 4 S. 18 I've...
  46. C

    I Derivation of splitting function from cross section

    Consider the real emission correction to the tree level process ##e^+ e^- \rightarrow q \bar q## involving a final state gluon emitted from either the outgoing quark or antiquark line. The differential cross section for producing a quark with fractional energy ##x_1## in the ##q \bar q g##...
  47. Kara386

    I Calculating Cross Section for ZZ* to Electrons/Muons: Issues & Solutions

    If given the cross section for ZZ* to two leptons, is it possible to work out the cross section for ZZ* to just electrons or muons from it? So excluding the tau decay mode, unless ##ZZ^{*} \rightarrow \tau + \tau## can't happen anyway? I'm guessing it wouldn't just be two thirds of the ##ZZ...
  48. H

    Trajectories never cross in phase-space

    I heard this statement from time to time, but what does it really mean?
  49. E

    B Does the following cross product identity always work?

    Mod note: Reproduced contents of image with broken link: i = j x k j = k x i k = i x j Wikipedia says this about the standard basis vectors. Does this work for all (i.e, non basis) vectors? For example, if you know A = B X C does that mean C = A X B and B = C X A?
  50. T

    I Motivation and proof behind cross products

    this question is a repost from math stackexchange because that guy worded the question so perfectly the question i really wanted to ask about cross products. *please see image below* as far i can understand, the formula for the cross product is basically that the idea of a cross product is sort...
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