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.

View More On Wikipedia.org
Recent contents Top users
  1. S

    Cross section of the photodissociation of CH

    Does anybody know how to calculate cross section of the photodissociation of CH?
  2. Q

    Drive Cross Country: Share Your Adventure

    Have you ever driven cross country? What was it like? How long did it take you? Did you enjoy it? What was your reason for doing it?
  3. P

    Cross Product Vectors: Finding Direction and Magnitude

    Homework Statement You are looking down at a map. A vector u with ||u|| = 5 points north and a vector v with ||v|| = 10 points northeast. The cross product u×v points: A) south B) northwest C) up D) down Please enter the letter of the correct answer: The magnitude ||u×v|| = Homework Equations...
  4. N

    Understanding the Sign Flipping Rule for Cross Multiplication of Vectors

    Homework Statement ^ ^ ^ i = i; j = j; k = k; i = x j = y k = z So this is the result after cross multiplication. -2ij - 8ji = 6k -ik + 4ki = 5j -4jk +4kj = -8i Unfortunately I am a little confused with the 5j It seems to me that it would be -ik + 4ki = -j...
  5. T

    Cross product, dot product concepts

    Hi, I'm very ashamed about getting a fully understanding of these vector product concepts. But i did a lot of search and get an idea about them. I read Feynman's lecture on physics and almost completely understand the mathematics behind their properties. In that book feynman proves that cross...
  6. N

    Circle with square cross sections

    Homework Statement The base of S is a circular disk with radius 2r. Parallel cross-sections perpendicular to the base are squares. Find the volume V of this solid. Homework Equations The Attempt at a Solution I'm clueless
  7. naima

    Understanding Cross Section in Physics: Dimension and Formula Explanation

    Hi all I found http://en.wikipedia.org/wiki/Cross_section_(physics)#Relation_to_the_S_matrix" on wikipedia: {d\sigma \over d\Omega} = (2\pi)^4 m_i m_f {p_f \over p_i} |T_{fi}|^2 Has it the dimension of a surface? (I only see M*M) It is the first time I read this formula about...
  8. K

    Please suggest a book including cross section

    I am reading some material on laser physics and quantum physics, there involes something called absorption cross section, collision cross section, differential cross section and so on quite often. I try to google the definition online but many website or book just give the formula instead of...
  9. R

    Varying cross section superconductor

    Here's the question: How can we describe a sudden change in the cross sectional area of a super conductor carying a current i from bigger A1 to smaller A2, such that the current density J after change is higher than Jc ? I have one scenario in my mind, the tempreture rises in the...
  10. V

    Proof that e-field lines don't cross

    I understand that electric field lines do not cross and a simple explanation is that it violates existence-uniqueness. I'm thinking there is a more complete explanation out there though. I think the answer lies in the realm of dynamical systems and non-linear dynamics, but it's been a whlie...
  11. L

    Proof involving cross products

    So I'm an engineering student and we're doing some work with tensors and indicial notation, and I came across something that I know is true but couldn't think of how to prove. I don't need it for homework or anything it's just a curiosity thing. OK, so Basically take a set of axes, 3...
  12. K

    What is the Result of i X i in a Cross Product?

    in the end of this http://i29.tinypic.com/5fktjo.jpg i got to the point where it asks what B is i think that because i X J = K then b should be in the direction of J but they have a different conclusion why??
  13. K

    What is the size and direction of the force on each segment in this gif?

    i need to figure out the size and direction of the force on each segment http://i32.tinypic.com/1075g8g.gif in cd segment there is a 45 degree between the electricity and magnetic field so i need to have size 40*sin 45 why they write only 40 ??
  14. K

    Cross product question of j ,k

    http://i26.tinypic.com/27zdaiv.gif i know that i should do right hand rule to know the result but here the vectors are weird ??
  15. K

    Another cross product question

    http://i25.tinypic.com/34s2flh.gif they say that the magnetic field is paralel to the plane so cross product is 0 (sin 0 =0) where is the mistake in my logic
  16. M

    Nuclear Collsions fusion cross section

    Hello, I am curious about fusion reaction cross sections, and fusion power in general, and have been trying to self-teach myself in this area. I saw some emprical data for cross section curves in a paper that I read recently, and thought I understood enough of what made the curve (cross...
  17. B

    Question about neutron cross sections

    I am looking for thermal neutron scattering cross sections (0.025eV) for individual isotopes, having found non isotopic data for most elements in a textbook. The most comprehensive cross section database appears to be http://www.nndc.bnl.gov/sigma/index.jsp?as=238&lib=endfb7.0&nsub=10"...
  18. D

    Photodetachment Cross Section Formulas: Explained

    I know the formulas for the length and velocity forms of the photodetachment cross section for an electron with energy \hbar\omega are, respectively, \sigma_{L}(\omega)=\frac{4\pi^{2}\alpha a_{0}^{2}\omega}{3}\sum_{f}|\langle\Psi_{f}|\sum_{j=1}^{n}z_{j}|\Psi_{i}\rangle|^{2} and...
  19. R

    Understanding the Cross Product and Its Role in Magnetism

    I just saw an instructor of EEE (YouTube) talk about Cross Product. He gave an example of when cross products are used. Here's what he said. He drew a vertical arrow and said this is current. Then he drew a circular arrow around the vertical line and said it is B. Then he drew a smaller...
  20. J

    Dot and Cross Products (of Gradients)

    Statement: I was wondering if the following are identical, \nabla \bullet \nabla \times \vec{A} = \nabla \bullet (\nabla \times \vec{A}) ? (#1) Also, more importantly I was wondering if someone could explain to me why the following is zero for any vector, \nabla \bullet \nabla \times \vec{A} =...
  21. M

    Is Section Modulus the same as a cross sections bending strength modulus ?

    Is "Section Modulus" the same as a cross sections "bending strength modulus"? Good afternoon everyone, I am following an ECE directive that sets out the requirements for the properties of a "Rear underun protection device" for the rear of a lorry. It states that the underun bar must...
  22. D

    Inner Product in $\mathbb{R}^2$: Not a Match for u x v

    Homework Statement Which of the following functions is not an inner product in \mathbb{R}^2, where u=(a, b), v=(c,d)? a. <u,v> = ac+bd b. <u,v> = 4ad+7bc c. <u,v> = ac/2+bd/3 d. <u,v> = 2ac+9bd Homework Equations None The Attempt at a Solution u x v = det(u v) = ad-bc I...
  23. N

    Cross product of a vector and its derivative

    Hello guys, this must be a very trivial question, but I just don't see it. Ihave a "vectorized quaternion" (quaternion with a null scalar part) : u(t) = (0, u_x(t), u_y(t), u_z(t) ) We also have it's time derivative : \frac{d}{dt}u(t) = \dot{u}(t) = (0, \dot{u}_x(t), \dot{u}_y(t)...
  24. H

    How can the boatman cross the river in the shortest time?

    hi! here is a problem for which I cannot really feel the answer though I know the answer here it is A boat can travel at a speed of 3m/s on still water.A boatman wants to croo a river whilst covering the SHORTEST POSSIBLE DISTANCE.in what direction should he row w.r.t the river bank if the...
  25. R

    Does Zero Volume Indicate Zero Curl in Vector Fields?

    Regarding the equation for curl: Nable E literally means the sum of the differences of certain rates of change with respect to certain coordinates i hat, j hat, k hat. Since Nabla Cross E also is interpreted as the volume of a paralleliped in 3D space... 1. when the volume is zero, does...
  26. R

    How do vectors relate to the area and length of a rectangle in vector calculus?

    I've been reading a bit on vector calculus trying to get a solid grasp on it. I am wondering how it can be stated that given C=AXB (all vectors), that C is equivalent to the area of the rectangle formed by A and B in the A,B plane. How can you equate length and area. One is in say...mm and the...
  27. D

    Can Scalars Be Factored Out in Vector Cross Products?

    Hello, this is pretty straightforward. I need to take cross product: kq(x/(x^2+y^2),y/(x^2+y^2)) x (d/dx,d/dy) since kq is a scalar can i just leave it outside the calculation until the very end and for now just calculate the cross product of the two and once i get a definite answer...
  28. M

    Does the Cross Product of -k and -i Result in +j?

    simple cross product question. Please help! Homework Statement Hey guys, I know how cross product works, i-->j-->k : positive numbers i<--j<--k : negative number Homework Equations The question that I have is, say I have -k x -i, I am supposed to get a positive j since the two...
  29. L

    How to find out the effective cross area of a molecule in a monolayer film?

    hi guys, i need some help with the following questions relating to cross sectional area. thanks in advance! 1) when n-hexatriacontanoic acid, CH3(CH2)34COOH was placed in water, a complete monomolecular film was formed. if the length of a carbon-carbon single bond in the chain is 1.54...
  30. L

    How to find out the effective cross area of a molecule in a monolayer film?

    hi guys, i need some help with the following questions relating to cross sectional area. thanks in advance! 1) when n-hexatriacontanoic acid, CH3(CH2)34COOH was placed in water, a complete monomolecular film was formed. if the length of a carbon-carbon single bond in the chain is 1.54...
  31. R

    The Dot Product and Cross Product: Finding the Angle Between Two Vectors

    Homework Statement If theta is the angle between two non-zero vectors A and B, then which of the following angles theta results in A dot B = |A x B|? Homework Equations A dot B = ABcos(theta) A x B = ABsin(theta) The Attempt at a Solution There were two choices in the multiple choice answers...
  32. V

    How to deal with non uniform cross section bending?

    How to deal with non uniform cross section bending? How does one approach a solution for bending of a shaft having multiple diameters. I have one particular shaft in mind that has 11 diameters to get to the midpoint. I know the deflection at the midpoint, and I would like to calculate...
  33. C

    Volume of a solid with known cross sections

    Homework Statement Any cross sectional slice of a certain solid in a plane perpendicular to the x-axis is a square with side AB, with A lying on the curve y^2 = 4x and B on the curve x^2 = 4y. Find the volume of the solid lying between the points of intersection of these two curves...
  34. J

    Determine how long puck takes to cross ice

    i have no clue where to begin to solve this problem. Can anyone help me out and get me started please and thanks. A hockey puck, sliding on an outdoor rink, has a velocity of 19 m/s forward when it suddenly hits a rough patch of ice that is 5.1 m across. Assume that the coefficient of...
  35. S

    Cross Product of Antiparallel Vectors: qvXb Maybe Not?

    qvxb maybe not?? Homework Statement consider the example of a positive charge moving in the -z direction with speed with the local magnetic field of magnitude in the +z direction. Find , the magnitude of the magnetic force acting on the particle. Express your answer in terms of , , , and...
  36. C

    Integration for Volume Given Cross Sections

    Homework Statement The roof and walls of a storage building are built in the shape modeled by the curve y(x) = 20 - \frac{x^6}{3,200,000}. Each cross section cut perpendicular to the x-axis is a rectangle with a base of 50 feet and a height of y feet. In cubic feet the volume of the...
  37. C

    Understanding Cross Product: Moment, r and F

    Say A,B and C are points on a plane. By taking the magnitude of the cross product of AB and BC gives you the area of the parallelogram. The direction of the answer will be perpendicular to both AB and BC, but what I don't understand is why we are getting the area? Is the area pointing in that...
  38. D

    Compton Scattering cross section calculation

    Not technically homework, just something I wanted to see if I could do. Homework Statement Find the differential cross section for the interaction between an electron and a photon via compton scattering. Basically, I'm just after calculating the matrix (s-matrix?/amplitude?) for the s-channel...
  39. Y

    Proof for cross product (please)

    TO be very honest i only know the definition of cross product of two vectors but i would like to prove.....in fact all textbooks just define the cross product how can i derive it starting from the first principles i,e, i must start with something and finally arrive at the expression given...
  40. P

    Questions on torque and cross products

    Homework Statement Hello. I have three questions on vector product that I am having problems with. I have attached the images for these questions. Homework Equations Vector Product: a x b = lal lbl sin (theta) n or i, j, k method Area of a triangle: (1/2) l a x b l...
  41. M

    Center of mass and cross product

    Let vectors vec{A}=(0,0,-3), vec{B}=(2,0,0), and vec{C}=(0,1,1). Calculate the following, expressing your answers as ordered triples (three comma-separated numbers). Calculate A x C. For this part, remember that only perpendicular components contribute to the cross-product. The vector C has...
  42. I

    What is the Cardinality of Multiple Cross Products?

    Since Tex is giving me a hard time this question is going to be a lot more undeveloped than it was going to be. We were discussing in class the cardinality of the Natural numbers being aleph null and that the cross product was also of cardinality aleph null. Is it true then that...
  43. D

    Cross Product in Spherical Coordinates - Getting conflicting oppinions

    Hey all, I really need some clarification here. I've seen problems dealing with the Angular Momentum of a particle, working in spherical coordinates. Wolfram says that there is no simple way to perform this and do the determinant, and you will find many people and other websites claiming...
  44. B

    VERY quick cross product question

    Homework Statement p X q - r Homework Equations The Attempt at a Solution In order to evaluate this expression, I must first evaluate q - r and then perform the cross product, right? I'm trying to think about X as an operator, like del, that operates on the things that appear...
  45. Y

    Can Cosine Affect Whether Three Nonzero Vectors Must Lie in the Same Plane?

    If there are three nonzero vectors.. Do you think cosine effects this example:show the three vectors must lie in the same plane? ------------- * -->dot product X -->cross product -------------- A*(BXC)=0 so we can change as.. |A||B||C|sin\alphacos\beta=0 then... we can meet...
  46. H

    MATLAB Matlab cross referencing matrix and table elements

    hi all, here's an interesting problem I've been working on in MATLAB. any help would be appreciated. i have four column vectors of length ~11000 (table1): in1 in2 out1 out2 11 4 101 47 1 8 111 9 38 1 1 39 ... i also have a matrix with dimensions 111x111 (matrix1): c1 c2...
  47. snoopies622

    Cross Product Force: Exploring Magnetism's Role

    To me the idea of a force that pushes an object only in a "sideways" direction - that is, always at a right angle to the direction of motion - has always seemed strange. I can see how it comes about in the case of the Coriolis force because there the frame of reference is rotating and the force...
  48. R

    Find the Moment Inertia of a Cross Section: y = 30 & 35

    Hi.. I've got a problem about moment inertia. I don't understand about looking the distance of centroid (y). I am still confucing about the formula of "y" in Moment Inertia. Can you shows me about the formula of y or if any thread for this before, I am glad to see it too.Thanx's for your help...
  49. N

    Vector cross product identity proof

    Homework Statement \bigtriangledown\times\\(v\times w)= v(\bigtriangledown\cdot w) - w(\bigtriangledown\cdot v)+ (v\cdot\bigtriangledown)w - (w\cdot\bigtriangledown) v I've tried expanding left side and get [v1(dw2/dy+dw3/dz)-w1(dv2/dy+dv3/dz)]i +...
  50. I

    Surface Energy of Miller Index Cross Sections

    ℽ = ½Nbερa ℽ = surface energy Nb = number of broken bonds ε = bond strength ρa = surface atomic density ℽ{100} = (½)( 2 / a2 )(4)(ε) ℽ{110} = ( 5 / sqrt(2) )( ε / a2 ) ℽ{111} = (2sqrt(3))( ε / a2 ) the supplemental picture shows a cube with atoms at each corner and one atom in...
Back
Top