Concept Definition and 1000 Threads

Hi guys. Just out of curiosity I want to know that why the concept of stress evolved? Why do we need stress?

I have a question about the increasing behavior of entropy. Suppose we heat a metal (Take for instance Fe) till it radiates energy and then put it in space (No medium). So the metal radiates energy in electromagnetic waves which decreases the entropy of the metal (Due to decrease in internal...

A wave packet is formed when a multitude of sine waves of different periods are linearly added. This localizes the pdf, and hence creates certainty in a particle's position. However, basic sinusoidal functions are cyclical, hence the additton of two sinusoidal functions are always...

if we weigh a metal object which is immersed in water, we have to correct the buoyant force for water. Air is a fluid. Why don't we correct for the buoyant force of the air when we weigh it in the air? What i think it may be is because we have a set atmospheric pressure being 1.013 Kpa in...

Hello! Which is the most difficult to comprehend scientific concept you have come across? How you managed to master it? Thanks!

Hi comrades. According to spivak, the defition of limit goes as follows: " For every ε > 0, there is some δ > 0, such that, for every x, if 0 < |x-a| < δ, then |f(x) - l |< ε. " After some exercices, I came across with a doubt. Say that I could prove that | f(x) - l |< 5ε, for some...

I'm having difficulty understanding the nature of a plane in complex space. Specifically, I have two complex (N \times 1) vectors, \underline{u}_1 and \underline{u}_2 , which are orthonormal: \underline{u}_1^H\underline{u}_2 = 0 \\ \Vert \underline{u}_k \Vert = 1 \ \ \ (k=1,2) So...

I'm reading a tutorial about the basics of circuits and I don't understand this concept. In the three last circuit diagrams in the link located below, there is a break in each of the circuits. The ends of the breaking points are labeled + and -, but how can that be? My thoughts: If there...

1mole of diatomic element X2 contains 34 and 40 moles of electrons and neutrons respectively, the isoptopic formula of the element is? Homework Statement 1mole of diatomic element X2 contains 34 and 40 moles of electrons and neutrons respectivelyHomework Equations all the basic ones! The...

Sometime I read that the helium atoms can be considered as boson, but I don't understand why. I know that its nucleous has a spin of 2 (integer) and that its 2 electrons gives the atom a total spin of 3, an integer. But then why isn't hydrogen considered also as a boson? I think it's considered...

I'm a little confused. Lets say i have a rigid,non-expandable container. If i heat the container,the gas molecules gain Kinetic energy. I initially learned that the gas molecules will hit the wall with a larger force,hence larger pressure.But F=MA,so the gas molecules are accelerating?But...

From what I've gathered about quantum mechanics, it's based on probability, correct? (Yes, it's true that I know next to NOTHING about this subject) Could someone explain to me how we've come to discover this and how it contradicts classical mechanics?

Hi all, For my studies I'm currently looking into Weak anti localisation. I've come across a lot of articles making use of this effect, but none of them describe the concept of WAL. So far I've been unable to find it in my books. I hope one of you can help me with this.

This is more of a conceptual question dealing with a homework problem than with the problem itself... So I am asked to find various limits for the function (x2-9x+8)/(x3-6x2). All well, no problems... I am asked to find the left and right hand limits as x→0, which is -∞ for each. At the end...

Hi guys ^^ I am still at A level physics, so don't flame me for my lack of knowledge Now i have been taught that for ex: We have a block of wood named A at rest on a perfectly smooth surface. If we analyse a free body diagram, we have a weight mg downwards acting on the block and a normal...

I understood that Newton has introduced a concept called "Force" which is basically a cause for an effect i.e. if an object is in a state of rest and if applied a "force" then the object moves (change in velocity, ∴ accelerates) also if an object moves with a constant velocity and is disturbed...

In a recent thread, language was described as an activity where: a set of rules is used to define constrains on the world. For instance the word car limits they type of objects in the world which are likely to be denoted by the word car because one might not normally expect one to call a house...

hi everyone, i am not a math geek and don't know why the hell i chose thermodynamics this semester with this little math knowledge, but can you please help me figure out the basic difference between discrete and continuous probability? in thermodynamics we are sometimes taking the probability...

I thought it would be interesting to do a visualisation of Brian Greene's concept that "everything moves at the speed of light". For a preamble see this old thread https://www.physicsforums.com/showthread.php?t=398590 First I should warn that speed or velocity is not defined in the conventional...

In the way we represent an electromagnetic wave (click on http://upload.wikimedia.org/wikipedia/commons/4/4c/Electromagneticwave3D.gif) we limit the magnetic field vector and electric field vector till the limit of the amplitude of the wave. But how is it possible ? because, Magnetic and INDUCED...

Homework Statement See attachments Homework Equations The Attempt at a Solution In the explanation attachment, I don't get the last paragraph "Note that force Fce acts downward on bar...compression." Since from the figure Fce is pointing down, and Fbd is pointing up, shouldn't...

Hello all I have a quick concept question concerning electrical current. For real life components, would they only draw the current needed...In other words, if the current of a voltage source of 12V happened to be 500A going through a resistor of 2 ohms, would the resistor pull only 6A or...

Really couldn't catch the concept on epsilon and delta in limits. Let ∂x=x2 - x1 In finding a gradient the value ∂y is taken at certain value. But in finding area using integral, the ∂y is seen to taken as zero. F(x2)=F(x1) Maybe one multiplication and the other is division.

Why would we assume matter is spread homogeneously throughout the universe if its only 4.6% of universe :confused: The universe is believed to be mostly composed of dark energy and dark matter, both of which are poorly understood at present. BBT explains about CMBR, redshift, expansion and...

i need to get the hang of the mole concept that , 1.008 g of hydrogen constitutes 1 mole atom, whereas 12g of carbon constitute that 1 mole atom! does it mean that these amounts for H & C contains the Avogadro number of atoms?? does it imply the hydrogen is more denser (or heavier or something...

The concept of "on-shell mass": what is it? Following the recommandation of MTd2, but after some hesitations, here are my questions.So, reading rapidly a first time the document “Curved Momentum Space and Relative Locality” [arXiv: 1205.1304v1 [hep-th] 7 May 2012 on another part of this forum]...

Four Wires form corners of a square. Force exerted by 3 of them on 1. Concept help! Homework Statement side length "a"= 5.5 cm current through wires "I" = 1.6A "L" of lower right wire= 1 cm upper and lower left currents are out of the page upper and lower right currents are into the...

Homework Statement I'm confused by the different equations textbooks use for Q=mcΔT I understand for solids/liquids its always Q=mcΔT but for gases am I right to say ΔH=Q=ΔU+pΔV=m(c_p)ΔT for constant pressure and Q=ΔU=m(c_v)ΔT for constant volume? So if my textbook says "ΔU=n(3/2R)ΔT for any...

Guys i really need your zealous help! It is a simple concept but i have some doubt on it. A vector can be resolve into its component vectors like unit vectors i,j & k. A single vector can be expressed into its i-components and j-components if it is a 2-dimensional vector. Is it when 2 vectors...

Is infinity a mathematical concept or a number? Please elaborate. Is there any debate over this topic or is there a consensus amongst academia?

How is Einstein's concept of Individual time different from a second dimension of time? I am thinking of the example of the twins; one twin goes into space at a velocity approaching the speed of light and the other remains on Earth. When the traveler returns he is actually younger than his...

Hello, I'm sorry if I'm posting in the wrong place. As you can see this is my first post and there's something that's been irking me so much that I need clarification. I'm sure some people will be disgusted at some of the logic, but I just want clarification. I do realize that it's all...

Okay, say a car is moving at a constant velocity and crashes into a wall. Now, observation would clearly illustrate to us that a destructive force was imparted onto the wall and the car. But how can that be if there there is no force, because there was a constant velocity? I must have some...

Homework Statement Fluid is flowing from left to right through a pipe (see the drawing). Points A and B are at the same elevation, but the cross-sectional areas of the pipe are different. Points B and C are at different elevations, but the cross-sectional areas are the same. Rank the...

Homework Statement This is not actually a homework question, just a question I ran into while studying for my math final. When I am using stokes theorem: ∫∫(curlF ° n)dS I have listed in my notes from lecture that there are time when it is applicable to replace dS with an easier...

Hi, I want to clear my concept about the direction of torque for a sin curve. Assume the torque can be represented by a sin wave (T=sinx, where x is from 0 to 2π and it starts from the origin(0,0) with an amplitude of 1. My understanding of the torque direction is as follows (assumuing...

Say you have a rectangle with -Q charges at top left and bottom left corners, and +Q charges at top right and bottom right corners. Any point along a horizontal line in the middle will have electric field going to the left, but any point along this line will have a potential of 0. I've been...

Homework Statement For decades, people have dreamed of cars that can fly. You are part of a team which hopes to build the worldʼs first commercial flying car. Your proposed design has four fans, each of radius 50cm. Each fan sucks in air from above the car and blows it downwards. The...

Homework Statement I just wanted to know if I was correct, when dealing with a Newton law problem that has components. Ʃfx=0 and ƩfY = zero However when we add the forces together then we can use Ʃf=ma thanks Homework Equations The Attempt at a Solution

What is the theory behind dividing human society into Countries and States? Do the taxes collected from people on a state are meant to be spend for the welfare of the State alone or we sometimes spend the money to help other 'poorly doing' states? In a more general perspective, are we trying...

I came across an interesting combinatorics optimisation description for 14numbers. Maybe someone good in combinatorics can expand this fano plane concept to a full 45 numbers lottery design. Any suggestions? http://en.wikipedia.org/wiki/Transylvania_lottery

How to make an electric field?

Homework Statement An object weighting 100N is traveling vertically upward from the Earth in the absence of air resistance at a constant velocity of 5 m/s. What is the power required to keep the object in motion? Homework Equations P = ΔE/t and P = Fvcosθ The Attempt at a Solution...

Homework Statement Its not homework, i have the answer I am just having a hard time wrapping my head around the concept of differentiating implicitly defined functions. the question was: x^3+y^3=3xy, find the equation of the tangent line at the point (3/2,3/2). Homework Equations...

Hello, As you all know, if a crystal contains a phonon of frequency \omega, it means that the Fourier decomposition of the physical vibration of that crystal lattice contains a plane wave of frequency \omega, and the more phonons of a given frequency, the bigger the amplitude of that component...

Homework Statement A piece of fruit falls straight down. As it falls, A. the gravitational force does positive work on it and the gravitational potential energy increases. B. the gravitational force does positive work on it and the gravitational potential energy decreases. C. the...

Hi guys! I think I need to explain myself before I ask for the advice :) I'm 27 (so been in grade 12 ages ago) - I didn't have science, I wrote it years later. Now I'm enrolled for a Bsc degree in the hope of getting into medical school (very tough in South Africa). I'm putting a lot of time...

Hello, I am somewhat confused about the concept of pressure in Bernoulli’s principle, which states that velocity and pressure are inversely related. When I think of pressure, I think that there is higher pressure acting on a liquid when a pipe is smaller rather than larger. I would appreciate...

Hello, a friend of mine has a "revolutionary" idea about harvesting wind power by using a "translational turbine". however, it all sounds silly to me. this is what it should look like: is this a good idea for a wind "turbine"?

Throughout all the cultures in the world there is the concept of spirits, angels, fairies, ghost, gnomes, totems etc. A dismissive view is that people who have seen these things are crazy, but a more accepting view is that people have actually seen such things. I myself have had sleep paralysis...