What is Principle: Definition and 1000 Discussions

A principle is a proposition or value that is a guide for behavior or evaluation. In law, it is a rule that has to be or usually is to be followed. It can be desirably followed, or it can be an inevitable consequence of something, such as the laws observed in nature or the way that a system is constructed. The principles of such a system are understood by its users as the essential characteristics of the system, or reflecting system's designed purpose, and the effective operation or use of which would be impossible if any one of the principles was to be ignored. A system may be explicitly based on and implemented from a document of principles as was done in IBM's 360/370 Principles of Operation.
Examples of principles are, entropy in a number of fields, least action in physics, those in descriptive comprehensive and fundamental law: doctrines or assumptions forming normative rules of conduct, separation of church and state in statecraft, the central dogma of molecular biology, fairness in ethics, etc.
In common English, it is a substantive and collective term referring to rule governance, the absence of which, being "unprincipled", is considered a character defect. It may also be used to declare that a reality has diverged from some ideal or norm as when something is said to be true only "in principle" but not in fact.

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  1. C

    Electromagnetic waves and the correspondence principle

    Hi! In classical electromagnetic theory the energy of an electromagnetic wave is proportional to its amplitude squared. In contrast, the quantum mechanical equation ##E = \hbar \nu## states that the energy is proportional to the frequency of the wave (photon). Now, according to the...
  2. M

    Pauli exclusion principle question (again)

    I've seen it stated in many places that the reason why atoms don't collapse is due to the pauli exclusion principle. The exclusion principle is given as a required anti-symmetry in the wavefunction of electrons. I don't understand how this principle was derived, or where it comes from. (I've...
  3. T

    Correct Representations of the Momentum Principle

    Homework Statement There are two parts to this question... a)Which of the following are correct representations of The Momentum Principle? (assuming a small-enough Δt whenever it shows up) 1) \frac{Δ\vec{p}}{Δt} = \vec{F}|| + \vec{F}⊥ 2) For every action there is an equal and opposite...
  4. I

    Holographic Principle Projection

    Hi. I have a question about the Holographic Principle. I've been looking up things about it for a while now, and I think I understand it. The total content of a space is propositional to the area surrounding it and not the volume. The thing I'm having trouble with is how everything seems 3d...
  5. StevieTNZ

    Duality principle is 'safe and sound'

    http://phys.org/news/2014-08-duality-principle-safe-apparent-violation.html It seems an experiment was performed back in 2012 that measured which-way information, but also found interference. (I may have come across it at the time, but cannot recall reading about it from memory.) How could this...
  6. M

    Virtual displacement and D'Alembert's principle

    While deriving lagrangian equation using D'Alembert's principle, in Goldstien, they defined a variable called "virtual displacement". Why do we need a concept called virtual displacement? What is its signigicance in classical physics?
  7. J

    Equivalence principle to argue mass and weight are the same?

    Hi pf, I have recently watched a YouTube physics video from SixtySymbols channel which is a channel of short physics videos presented by professors of physics from Nottingham University. The video (pasted below) argues that because of inertial mass and gravitational mass being equivalent this...
  8. microsansfil

    Relevance of Karl J. Friston's Free Energy Principle

    Hello, Does this concept of "Free energy principle ( https://en.wikipedia.org/wiki/Bayesian_approaches_to_brain_function#Free_energy )", developed by Karl J. Friston, showed its relevance ? ie is it a fruitful concept in the field of neuroscience and in biology ? Patrick
  9. B

    On constraint forces and d'Alembert's Principle

    According to d'Alembert's Principle, the virtual work done by constraint forces must be zero. I have a few things needing to be clarified. First, as we know from friction, d'Alembert's Principle is not always true (friction usually does work, and is not normal to the constraint surface). On the...
  10. P

    Aircraft lift - is it all Bernoulli's principle?

    I find it hard to believe that the only factor important in computing aircraft lift is Bernoulli's principle. Doesn't good old Newton's Second Law play an effect? In other words simply deflecting the airflow downwards. Does anyone know the relative importance of these factors? (EG for a...
  11. N

    Proof of Fermat's principle of least time?

    I've been looking for this proof for months but I wasn't able to find something...
  12. Quarlep

    Hesienberg Uncertanity Principle

    I watched a video about particle physics and there I saw something strange(for me) (link is here) and there in 13:44 I saw [x,y] I didnt understand because I think uncertanity principle exist only between position and momentum but he make uncertanity between two coordinates .Can somebody...
  13. N

    Why does light take an extremum path according to Fermat's Principle?

    According to Fermat's Principle (modified version) a light ray takes an extremum path. Can anyone explain the physical reason behind this? I mean, sure the light can take the least path but how can it take the longest path? And if it can indeed take the longest path, why not the paths shorter...
  14. PcumP_Ravenclaw

    Don't understand a principle from Beardon, Algebra and Geometery

    Hi all, I Dont understand a principle from beardon, Algebra and Geometery chapter 2 real numbers... Please answer In the principle of Induction what is 'm'? and what is 'm+1'? are m elements of A how can m+1 also be an element of A?? if A is a subset of natural numbers how it be all...
  15. E

    Bernoulli's Principle Problem/Energy Conservation

    Homework Statement Hi! The problem states: Water through a certain sprinkler system flows trhough a level hose connected to a nozzle which is directed directly upwards. The water leaves the nozzle and shoots to a height, h, before falling back down again into a pool. The hose is connected to...
  16. N

    Energy Principle for Motion in Space

    Homework Statement A 0.2 lb gear is released from rest in the position shown (Okay so this essentially a circular gear in the groves on an adjacent wall that is also circular. Think perpendicular circles with different radii. The gear is a circle, and the gear teeth are in a circular...
  17. R

    What is phase congruency principle?

    Hello everyone,can anyone help me to understand the phase congruency principle. I have read that,in image processing if we reconstruct an image using its phase only,we can get obtain edges and lines and this is because of phase congruency principle. I am searching more about phase...
  18. kq6up

    Fermat's Principle with Calculus of Variations

    Homework Statement This is problem 6.3 in Taylor’s Classical Mechanics. It is in context of the calculus of variations. Consider a ray of light traveling in a vacuum from point P1to P2 by way of the point Q on a plane mirror, as in Figure 6.8. Show that Fermat's principle implies that, on the...
  19. Greg Bernhardt

    What is the uncertainty principle

    Definition/Summary This is a derivation of the Uncertainty Principle based on the properties of non-commuting Hermitian operators. Equations \langle (\Delta A)^2 \rangle \langle (\Delta B)^2 \rangle \geq \frac{1}{4} |\langle [A,B] \rangle | ^2 \langle (\Delta x_i)^2 \rangle...
  20. M

    What is the meaning of the delta in Fermat's principle integral?

    I am unable understand this Integral, what does it actually saying? What does that "δ" means here? I haven't learned Calculus of variations, explain me with diagrams with possible.
  21. M

    Why Is the Denominator in Fermat's Principle (c/n) Instead of (cn)?

    The expression gives time taken to reach a a distance ds in a medium. The expression has the dimension of time, But my question is how come the denominator is (c/n)? why it can't be (cn)? explain me.
  22. J

    Time period of a conical pendulum by D'Alembert's principle

    Homework Statement Finding the time period of a conical pendulum by D'Alembert's principle. The string is of a constant length and all dissipations are to be ignored. Homework Equations The time period of a conical pendulum is 2\pi \sqrt{\frac{r}{g\tan\theta}}. I need to arrive at this result...
  23. matineesuxxx

    Work-Energy Principle: Pulley System

    Homework Statement (see attatchment) We know masses M and m, and I am trying to describe the downward speed of m in terms of the hight from the ground. There is no friction Homework Equations \sum \text{F} = ma W_{\text{total}} = \Delta K = \int \text{F} \cdot d\text{s} The Attempt at...
  24. S

    Holographic principle in reverse

    I'm getting a rough idea of the holographic principle relating the shannon entropy of a boundary surface to the thermodynamic entropy contained within the bounded volume. So far as I understand the primary claim is that the total information needed to describe the entirety of the internal...
  25. C

    Variational Principle for Spatially Homogeneous Cosmologies/KK-theory

    These questions applies to both spatially homogenous cosmological models, and multidimensional Kaluza-Klein theories: Suppose we have a manifold M, of dimension m, for which there is a transitive group of isometries acting on some n-dimensional homogeneous subspace N of M. Thus there exists a...
  26. I

    Interesting book explaining the principle of least action in detail

    Hi, I'm doing further maths and I would like to study maths at university. I have been asked to read a number of books to put on my personal statement, and as I am finding it difficult coming to terms with the fact that mechanics is taught as maths and not physics, a maths don at Oxford...
  27. B

    When does ΔxΔp equal h/4pi in the uncertainty principle?

    When it actually 'equal to' in uncertainty principle? For example under what conditions: ΔxΔt = h/4pi
  28. N

    Proof of Fermat and Huygens–Fresnel principle

    Hey guys, I don't know if here is the right section for this question but anyway. So I was looking for the proof or derivation of the Fermat's principle and all I got was the Huygens–Fresnel principle. However, neither could I understand how the Huygens–Fresnel principle derives the Fermat's...
  29. Demystifier

    Haggard and Rovelli against the Eddington principle

    In a new paper http://lanl.arxiv.org/abs/1407.0989 Haggar and Rovelli offer a new solution to the black-hole information paradox, by proposing a time-symmetric scenario in which black hole tunnels into a white hole. I think the main problem with this proposal is violation of the second law...
  30. D

    Uncertainty Principle and the Infinite Well

    For the infinite square well in one-dimension the wavefunctions have the form Acos(kx) where k is the wavenumber which is proportional to momentum. Now due to H.U.P. if Δx is fixed as the infinite well size we can't know the exact momentum. I presume this is because the wavefunction exists as a...
  31. L

    Archimedes Principle and Buoyancy

    Homework Statement A 100g object having a density less than that of water is placed into a beaker half full of water. The beaker is then placed on a triple beam balance. Would be measured weight be equal to, greater than, or less than the weights of the water, the beaker, and the object added...
  32. B

    What's the meaning of "totally antisymmetric" in Pauli's Principle?

    Pauli's Principle says: "The wavefunction of two identical fermions must be totally antisymmetric". I know that, for a antisymmetric wavefunction, (-1)^L*(-1)^{S+1}*(-1)^{I+1}=-1 "totally antisymmetric" means this relation or it means that these 3 relations: (-1)^L=-1 and (-1)^{S+1}=-1...
  33. H

    Archimede's Principle Question?

    Homework Statement To verify her suspicion that a rock specimen is hollow, a geologist weighs the specimen in air and in water. She finds that the specimen weighs twice as much in air as it does in water. The density of the solid part of the specimen is 5.0 x 10^3 kg/m3. What fraction of the...
  34. M

    MHB Proof of the strong maximum principle

    Hey! :o I am looking at the proof of the strong maximum principle: If a function $u$ satisfies the Laplace's equation at the open space $D$ and is continuous at the boundary $\partial{D}$ and achieves its maximum at $\partial{D}$ and at a point of $D$ then the function is a constant. which is...
  35. N

    Uncertainty principle & simultaneous measurement

    I have been reading and would be interested in feedback to the comments that I make, below. One of the points made in this paper is that the interpretation of the uncertainty relation needs to be re-examined in its relation to the viability of having simultaneous measurements of A & B. For...
  36. A

    Derivation of Kinetic energy formula and energy principle

    Hi all, Here is the derivation of kinetic energy from Work: W = ∫Fds From the second law of motion F = dp/dt, which is equal to mdv/dt, so: W = m∫dvdx/dt which = m∫dv x v because dx/dt = v Therefore W = 1/2mv2, when integrated. However from simple algebra derivation, W = Δ1/2mv2...
  37. I

    Principle Determining the Loudness (Intensity) of Sound

    A test question I was asked earlier in the year has been hanging around in the back of my mind: "If a radio is moved 3x further away from you, how is its loudness affected?" To answer this question one needs to know what determines the loudness of sound. What determines the loudness of...
  38. W

    Navigating Wikipedia for Scientific Answers: Tips and Tricks for Laymen

    I have a question: can the mechanism behind Pauli's exclusion principle be considered a fundamental force, like gravitational, electromagnetic, nuclear weak or strong? Why? Thx.
  39. F

    Basic Probability Theory (Equaly Likely Principle)

    Homework Statement Five cards numbered 1 to 5 are shuffled and placed face down on a table. Two of the cards are picked at random. [Hint: find all of the possible outcomes of this experiment which form the sample space S and use the Equally Likely Principle.] Find the probability of the...
  40. Maxo

    What is the energy principle and how does it apply to this problem?

    Homework Statement A wooden block with mass 1.25 kg is hit by a bullet with mass 5.00 g and the speed vi according to the figure. The bullet sticks in the block and together they go up to the maximum height of 22.0 cm. Calculate the initial speed of the bullet. Homework Equations...
  41. V

    Heisenberg's Uncertainty Principle

    I was watching a YouTube video about Heisenberg's uncertainty principle and was wondering if it only works for position and momentum or does it work for any other two measurable quantities?
  42. R

    Does the uncertainty principle apply at all to photons?

    I know this may sound strange, given that we cannot really work out where in space a photon is because it cannot be completely stopped. But here's a thought. Let us assume that a photon has been emitted in vacuum going in a straight line. At any given moment in time, this photon will have...
  43. R

    What does the symbol \mathscr{P} in Atom-Photon Interactions stand for?

    Hi. I am reading a book entitled "Atom-Photon interactions" by Claude Cohen-Tannoudji, et al. There is a symbol which looks like \mathscr{P}. The authors state that this symbol stands for the principle part. e.g. \hat{\Delta}_b(E)=\frac{1}{2\pi} \mathscr{P} \int dE'...
  44. B

    Heisenberg's Uncertainty Principle

    Hello, i'm solving some quite simple problems using the uncertainty principle, but I don't have access to the solutions and I really don't have a feel for what a 'sensible' answer is... When finding the minimum uncertainty in velocity, I end up with things greater than the speed of light, so I...
  45. S

    Bernoulli's principle, flow rate, velocity and pressure

    Hello, I need some help understanding Bernoulli's principle, flow rate, velocity and pressure. I understand that when the diameter of a pipe decreases, the velocity will increase and the pressure will decrease. But I am having a hard time applying this to a practical application. For...
  46. H

    Fundamental forces and Pauli's exclusion principle

    To which type of Fundamental forces below the Pauli's exclusion belong? Strong nuclear forces Coulomb forces Weak nuclear forces Gravitational forces
  47. F

    Uncertainty principle - Nature of observer

    Uncertainty principle -- Nature of observer About the fact that electrons have a determined position only after having been observed/measured, I don't understand how they make the difference between being watched by an eye/instrument or any non-measuring object just sitting there.
  48. M

    Susskind process and holographic principle

    Why is it that Susskind in describing the holographic principle - why is it that he considers a quantized universe?
  49. T

    Solid Mechanics Stress Blocks- Principle blocks

    Hey guys, I would just like some confirmation that I got this question correct. Homework Statement For the stress block shown in Figure 3a, using the stress equations; a) Calculate the stresses sigmax' sigmay' and taux'y' on a stress block rotated 15° clockwise from the x-y axes...
  50. Q

    Finding Angular Velocity of a Torsion Spring Assembly

    Homework Statement When Θ=0, the assembly is held at rest, and the torsional spring is untwisted. If the assembly is released and falls downward, determine its angular velocity at the instant Θ=90°. Rod AB has a mass of 6kg, and disk has a mass of 9kg. Rod is 450mm and disk has a radius of...
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