Reference Definition and 1000 Threads

Reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to refer to the second object. It is called a name for the second object. The second object, the one to which the first object refers, is called the referent of the first object. A name is usually a phrase or expression, or some other symbolic representation. Its referent may be anything – a material object, a person, an event, an activity, or an abstract concept.
References can take on many forms, including: a thought, a sensory perception that is audible (onomatopoeia), visual (text), olfactory, or tactile, emotional state, relationship with other, spacetime coordinate, symbolic or alpha-numeric, a physical object or an energy projection. In some cases, methods are used that intentionally hide the reference from some observers, as in cryptography.References feature in many spheres of human activity and knowledge, and the term adopts shades of meaning particular to the contexts in which it is used. Some of them are described in the sections below.

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  1. Astronuc

    How Does the Jacchia Model Predict Atmospheric Drag for Satellites in LEO?

    The subject was part of a presentation today on orbital determination and orbital mechanics. Lots of variables to consider, but in particular for satellites in LEO, how much drag occurs in the atmosphere, and how does it vary peridically (day/night during an orbital period), seasonally (as the...
  2. D

    B Two-body Kepler problem where the Sun is at rest in a coordinate system orbited by another body

    The two-body Kepler problem where the Sun is at rest in a coordinate system orbited by another body: is the coordinate system an inertial reference system or not? Please no yes/no answers. A bit of elaboration is appreciated towards why and which principles apply.
  3. MatinSAR

    Problem about non-inertial reference frame

    Picture of the problem: I wanted to use this formula: (From Classical dynamics of particles and systems Book by Stephen Thornton) O is inertial observer. O' is non-inertial observer. I think ##F## and ##\ddot R## and ##\dot \omega## are ##0##. According to O' the point mass has velocity of...
  4. A

    I Question about electromagnetism and frames of reference

    in this text: my question is in highlighted line: "The two rods have the same length (in S) and contain the same number of charges." why? Considering that the negative rod has movement, it should have a shorter length than the positive rod according to a relativity!
  5. K

    Differential geometry of singular spaces

    TL;DR Summary: Reference request Hello! Reading the book "Differential geometry of Singular Spaces and Reduction of symmetry" by J. Sniatycki https://www.cambridge.org/core/books/differential-geometry-of-singular-spaces-and-reduction-of-symmetry/7D73498C35A5975594605428DA8F9267 I found that...
  6. Ranku

    I Locally Inertial Frames: Freefall & Relative Velocities

    When an object is in freefall, it is in a locally inertial frame of reference. If two objects are in freefall, can their locally inertial frames of reference have different relative velocities?
  7. AronYstad

    Special Relativity - Which reference frame experiences which time?

    This was a practice question, so it had the answer with it, which is 31 minutes. However, I'm confused as to why Lisa experiences T0. It isn't exactly an event happening in Lisa's rocket, but rather her just moving through space. From her perspective, it should look like Earth is moving at the...
  8. shivajikobardan

    Comp Sci Call by reference C++ confusion

    Full code goes here: The goal of the program is to overload the extraction operator for cout. #include<iostream> using namespace std;class Time { private: int hour; int min; public: Time() { hour=min=0; } Time(int h,int m) { hour=h; min=m...
  9. homeworkhelpls

    How is Wave Intensity Affected by Half Amplitude and Double Frequency?

    TL;DR Summary: How do i find the intensity of this wave? I know I is proportional to amplitude / frequency squared, but I don't know what equation this comes from. And I don't know how to answer this.
  10. shivajikobardan

    C/C++ Why should the copy constructor accept its parameter by reference in C++?

    https://stackoverflow.com/questions/2685854/why-should-the-copy-constructor-accept-its-parameter-by-reference-in-c I've read these answers, but most of them aren't satisfactory. They just throw a big wall of text. I get they're trying to say something about infinite recursion, but I'm failing...
  11. Lotto

    B What is x' for Moving Rocket from P?

    I have a rocket and it is moving straight from a point P with a velocity ##v##. When I say that ##x'=0## is at the place we sit in the rocket, then when the event happened outside his rocket at the point P, can I say that the coordinate of the event is for him negative, so ##x'=-vt'##, although...
  12. chwala

    A The boundary conditions in reference to Laplace's equation

    We have inhomogenous dirichlet boundary conditions (well understood)....the laplace equation is a steady state equation and we can clearly see that in 2D..it will be defined by 4 boundary conditions and NO initial condition...having said that; kindly have a look at the continuation below... I...
  13. Haorong Wu

    A Propagation Vector of Light in Kerr Spacetime: Reference Needed

    Hi, there. I am currently reading the paper, Gravitational Faraday rotation induced by a Kerr black hole (https://doi.org/10.1103/PhysRevD.38.472). After Eq. (2.4), it reads that The paper does not provide the derivation of the equations and no related reference is listed. Also, ##k^i## is not...
  14. A

    B Time Shift Reference Point: Earth or Other?

    I have been thinking about the classic example of two twins traveling at different speeds and the effect of time passage. One twin that goes off to travel near the speed of light while the other twin is left on earth. We theorize that the traveling twin will pass through less time and be...
  15. Drakkith

    Passing a Reference to a String in C#

    I'm using C# to program in Unity and I'm having some trouble with getting a tooltip box to update every frame. Simplified, Class A (and several other classes and instances of classes) has a string that stores the tooltip text and updates it every frame. Class B contains the logic to generate the...
  16. S

    Finding the position vector for translated frame of reference

    what would be the y'-x' ##\vec r## vector be? I think it is ##\vec r = (8t - 1) \hat i + (6t - 2) \hat j## (not sure whether it is correct or not.) I thought about it as at t = 0 the position needs to be -1i -2j so that is why I took the signs in the y'-x' frame position vector as a - instead...
  17. S

    Questioning a Physics Problem: Taking Different Reference Frames

    for (a): I basically got the correct answer, but when resolved with taking different reference lines/frames I got a different answer. for the 1st attempt I took y = 0 (for both ##U_{el}## and ##U_{grav}) at the position where the spring is uncompressed. for the 2nd attempt (with wrong solution)...
  18. ohwilleke

    B Where Can I Find The New Daya Bay Results?

    On May 31, 2022, the Daya Bay experiment announced in a press release that they had made a new ultra-precise measurement of the neutrino oscillation parameter theta13 (but did not provide the half line of text necessary to fully summarize the new result). The press release also doesn't provide...
  19. Buzz Bloom

    I Seeking reference for math related to the age of Recombination

    The Wikipedia references is https://en.wikipedia.org/wiki/Recombination_(cosmology). It says: Recombination occurred about 370,000 years after the Big Bang (at a redshift of z = 1100), and the cosmic background radiation is infrared [and some red] black-body radiation emitted when the universe...
  20. curiousPep

    When to consider body fixed reference frame

    Hello I am studying mechanics and I have been reading about having the reference frame fixed at a certain point, body fixed and also the gyro equations. I an identify the gyro case easily as I am looking for an AAC body which rotates about an axis. I am confused about the other two cases in...
  21. AzAlomar

    A Reference for empirical Tight-binding Hamiltonian of spds* vs sps*

    Is there a clear reference article/note for the 20X20 Hamiltonian matrix of the spds* Zinc-Blende system similar to the sps* reference in [1] Table (A) of Vogl P, Hjalmarson HP, Dow JD. A Semi-empirical tight-binding theory of the electronic structure of semiconductors†. J Phys Chem Solids...
  22. U

    Help with identifying a reference for the time-invariant Kaluza-Klein metric

    Homework Statement:: Please see below. Relevant Equations:: Please see below. I am trying to find a reference to a textbook or a paper that details the following time-invariance Kaluza-Klein metric: \begin{equation}...
  23. J

    B A difficulty with the equivalence of all inertial reference frames

    Although I am not a physicist, I am interested in physics, and recently I've been reading about special relativity. I have a doubt about it, a difficulty I see in the equivalence of all inertial reference frames which I haven't found solved anywhere, and I've thought perhaps you in this Forum...
  24. rudransh verma

    Classical Understanding Frames of Reference in Force and Motion for 10+2 Students

    I was starting out some problems on force and motion at 10+2 level. I was told you don’t need to know about frame of reference. But I want to. So which books are there to understand frames? Is it a part of relativity? If yes then which books? Thank you guys.
  25. LCSphysicist

    Charge density seen from a moving reference frame S' (SR + EM)

    There are some question involving the statement. One of them is about the charge density in S' frame. It asks to calc it. I thought that i could calculate the electric field in the referencial frame S' and, then, use the formula $$ E = \lambda / 2 \pi \epsilon l $$ In that way, i would obtain...
  26. shivajikobardan

    Python Best book to keep as reference for python?

    1) Basic python only no data science or machine learning or game development. 2) I have CS experience and some programming experience but I am really bad programmer, so you can discount my programming experience. But I can learn cs concepts on the go. 3) Contains lots of solved and unsolved...
  27. PainterGuy

    How to cite text from a reference without ambiguity

    Hi, Suppose I need to cite some information from a source. I think it'd be easier to explain using an example. In the text below titled "Example Text", I have added the sentences, "Hall effect encoders are also called magnetic encoders. They are very popular with hobbyists these days and have...
  28. morrobay

    B One way speed of light in one reference frame measurement

    Two clocks with photo detectors are 100 kilometers apart at A and B. On the center of AB axis two light pulses are sent to the clocks , synchronizing them. Then a light signal is sent from A to B. The two stationary observers record the time from event at A to event at B. Is there a one way...
  29. L

    I Reference frame vs coordinate system

    Just want to clarify some concepts. There seems to be difference between reference frame and coordinate system. See https://en.wikipedia.org/wiki/Frame_of_reference#Definition . A reference frame is something has physical meaning and is related to physical laws, whereas coordinate system...
  30. V

    B Fluid Continuity Equation in different reference frame

    If I have fluid with area 10 and velocity 10, if the velocity increases to 20 the area will become 5. But if we switch to a reference frame moving at velocity 1 opposite this motion, then it would be 10 and 11 to 5 and 21, violating the continuity equation. What is wrong?
  31. Monsterboy

    Is this a reference frame problem ?

    v1 - velocity of the rain with respect to the ground v2 - velocity of the man with respect to the ground v3 - velocity of the rain with respect to the man So, v1 = v3 + v2 is this right ? So, for the man moving with a velocity v2 with respect to the ground, the rain will have a horizontal...
  32. MattGeo

    I Reference Frame, Difference in Kinetic Energy, Fuel Consumed

    Say 2 cars are traveling side by side at 10 m/s in some flat, wide open space. Relative to each other they are stationary. Relative to someone on the ground they are both moving at 10 m/s. Now say you're in 1 of the cars and you see the other car accelerate, changing his velocity by 10 m/s in...
  33. cianfa72

    I Synchronous Reference Frame: Definition and Usage

    Hi, reading the Landau book 'The Classical theory of Field - vol 2' a doubt arised to me about the definition of synchronous reference system (a.k.a. synchronous coordinate chart). Consider a generic spacetime endowed with a metric ##g_{ab}## and take the (unique) covariant derivative operator...
  34. R

    Finding the velocity of a car in a different frame of reference

    Here's what I did so far. The velocity of the first car is ##v = v_0 +at## Frame of reference S = the road Frame of reference S' = the second car thus, v' is the speed of the first car in the frame of reference S' and v the speed in the frame of reference S. Here's what make me doubt. The...
  35. Rubberduck2005

    Simple conceptual reference frame question

    If a train is moving at some constant V with a bed sitting still on top. When the train decelerates at some rate -A then the bed will move at some acceleration A. I can't seem to get an intuition for how this comes to be. I have looked online and find no help.
  36. S

    I Does the CMB reference frame violate the Cosmological Principle?

    Studies of the Cosmic Microwave Background shows that the Earth is moving roughly 380 km/s with respect to it towards the constellation Leo I think. Yet (I think) the Cosmological Principle and the Michelson-Morely experiments suggest there is no preferred reference frame in the universe --...
  37. L

    B Inertial Frame of Ref & Forces: Understand Special Relativity

    In trying to understand a bit of special relativity, I want to make sure if I understand it correctly, and I came up with the following question: "Imagine you would know all forces in the universe acting upon an object, doesn't that give away the only real existing frame of reference (imagining...
  38. cianfa72

    I Principle of relativity for proper accelerating frame of reference

    Hi, I've a doubt about the application of the principle of relativity as follows. Assume as principle of relativity the following statement: It is impossible by any experiment performed inside a "closed" laboratory to say whether we are moving at constant velocity or staying at rest. Consider...
  39. M

    A Moments and forces in different frames of reference

    Good morning! I know this may sound a little odd, because there is a theorem regarding it, but i have this question. Basically, a CFD analysis gives me the value of the forces and the moments, as a function of fuselage's orientation, in a particular frame of reference. How can i calculate the...
  40. RUTA

    Insights No Preferred Reference Frame: Quantum Mechanics Interpretations

    Continue reading...
  41. yucheng

    Restrictions on the frame of reference

    Newton's laws only hold in intertial frames. In general, the center of mass (CM) is accelerating, so it cannot be used as a frame. However, 1. Suppose that CM is accelerating only in the ##\hat{z}## direction. Does this mean that the CM frame is still valid in the ##\hat{x}## and ##\hat{y}##...
  42. eden24

    Accelerating Rockets in Different Frames of Reference

    pls help me
  43. F

    Body-Fixed Reference Frame -- How is it useful?

    Hello Everyone, I am trying to understand the usefulness of a body-fixed (body-centered) frame of reference ##O'x'y'z'## versus a lab frame of reference ##Oxyz##. The body-fixed frame is attached to the moving body and changes orientation exactly as the body changes orientation. From the...
  44. B

    And An Excellent Keith Laumer Reference Goes Underappreciated

    My youngest is a fabricator for GM, working in their autonomous car division. She's doing so well she just opened a trade school to teach people how to use a five axis milling machine, with her own money no less AND still keeping her day job. (Lord! five axis milling machines cost over a...
  45. F

    Reference frame in collision problems

    2 balls (Ball 1 and Ball 2) collide fully elastically and their relative velocity stays the same as but in sign opposite to that before the collision. Is there any sort of reference frame in which Ball 2 is always fixed (at rest) so that one can look at their relative velocity always in that...
  46. D

    Chemistry Reference book on catalytic mechanisms of enzymes

    Hi everyone, I'm curious if anyone knows of a reference book on enzymes that discusses each enzyme's catalytic mechanism(s). I've consulted several books that explain the different categories of enzymatic catalysis but I'm looking more for an encyclopedic reference that explains such...
  47. J

    Kinetic energies in two reference frames

    A very basic and simple query, but I can't see my way through it. A mass m moves at speed v1 relative to a truck traveling at speed v2 , fig.a. All components except this mass are massless. In a truck-stationary frame, the mass collides with a barrier on the truck liberating kinetic...
  48. E

    Calculus Practical reference for integration on manifolds

    I was trying to look for something that works a lot of examples of integrals over surfaces, volumes etc. in general relativity. Tong's notes and some others are good on the abstract/theoretical side but it'd really be better at this stage to get some practice with concrete examples in order to...
  49. wrobel

    I Which Textbook References Hamiltonian Systems with Equal Orbit Periods?

    There is a nice fact. It approximately sounds like that: Let ##H=H(p,q)## be a Hamiltonian system with ##n## degrees of freedom such that all its orbits are closed. Then the periods of all the orbits belonging to the same energy level are the same. Please which textbook does contain this? I...
  50. cianfa72

    I Reference frame vs coordinate chart

    Hello, here on PF I've seen many threads about the concepts of 'reference frame' and 'coordinate system'. In the context of SR my 'envision' about the concept of 'frame of reference' is basically the 'rods & clocks latticework' as introduced in the book Spacetime physics (Taylor, Wheeler)...
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