What is Reference frames: Definition and 227 Discussions

In physics, a frame of reference (or reference frame) consists of an abstract coordinate system and the set of physical reference points that uniquely fix (locate and orient) the coordinate system and standardize measurements within that frame.
For n dimensions, n + 1 reference points are sufficient to fully define a reference frame. Using rectangular (Cartesian) coordinates, a reference frame may be defined with a reference point at the origin and a reference point at one unit distance along each of the n coordinate axes.
In Einsteinian relativity, reference frames are used to specify the relationship between a moving observer and the phenomenon or phenomena under observation. In this context, the phrase often becomes "observational frame of reference" (or "observational reference frame"), which implies that the observer is at rest in the frame, although not necessarily located at its origin. A relativistic reference frame includes (or implies) the coordinate time, which does not equate across different frames moving relatively to each other. The situation thus differs from Galilean relativity, where all possible coordinate times are essentially equivalent.

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  1. Muu9

    B Conservation of KE in a moving frame

    Suppose I accelerate a mass from rest to 1 m/s using n J of energy. From the sun's perspective, I've just accelerated it from (say) 29,785 m/s to 29,786 m/s, which would require 59571n J of energy. Where is the extra 59570n J coming from? If the answer is "nowhere, changes in KE are relative to...
  2. G

    I Reference frames in terms of homogenity/isotropy

    Question 1: in the non-inertial frame, space is non-isotropic. If we're in an accelerated train frame, and we face forward(the same direction where train is accelerating) and drop a ball, ball moves backward. If we face backward and repeat the experiment, dropped ball moves forward to us. So we...
  3. G

    I Using reference frames for derivation of Lagrangian

    I had an interesting thought and it just might be me, but I'm looking forward to hearing your thoughts, but not like the thoughts - "yeah, Landau is messy, complicated, don't read that, e.t.c". Just think about what you think about my thoughts. Landau first starts to mention the variational...
  4. Sciencemaster

    I Is an accelerating frame the same as an inertial frame at a point?

    If we have an observer that is accelerating in one direction (perhaps a rocket ship accelerating towards the sun), would its reference frame be identical to an observer at the same point that is not accelerating, but has the same instantaneous velocity? In other words, is an accelerating...
  5. J

    Switching reference frames between two intersecting lines

    I got no problem with the arithmetic and solving it, finding the intersection of 2 lines is nothing, but I decided to play around with the problem and explore it more and ran into a strange problem, why can't I shift the X axis up? Let me explain. The problem can be set up from 2 different...
  6. A

    I Drag equation - relative flow velocity

    Hello! I have a question about aerodynamic drag. It sounds simple but when trying to understand why the relative velocity can be used in calculations I have some trouble. The formula is 0.5*rho*u2*cd*A where u is the relative velocity between the object and the fluid. The cd value depends on the...
  7. 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)...
  8. 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...
  9. 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...
  10. cianfa72

    I Is acceleration absolute or relative - follow up

    Hello, Some doubt arose me reading this thread https://www.physicsforums.com/threads/is-acceleration-absolute-or-relative-revisited.999420/post-6454462 currently closed. Sorry, I have not be able to quote directly from it :frown: Your claim is not , however, asserting that the spacetime...
  11. B

    I Different reference frames in QM

    Hello! I am dealing with a problem of a 2 level system (an ion in my case) placed in a Penning trap. Basically the ion is moving inside the trap under the influence of the magnetic and electric field and I need to study its inner 2 level system (basically the lowest 2 energy states) while it is...
  12. cianfa72

    I About inertial reference frames and logical deduction

    Hi, consider the following in the context of classic mechanics and SR. We know there exist special "frame of reference" according to free objects stay at rest or keep moving with constant uniform velocities. Suppose you single out a such reference frame according to the Newton law of inertia...
  13. S

    B Why is "time orthogonal to space" in inertial reference frames?

    I'm reading about the geometry of spacetime in special relativity (ref. Core Principles of Special and General Relativity by Luscombe). Here's the relevant section: ----- Minkowski space is a four-dimensional vector space (with points in one-to-one correspondence with those of ##\mathbb{R}^4##)...
  14. Lynch101

    B How Many Reference Frames Can Be Coincident at the Origin?

    In a previous thread, I referenced an argument from the book the Scientist as Philosopher by Friedel Weinert, in which he talks about the reference frames of two relatively moving observers. He made a statement that I had't thought about before, but when I was reading it this time around a...
  15. Arman777

    I Cosmology: Inertial Reference Frames & Misconceptions

    I was reading an article about the misconceptions in cosmological horizons and I wanted to clarify an idea. https://arxiv.org/abs/astro-ph/0310808 In cosmological perspective, is there a global inertial reference frame (maybe CMB) ? Or all inertial frames are defined locally and there is no...
  16. x-vision

    B Reference frame symmetry in Special Relativity

    Hello, I have a couple of questions related to reference frames in Special Relativity. Let's consider a rocket that is inertially moving towards a star with a relative velocity 0.9c. I'd like to look at this example from both the rocket's and the star's perspectives. In the reference frame of...
  17. T

    Is the Earth's frame considered inertial in a moving car?

    I have a really stupid question. Suppose I am in a car and the car is going around a loop. Yes, the Earth is not inertial -- it rotates -- but let me ignore that and, momentarily, designate the Earth as inertial. Now, we know the frame of the car is not inertial--the car is turning. We...
  18. E

    B Confusion over relative velocities and reference frames

    I will refer to the example given in 'On the electrodynamics of moving bodies' concerning a rod moving in a coordinate system, in which a beam of light is sent from one end of the rod to the other and is then reflected back. Usually when calculating relative velocities, we may simply consider...
  19. E

    Difference in energy not the same in different reference frames

    These quantities are evidently not equal unless v1 = v2, but surely the change in kinetic energy should be the same in different frames of reference? I was wondering what mistake or misconception I have made because this has been bugging me for a little while. Thank you in advance!
  20. T

    Are vectors independent of reference frames?

    Ok, this is the notation I am going to use in this thread: uppercase means vectors, while "[V]c" means coordinates of vector V in frame c. I'm reading from a book: i have a reference frame "a" and a reference frame "b" rotated with respect to "a", the vector connecting the frames origin is R. We...
  21. JD_PM

    I Simultaneity of two light rays in different reference frames

    I am studying the fact that two events that are simultaneous in a frame aren't (in general) simultaneous in another. The lamp is equidistant from the two ends. When the light is switched on an observer on the train sees how both light rays hit the back and the front of the train...
  22. B

    I Gravitational potential gradient in accelerated reference frames?

    Hi, Could you please help me to clarify the following problem? In the gravitational field of a mass, the force on a body in steady state comes from the gradient of the gravitational potential - or the gradient of speed of time. But what about accelerated reference frames? I assume that there is...
  23. George Keeling

    A Second order partial derivatives vanish?

    At the end of a long proof I came across something in tensor calculus that seems too good to be true. And if something seems too good to be true ... The something is that a second order partial derivative vanishes if one of the parts in the denominator is in the same reference frame as the...
  24. J

    I Relativistic Reference Frames and the Big Bang?

    Peeling this out into its own thread for clarity: How is time dilation of extreme reference frames (photons, black holes, intergalactic space-time) taken into account in Big Bang cosmology? Since from the POV of a singularity or a photon, their clocks have effectively stopped and any lower...
  25. L

    Relativistic Dynamics Problem - Reference Frames

    Homework Statement Two images are attached. The first image details the problem. The second image has an x',y' coordinate system depiction of the problem. Homework Equations The total energy of a particle is defined as E = mc^2, with m = γ*m_0. The Attempt at a Solution If the x', y'...
  26. T

    Reference frames and Galilean transformation

    Homework Statement I am having a issue relating part of this question to the Galilean transformation. Question Relative to the laboratory, a rod of rest length ##l_0## moves in its own line with velocity u. A particle moves in the same line with equal and opposite velocity . How long dose it...
  27. C

    Understanding meaning of inertial reference frames

    Homework Statement Which of these situations describes you observing from an inertial reference frame? a. you are in the car of a train, seated at a table, and the train suddenly slows down b. you are in the car of a train, going north, and the train goes around a curve to go west c. you are in...
  28. F

    I Non-inertial reference frames question

    Do you really need to absorb an advanced paper to answer the question? Let's view the problem this way: The ship is traveling at a constant .999c relative to Earth just prior to reaching Earth The ship puts on the brakes moments before reaching Earth, so that it is now at rest relative to Earth...
  29. K

    Math of Reference Frames: Spanning Vector Space

    I'm aware that there are definitions of how reference frames translates to mathematics. But I've came to the following. How incomplete would be to say that, mathematically speaking, two Lorentz (or whatever) inertial frames are two subspaces of a given vector space whose span is the same vector...
  30. B

    Elastic Collision in Two Reference Frames

    Homework Statement 1. Two skateboarders start from rest on opposite sides of a ramp like the one in the image, roll down and collide elastically on the level part of the ramp. The masses of the skateboarders are m1 = 48 kg and m2 = 55 kg and they both start from the height h = 4.70m. Ignoring...
  31. F

    Synchronized clocks in two moving reference frames

    Homework Statement My professor gave us a werid question which is as follows In the laboratory frame, two clocks are synchronized and measure two light sources at the same time situated at 3 years in time axis and 3 light years and 2 light years in x-axis >At what velocity is the frame ##S'##...
  32. D

    I The uncertainty principle and moving reference frames

    To what extent is the Heisenburg Uncertainty principle a statement about moving frames of reference? The ill-defined position of a particle seems to imply that one can never find an inertial frame of reference in which the velocity of a given particle is constant.
  33. J

    Circular Motion - Newton's Laws in different reference frames

    Homework Statement A child stands near the middle of a roundabout that is rotating with some angular velocity w. The child moves towards the edge of the roundabout in a straight line from the child's perspective and at constant speed. Explain in as much detail as you can (and using equations)...
  34. Hiero

    Absolutely rotationless reference frames?

    So there are no "absolutely motionless" reference frames, but is there a set of reference frames which could be described as "absolutely rotation-less"?
  35. X

    I Relativity Paradox w/ Charged Spheres

    There is a similar thought experiment I imagined to help me begin to understand the Bell's spaceship paradox: Consider two positively charged spheres, placed side-by-side inside a frame S' with a string stretched to the point where it balances the repulsive electrostatic force between them. For...
  36. L

    I Reference Frame Usage in General Relativity

    In the book General Relativity for Mathematicians by Sachs and Wu, an observer is defined as a timelike future pointing worldline and a reference frame is defined as a timelike, future pointing vector field Z. In that sense a reference frame is a collection of observers, since its integral lines...
  37. F

    I Derivative of Lorentz factor and four-acceleration

    As far as I understand it, the Lorentz factor ##\gamma(\mathbf{v})## is constant when one transforms between two inertial reference frames, since the relative velocity ##\mathbf{v}## between them is constant. However, I'm slightly confused when one considers four acceleration. What is the...
  38. L

    A How these notions relate to the usual SR approach?

    In the context of General Relativity spacetime is a four-dimensional Lorentzian manifold M with metric tensor g, its Levi-Civita connection \nabla and a time orientation vector field T \in \Gamma(TM). In this context I've seem the following three definitions: A coordinate system is a chart...
  39. ElPimiento

    Puzzled by an equation for relativistic time difference....

    Homework Statement Suppose that A', B', and C' are at rest in frame S', which moves with respect to S at speed v in the +x direction. Let B' be located exactly midway between A' and C'. At t' = 0, a light flash occurs at B' and expands outward as a spherical wave. (A', B', and C' are all on...
  40. Q

    Simple Algebra problem, but done with reference frames

    Homework Statement Sonia walks up an escalator which is going up. When she walks at one step per second, it takes her 20 steps to get to the top. If she walks at two steps per second, it takes her 32 steps to get to the top. She never skips over any steps. How many steps does the escalator...
  41. smodak

    I Spacelike hyperbolae -- accelerated reference frames

    Uniformly accelerated reference frames in special relativity are represented in the space-like region of a lightcone as hyperbolae. These hyperbolae represent worldline of accelerated observers. However, there could not be be causal relationship between two events on the spacelike section...
  42. J

    I Deriving distances from reference frames

    Hello, I have a question regarding the following picture: Here, person B is moving in his S’ reference frame with a speed of u relative to Person A, who is in his reference frame S. An event shown by a star (*) happens after tA time according to A. The distance of that event is a distance...
  43. E

    What percentage of the original kinetic energy is convertible?

    Homework Statement Ball 1 has an inertia of 0.500 kg and ball 2 has an inertia of 0.600 kg . Ball 1 is moving away from you at 5.0 m/s, and you decide to throw ball 2 at it to make it go faster. The balls collide head-on, and the coefficient of restitution for the collision is 0.95. Part A)How...
  44. E

    What is the magnitude of the girl's change in momentum?

    Homework Statement A 50-kg ice skater moves across the ice at a constant speed of 2.0 m/s. She is caught by her79-kg partner, and then the pair continues to glide together. He is at rest when he catches her, and immediately afterward they both coast. Part A) What is their velocity just after...
  45. E

    How Fast Must the Woman Walk to Match Her Friend's Speed?

    Homework Statement A woman is on a train leaving the station at3.5 m/s , while a friend waving goodbye runs alongside the car she's in. Call the train's direction of motion the +x direction. Once the train has reached a speed of 10 m/s, how fast must the woman walk, and in which direction, to...
  46. Aswin Sasikumar 1729

    I Will the velocity of light be the same in all reference frames?

    If yes then imagine what I am going to say... From a source two photons are emmutted symultaniously. If one of the photon had eyes to see what will 'he ' measures the velocity of the other photon which is moving with 'him'? Won't it be zero?!
  47. P

    Sound waves and speed of it in different reference frames

    Think at a cop car with a siren that moves with a velocity Vc, it emits a sound with a velocity C Now think about a person that doesn't move, in front of the cop car, shouldn't he register that the speed of the sound emited is Vs= Vc+ C? According to the galileian equations (true for v <<c...
  48. R

    Why is acceleration independent of reference frames?

    I want to know why is the measurement of acceleration independent of inertial reference frames? I mean if displacement, velocity varies with change of inertial reference frames, acceleration should vary. And, one more question: When we say that displacement or velocity varies with change in...
  49. SophiaSimon

    Classical Mechanics: Inertial Reference Frames

    Homework Statement Classical Mechanics: John Taylor[/B] (1.27) The hallmark of an inertial reference frame is that any object which is subject to a zero net force will travel in a straight line at a constant speed. To illustrate this, consider the following experiment: I am standing on the...
  50. B

    B Distinguishing inertial reference frames

    As described in Wikipedia as well as this entry http://physics.stackexchange.com/questions/172739/is-the-lay-explanation-of-the-equivalence-principle-wrong, "being at rest on the surface of the Earth is equivalent to being inside a spaceship (far from any sources of gravity) that is being...