# Questions tagged [inertial-frames]

A specific reference frame that describes its coordinates in a manner that does not depend on time and is isotropic.

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### Do these two refer to the same reference frame? [duplicate]

What exactly is the difference between a reference frame and a coordinate system (with respect to classical mechanics only)? Can we claim that these two coordinate systems is from same reference frame?...
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### Can a non-inertial frame be viewed as an inertial frame?

Let's consider a non-inertial frame with an acceleration of $a$ relative to an inertial frame, if $a$ is really small and we don't need extreme accuracy, can we ignore this acceleration and treat this ...
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### In SR, why do we claim length contraction rather than faster than $c$ travel in the rocket frame? [closed]

In special relativity, a rocket traveling at .5c will take .86 years (in the astronauts frame) to travel 1 light year (in a stationary observer frame). There are two possible ways to look at this ...
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### Meaning of of inertial frames in special relativity [duplicate]

One of the postulates of special relativity is The laws of physics are the same in all inertial frames There is a mention of inertial frames in the postulate. But the predictions of relativity are ...
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### Linearity of Amplitudes and Lorentz Frames

I know that this question may be a bit weird, but I decided to ask. Assume I have an amplitude, say in QED, which depends on a set of four-momenta $\{p_1,\ p_2,\ p_3,\ ...,\ p_n\}$. Further assume ...
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### Do we need a notion of the center of mass in special relativity?

I don't see much mention of the center of mass in discussions of special relativity. I understand that there is a definition for it, but it is relatively obscure and seems to depend on mass-energy ...
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### Differential form of Lorentz equations

A Lorentz transformation for a boost in the $x$ direction ($S'$ moves in $+x$, $v>0$) is given by: $$t'=\gamma\left(t-v\frac{x}{c^2}\right),~x'=\gamma(x-vt)$$ In the derivation of the addition of ...
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### Do magnetic force depend on frame of reference? [duplicate]

Consider two point charges If both charge are moving with velocity v in same direction with respect to a inertial frame of reference. Will the magnetic force would be zero between them? If one charge ...
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### ${}$Doppler shift

If observer is moving and source is at rest then i can treat the scenario with approach in which observer is at rest and source is moving since this is what will be happening according to the observer ...
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I am reading Schwartz's book, Quantum Field Theory and Standard Model. I have some questions about invariance under Lorentz transformations. In the book, the scalar fields are functions of space-time ...
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### Lorentz Boosts and Hyperbolic Quaternions

How do you use the Hyperbolic Quaternion formula for a Lorentz Boost? I'm refering to https://en.wikipedia.org/wiki/Hyperbolic_quaternion : t' + x'r = (\cosh(a) t + x\sinh(a)) + (\sinh(a) t + x\cosh(...
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### Does relative velocity apply before or after the impulse?

I'm solving the following question from Kleppner and Kolenkow: My solution differs from the textbook solution not only in the magnitudes in parts (a) and (b) but also the conclusion in part (c). ...
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### Invariance of Acceleration vs Invariance of Magnitude of Acceleration and help with proof

This question is a half-rant, half-question, as I am genuinely curious as to what the standard physics view is on this question. As someone who has studied math extensively (but not physics), please ...
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### I am basically having doubt about where Lorentz transformation is being applied?

I was solving example 12.14 in Griffiths' Introduction to Electrodynamics) The question was: A point charge $q$ is at rest at the origin in system $S_{0}.$ What is the electric fieldof this same ...
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### Can you tell who is moving through time? [closed]

In relativity, there is no way to tell if you are moving through space. So, if you were inside of a box, there would be no way for you to tell if you were moving or not. However, can you know who is ...
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### If velocity is relative, why does time dilate for some observers and not others? [duplicate]

I'm trying to get my head around relativity and time dilation. What I can't figure out is how time can dilate for fast-moving observers if all velocity is relative. Here's a scenario: Alice and Bob ...
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### If an observer was trapped in a closed box with no way to interact with the external surroundings how will he know if he is moving or at rest [duplicate]

I am a high-school student. Recently we learned the concepts of relative motion and velocity. The idea that anything in motion can subsequently be at rest depending on the frame of reference ...
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### How is it that energy of matter yields gravity if the amount of energy in a system is frame dependent while the force caused by gravity is not?

I've been told that the gravitational field arises due to the energy density terms in the stress-energy tensor of matter and therefore that all energy of matter exerts a gravitational field effect, ...
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### Seemingly equivalent linear form of the Sagnac effect

This is a derivative of the question regarding the Sagnac effect. Judging from the metric $ds^2=-dt^2+(rd\phi)^2$ for a constant $r$ for this question, it should be no different from that on a line ...
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### Special relativity and accelerating twins

Imagine two twins synchronise their clocks and then twin A quickly accelerates to velocity v. After a time T twin B quickly accelerates to 2v and catches up with twin A. Which one will be younger? How ...
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### Transformation of wavefunction

While learning QM, I was wondering how would the wavefunction of a particle, suppose charged particle, look for different observers moving with respect to each other. To begin with, let the electric ...
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### The principle of relativity and why Inertial frames attribute the same velocity to one another

In introductory texts introducing relativity, it is always assumed that frames measure the same velocity for each other. For example if frame S' moves at velocity v with respect to respect, then S ...
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### On Landaus&Lifshitz's derivation of the lagrangian of a free particle [duplicate]

I'm reading the first pages of Landaus&Lifshitz's Mechanics tome. I'm looking for some clarification on the derivation of the Lagrange function for the mechanical system composed of a single free ...
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### Length contraction doesn't affect the perpendicular direction...but it seems as if it can

A is a stationary observer watching B who is moving relative to A. Both of them hold two identical light clocks and each shoots light rays to estimate the lengths of their clocks. A's light ray will ...
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### Unsubstantiated assumption in a problem about two particles rotating around each other

In solving the following problem from Kleppner and Kolenkow the solution assumes there is a point $C$ along the line joining $m$ and $M$ such that Both $m$ and $M$ execute uniform circular motion ...
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### How Fast Must a Man Run to See Rain Falling horizontally?

The fact that tan 90° is undefined indicates a key issue in the problem. Does this mean it is impossible to observe rain falling at 90 degree? When rain is observed to be falling at 90° with the y-...
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### Sagnac effect viewed in the rotating frame

Consider the Sagnac effect. It is quite simple viewed from the rest inertial frame what the time difference should be. However, if viewed from the frame fixed to and rotating together with the emitter/...
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### Time Dilation when they meet

I am starting over because what I was asking was unclear. I have read many articles, such as How can time dilation be symmetric? and others suggested by people here and understand the overall concepts ...
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### Why $\sigma$ wouldn't change under standard Lorentz transformation in Weinberg QFT V1 Ch2

Weinberg defines one particle via state vector $\Psi_{p,\sigma}$ that is eigenvector of four-momentum and a label $\sigma$ to denote all other degrees of freedom. This is physically understandable, ...
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### Centre of mass and reduced mass in classical mechanic

Can we derive reduced mass from Cetre of mass equation if CM is not taken as origin? And Under what conditions can reduced mass concept be used or not?
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### Suppose there is no friction force to balance Coriolis force? [closed]

Question 1: Why in rotating frames, is it necessary that an object's acceleration is always directed radially outwards in the $\hat{r}$ direction and why is there no component in the tangential ...
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### Special Theory of Relativity Contradiction [duplicate]

I’m struggling with an apparent contradiction of special theory of relativity (STR) that I cannot resolve. Imagine two twins flying by each other in space along the same straight line but in the ...
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### Is a Lorentz transformation allowing an infinite value $c$ still a proper Lorentz transformation?
Is it correct to say that inertial systems are related by Lorentz transformations even if we do not know if the "invariant speed" is finite or infinite? To me, this is incorrect because $c$ ...