A reference frame is a particular coordinate system chosen to represent physical entities. The notion is most often used in special and general relativity to dentoe particular coordinates chosen on the spacetime manifold.

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Does a person inside a falling bus fall to the front of it?

So I was watching Final Destination 5 and something caught my attention. There's a part where a bridge collapses and everything falls apart, so there's this bus that has a person inside (unaware of ...
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Kinetic Energy and reference frame

Two space ships A and B moving through space in the same direction. Space ship A moves 2m/s faster than B, its weight is 1kg and it has a 2 Watt engine. It is accelerating with full power for one ...
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How to calculate Eulerian velocity from Lagrangian velocity [on hold]

I'm doing research involving particles travelling through a viscous liquid. I've calculated the Lagrangian velocity, and I was wondering what would need to be done to derive the Eulerian velocity from ...
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Does an orbiting body accelerate?

Wikipedia says that : "Transverse acceleration (perpendicular to velocity) causes change in direction. If it is constant in magnitude and changing in direction with the velocity, we get a ...
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36 views

Don't photons pass photons move away at speeds faster than light? [duplicate]

I understand there is a longstanding rule that nothing travels faster than the speed of light... but if you had two flashlight aimed at each other and turned them on inside a vacuum the photons from ...
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Can we find actual rest mass of things on Earth

Earth moves around the Sun and the Sun moves around the galaxy and the galaxy moves with unknown speed and direction. We have speed so the mass of us all altered. Can we know the real rest mass? If ...
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55 views

Ηow does a current density becomes a charge density?

The transformations in special relativity of current and charge density fields are given as follows: $\bar J' = \begin{matrix} \gamma(J_x - Vρ) \\ J_y \\ J_z \end{matrix}$ $ρ' =\gamma (ρ - {V J_x ...
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Is the fourth dimension of Minkowski Spacetime viewed as an axis? [duplicate]

In the three axes of space, I can move in a positive or negative fashion, up or down, side to side, etc. Why is this not the same for the axis of time, if it is an axis? If it's not an axis, what is ...
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1answer
33 views

Is distance always 0 relative to an object moving at speed of light $c$?

As I understand it, when an object is traveling at the speed of light, relative to itself all travel is instantaneous and the distance is zero. If a photon traveling from the sun was aligned with the ...
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67 views

The invariance vs constancy of the speed of light in vacuum

This is perhaps as much a question of semantics as of physics but it is something I have been thinking about recently and was wondering if anyone else had a perspective on this. Now, it could be that ...
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2answers
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Falling into the sun, a thought experiment for trying to understand reference frames

If someone were transported out into empty space, idk like hundred of thousands of miles straight "up," would they fall into the sun? It would depend on their velocity right? If they were moving at an ...
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2answers
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Conservation of momentum and mechanical energy in different reference frames

I am a biologist developing an interest in physics. I am struggling with the implications of changing reference frames on momentum, mechanical energy and work done calculations. I invented the ...
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1answer
45 views

Proper acceleration of a stationary object

What is the proper acceleration measured for a stationary object? is it 9.8 or 0? Thank you.
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108 views

Shape of a rotating fluid surface

I was going through fluid dynamics and I came upon this section on the shape of a liquid surface in a rotating cylindrical vessel. The working in the book is something like this: It considers a mass ...
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1answer
72 views

Do Einstein's equations allow multiple solutions that agree in a neighborhood of a spacelike hypersurface?

This question is an extension of my a question that I have recently asked: Why doesn't a global frame of reference exist for GR?, where it was recommended that I post another question (so I am ...
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2answers
135 views

Why doesn't a global frame of reference exist for GR?

I only have at best a layperson's familiarity with GR, so forgive me if I am asking a basic question, but I have heard that in GR, we cannot have a global frame of reference, that is a frame of ...
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2answers
323 views

Do the planets really orbit the Sun?

This is a duplicate of this question on Space Exploration.SE. So why would I ask it again here? Read below. The question reads We often say that the planets orbit the Sun, which is usually a ...
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1answer
37 views

Calculating time dilation

I'm writing a science fiction story where the protagonist's experience requires long intervals between events. My plan is to put him/her/it on a spaceship between events at an appreciable percentage ...
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Exact trajectory of a ball projected vertically upward [duplicate]

Wht is the exact trajectory of a ball projected vertically upward at right angle to the earth's surface by a boy standing still on the surface.if observed by an observer 1. Standing on earth's surface ...
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2answers
188 views

Does a rotating disk develop a potential difference between the centre and rim?

This stems from thinking about the question If a perfect conductor where to move what happens to the electrons?. Suppose we have a rotating disk with no external magnetic field, so this is not a ...
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1answer
34 views

'Eulerian' description of a rigid body submerged in fluid

In this paper, equations of rigid body motion (eq 4 and 5 in the paper) are written in Eulerian form (eq 12 in the paper). The rigid body is submerged in a viscous incompressible fluid. ...
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2answers
60 views

Does special relativity happen in reverse when you freeze?

As you travel faster and faster you experience time at a slower rate, so if you travel slower and slower you should experience it at a faster rate right? My thinking is that even if you are ...
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0answers
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Test bodies general relativity

I'm studying section 82 of the Landau & Lifshitz Field Theory vol.2 In this page it's written that the relative position of test bodies can't remain unchanged during time. And ok with this. But ...
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1answer
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Is this one of the reason we can't travel in time? [closed]

I'd like to ask a question that is not in my field but it's bothering me. Based on that, I'm sorry if I make dumb mistake in my assumptions. The idea of travelling in time means "going" at some ...
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1answer
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How to find the period of a simple pendulum in an elevator going up with an acceleration?

How to find the period of a simple pendulum in an elevator going up with an acceleration of $a$. Don't just say, $T=2 \pi$ $\sqrt{ \frac l {g+a}}$ I want to know how the above equation is formed.
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1answer
60 views

Invariant equations of motion under Lorentz transformations

My question regards the statement that an equation of motion may be invariant under a Lorentz transformation I just finished watching the Stanford University special relativity lectures on special ...
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1answer
50 views

The physics behind tilting trains

So I'm trying to get an intuition behind the physics behind banked curves (and tilting trains). I know how to do the calculations, but I'm still struggling with the "why". If I'm sitting in a train, ...
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1answer
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Time dilation: reference frame [closed]

So I'm calculating how long it will take for half of some muons to decay for in a stationary observers frame of reference. They have a half life of 2.2 * 10^-6 and are moving at a speed of .98c ...
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1answer
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Coriolis Effect vs airplane

So, if I build a highway from north pole towards equator and sit in a car, speed pretty fast towards it, I should feel force with the Earths rotation due to the different rotation speeds of this ...
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2answers
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If you fall in a black hole, when do you go past the event horizon? [duplicate]

Say I fall into the event horizon of a black hole. As I cross the black hole, I would appear to outside onlookers to freeze in time, and would never move from that point again. In my perspective, time ...
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2answers
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How does a spacecraft enter Earth's atmosphere?

If Earth whirls around the center of our galaxy at some 220 kilometers per second, how can a spacecraft, which is much slower, enter Earth's atmosphere when returning from Mars for example?
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Path of puck through a turntable

I was looking a problem which tells me to consider myself standing on the ground beside a perfectly flat horizontal turntable rotating with constant angular velocity w. Then I lean over and shove a ...
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1answer
57 views

Superluminal speed?

I came by the quotation below. I'm confused: does it mean that it's actually possible for light and even material objects to move faster than light? Gravitational fields are present the velocities ...
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1answer
139 views

Why doesn’t the Sagnac Effect disprove relativity?

It was my understanding that the speed of light is supposed to be constant for every observer, as in the classical mental experiment of the guy in the train with the flashlights which explain the ...
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2answers
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How does the kinetic energy of a ballerina increase? [duplicate]

When a ballerina pulls her arms in, her rotational kinetic energy increases because angular momentum is conserved. That means that work must have been done on her. I saw somewhere that there is work ...
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1answer
47 views

Seeing in shell reference frame in general relativity

I'm trying to understand how an observer in a shell reference frame sees an object (star) near a black hole. I'm specifically trying to understand the equation: $$\tan \theta_{shell} = ...
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1answer
102 views

Understanding gravitational time dilation / Schwarzschild metric

I've had a look at the answers to these sorts of questions already, but feel like I'm still missing something. Starting with this question, and this one and even this one here. I'm looking at this ...
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2answers
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Why do we feel resistance to gravity rather than the acceleration of gravity?

What's the reason it behaves differently from all other forces? What I mean is, if you're in orbit you're accelerating toward the earth at almost 9.8m/s^2, but you feel nothing. If you are riding a ...
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1answer
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Symmetric Time Dilation in Uniform Relative Motion

I feel (and hope) this is an easily answerable question among physicists versed in GR. I promise that I searched for other answers on the forum. Here goes: Observer 1 starts at X distance from ...
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3answers
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A very elementary question about centrifugal force

I was recently reading some Newtonian dynamics textbook, and then I came across with a problem about the centrifugal effect on mass free falling to Earth. I can mathematically appreciate the fact that ...
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Measuring Hawking radiation after the coordinate transformation

If an observer can measure the Hawking radiation or surface gravity in his co-moving/rest frame, what happens when we make a coordinate transformation to a moving frame of reference? Can we make such ...
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1answer
65 views

Will the Velocity of an object change if it moves fast,close to speed of light?

If an rocket moves with a speed of 0.5c. So, the speedometer in that rocket will show 0.5c. But there will be time dilation when someone sees it from the earth. Then it's Time dilation will be So, ...
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Absoluteness of Simultaneity?

Here is a very good proof that simultaneity is absolute, not relative. The question: Is this proof wrong, or is Einstein? Can you prove it? What is the mistake in the proof, if any? If there is none, ...
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Is there a constant/baseline for how fast time passes?

Say we have an object completely unaffected by the effects of gravity/velocity. Is there a way to measure the passing of time for this object? Since time moves slightly slower for us on or near ...
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2answers
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Speed of light and perception

So I'm reading a book called "The Elegant Universe" and here is a part of it Imagine two countries that have been at war are sitting down to sign a treaty ending hostilities while traveling aboard a ...
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1answer
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Does time really differs drastically?

As I heard/read time moves slower and slower for me as I speed up, but does it change drastically? Our current formulas say, If I go at a speed very close to light's, time almost stops. I've seen ...
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3answers
61 views

Why is the zero of electric potential taken to be $r = \infty$, rather than $r = 0?$

Wouldn't it make more sense if it was taken to be zero at $r = 0$? This seems to imply that with a negative test charge at $r = 0$ from a positive point charge, $V = -\infty$, which I have trouble ...
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3answers
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How can we define a frame of reference in general relativity?

I have started reading general relativity. (A First Course in General Relativity, Bernard Schutz). I am finding very hard to understand a frame of reference. When I was reading special relativity ...
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Explanation of different magnetic field measurements due to frame of reference

If there is an stationary infinitely long wire carrying current I, and we turn on a gaussmeter, it will record a magnetic field B. Since I know the Biot-Savart law, I am satisfied with this ...
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Geodetic effect and Frame dragging

Two gyroscopes pointing perpendicular to each other were housed inside Gravity Probe B which performed polar orbit around Earth to test Einstein's theory of relativity. As the probe is orbiting ...