A system of coordinates used as the basis for describing the position and motion of objects

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Will Earth clock be slower than aeroplane clock when viewed from another planet?

I know that a clock placed on an aeroplane will have slowed with respect to a clock placed on earth because the more our velocities are, compared to the speed of light, the more the time slows down. ...
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1answer
127 views

Does gravitational lensing violate Fermat's Principle that light must travel in straight lines?

Does bending of light due to warping of space violate Fermat's Principle or is it that in the principle light goes in a straight line with respect to space (taking space as the reference) and in ...
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2answers
159 views

Centripetal force in frame of reference of body moving In a circle

Suppose a body is moving in a circle about a fixed point. In the frame of reference of the body, is the centripetal force felt or is only the centrifugal force felt? More generally, does a body only ...
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45 views

Clocks tick steadily, so why is there no photon time? [duplicate]

Consider a photon bouncing left and right between two mirrors in a photon clock. Seen from inside the clock, the photon bounces at a constant frequency. Time ticks regularly. No matter whether the ...
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1answer
111 views

What is the significance of Einsteins postulate on speed of light?

Einstein postulated that the speed of light in free space is the same for all observers, regardless of their motion relative to the light source, where we may think of an observer as an imaginary ...
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1answer
128 views

What do subatomic particles look like for a 'speed of light - observer'?

Let's say that an observer is moving with the speed of light relatively to an atom that he wants to look into. He has equipment that precise that he can observe the atom and what is inside. From ...
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384 views

Throwing a ball upwards in an accelerating train

If I throw a ball upwards to a certain height in an accelerating train, will it end up in my hand? At the moment I release the ball, it will have a velocity equal to that of the train at that instant. ...
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1answer
126 views

What really happens with Time Dilation? [duplicate]

I know if you move your time moves slower than someone who is stationary, by Lorentz's transformation. However, I don't get how this happens. What does it mean when time moves slower? How does it ...
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4answers
209 views

Is there a distinguished reference system, after all?

The equivalence principle, being the main postulate upon which the general relativity theory rests, basically states that all reference systems are equivalent, because pseudo forces can (locally) be ...
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162 views

Do heavier objects fall faster? [duplicate]

This question has been asked multiple times here and all over the internet yet I can't find a conclusive answer: Some claim that heavier objects do fall faster: Don't heavier objects actually fall ...
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1answer
102 views

Why is body frame angular velocity nonzero?

This question is relevant to Euler's angles and Euler's equations for a rigid body. Why aren't $\omega_1$, $\omega_2$ and $\omega_3 = 0$ in the body frame? How can we measure $\vec\omega$?
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100 views

Paradox of angular velocity

For a torque-free symmetric top, the Inertia tensor has an inverse $I^{-1}$, and $L=I\omega$. Which implies that $\omega=I^{-1}L$. But since $I, L$ are constants, $\vec\omega$ is a constant. However, ...
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170 views

Will an object resting on a rotating platform move in a frictionless world?

Imagine that a pebble is placed on a uniformly rotating, frictionless disk. What will happen to this pebble? Will the disk slide under it and the pebble stay as is? Or will there be a centrifugal ...
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2answers
185 views

What does it mean to be stationary?

I'm looking for a simple answer. What do we regard a stationary. Do we mean an object that is not moving noticeable from the viewers perspective because then a parked car would be considered ...
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Is a photon “fixed in spacetime”?

From what I've read, according to relativity, a photon does not "experience" the passage of time. (Can we say there is no past/present/future for a photon?) Would it be better to say a photon is ...
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2k views

When does not Newton's 3rd law apply?

Is Newton's 3rd law valid in non-inertial frames? If so, then are there other cases for which Newton's 3rd law is not applicable?
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1answer
221 views

Will a free-falling rod (without drag) rotate?

When we consider a bicycle is turning on a flat plane, we know that there is friction, which provide centripetal force on the bicycle. And we know that the bicycle is no longer perpendicular to the ...
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2answers
160 views

Twin Paradox in case of two twins that don't meet

If there are two twins. such that one of them goes on a space voyage maintaining a constant velocity, and that one never returns to earth, and the twins decide to never meet but send information about ...
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2answers
247 views

Can we show that time is orthogonal to space?

It's easy to show that the time we measure is "in a different direction" from the space directions we measure. However, it's not immediately obvious to me that these directions are orthogonal. How do ...
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1answer
234 views

Frame of reference for moving charge in relation to a magnetic field?

We know that a moving charge produces a magnetic field. But which frame of reference it must move to produce magnetic field? And also if a charged particle moves in a magnetic field, magnetic force ...
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2answers
120 views

Time Slowing Down [duplicate]

I'd like to ask a question if I may about the relativity. When travelling at the speed of light(which is impossible for particles with masses) or very close to it, would it be me, myself, seeing my ...
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1answer
197 views

Why do you experience low-gravity outside the Earth (but near it), and experience more gravity on it?

Spacecraft, satellites, etc. They also experience zero gravity, weightlessness, or "micro gravity" when outside Earth, about 20-300+ miles beyond the escape velocity zone("atmosphere"). However, you ...
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2answers
145 views

How does artificial gravity reduction (weightlessness) work on Earth?

Some of us have been to those amusement park/carnival/theme park rides where you enter some capsule/machine, and you will float, etc. There is also some other thing where you go inside a big room ...
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1answer
325 views

Condition for circular orbit

I am a little confused about the condition for circular orbit. Goldstein's Classical Mechanics has the condition for circular orbit as $$f'=0\tag1$$ where $f'$ is the effective force. I understand ...
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156 views

Is isotropy a fundamental/invariant feature of our universe, or is it merely a convenient, albeit arbitrary, feature of some reference frames?

This is related to a previous post. Assuming that the Cosmological Principle is correct, does this imply that the universe possess an empircially privileged reference frame? What I am trying to ...
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Principle of relativity - a second, equivalent form, using invariants

Most people state the principle of relativity like this: "The rules of physics must take the same form in all inertial frames." Question: is this an equivalent way of saying the same thing: ...
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168 views

Internal/Rotational angular momentum

I have some difficulties to understand the relation between the internal and the rotational angular momentum of a rigid body which is also known as König's theorem, so what physical intuition lies ...
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211 views

Intuitive understanding of centripetal vs. centrifugal force

I am having trouble understanding how centripetal force works intuitively. This is my claim. When I have a mass strapped on a string and spin it around, I feel the mass pulling my hand. So, I want ...
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1answer
172 views

How do we know the Earth orbits around the Sun and not the other way around? [duplicate]

I know that describing the trajectory of all planets in the solar system around the Earth is much more complicated than if we take the Sun as the reference point. But besides this, what is the ...
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2answers
247 views

What would earth look like if traveling at 77% the speed of light?

I've been having a bit of trouble wrapping my mind around this. If I were in a ship going 77% the speed of light (enough to experience reasonable time dilation) would I see earth going in fast ...
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1answer
276 views

Frame of Reference Question

I'm having trouble understanding this frame of reference question. You are traveling in a car going at a constant speed of 100 km/hr down a long, straight highway. You pass another car going in ...
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2answers
227 views

Watching something fall into a black hole from far away

I am observing (theoretically) an object falling into a black hole from a safe distance away. My understanding is that from far away it appears as if the body will asymptotically approach the event ...
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2answers
145 views

Variance in speed of light

While discussing this question (Does light have an unending journey?) I stumbled on the fact that light's speed is constant only in inertial frame. What I happened to do was add up the expansion of ...
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Why rendezvous attempt failed on Gemini 4?

It is said in Wikipedia, that On the first orbit, McDivitt attempted to rendezvous with the spent Titan second stage. This was unsuccessful for a number of reasons: NASA engineers had not ...
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1answer
214 views

Reference Frame and Angular Speed Related?

I am given the following problem: If an airplane propeller rotates at 2000 rev/min while the airplane flies at a speed of 480 km/h relative to the ground, what is the linear speed of a point on ...
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1answer
283 views

Is General Relativity applicable for all coordinate systems?

My understanding was that relativistic physics can be expressed in any inertial coordinate system, but not arbitrary systems. That is, no experiment can determine if we are "still" or "moving" at a ...
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3answers
397 views

The reference frame of $c$

I don't have a lot of knowledge of special relativity and associated topics; some of the few things I know are that "all motion is relative" (that is, there is no 'stationary reference frame'), and ...
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1answer
189 views

If two observers pass each other in opposite directions at $.5c$ what would effect on each others clocks be?

Assume an alternate universe with same physical laws as here. In this universe nothing exists except 3 observers, each in a transparent box with a clock. Observer A is travelling at .5c. Observer C is ...
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4answers
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How should observers determine whether they can be described as being “defined on a Lorentzian manifold”?

Consider infinitely many distinguishable observers, no two of whom ever meet; and who generally "keep sight of each other", but not necessarily "each keeping sight of all others". How should they ...
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2answers
100 views

Whose reference frame to use for $d \theta$ near a black hole?

Using the Schwarzchild metric for a body circularly orbiting a nonspinning black hole (i.e. $dr=0$), the relation between $d\tau$, the time between two light pulses sent out infinitesimally close ...
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1answer
313 views

Reference frame involved in the Schrödinger's equation

I have a doubt as to in which frame of reference the Schrödinger equation is written? I think it is inertial but can't reason it out.
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129 views

Frames of reference: Inertial and accelerated - and jerked, snapped, crackled and popped?

There are inertial frames of reference and the accelerated frames of reference, but are there any frames of references w.r.t. higher order derivatives of velocity? [1] [2] For example, jerked frames ...
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1answer
103 views

Speeds and frames of reference

When reports state that Voyager is traveling at ~17kmps or that a comet is traveling at 600kmps, what are these numbers relative to? Given that the Earth travels at ~30kmps and orbits the Sun which is ...
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258 views

Why do we feel the passing of time?

Why do we feel the passing of time? Why do we feel the time is changing with increasing speed $d\tau=\gamma^{-1}dt$? In other word why Lorentz factor (or scientifically relativistic velocity function ...
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2answers
338 views

If I travel close to the speed of light and come back, why is everyone else dead, and not me? [duplicate]

Consider the following scenario: I get in a spaceship, and travel really close to the speed of light for a while, and then come back. A lot of time has passed on the Earth, but since I was traveling ...
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2answers
80 views

The speed of light through space and its effects on black holes

I was curious about a couple of things that don't quite make sense to me: (and probably won't when I get the technical answer, but I'll ask it anywise.) When light travels through space, is its ...
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2answers
2k views

Why does light always travel in a straight line?

No matter the frame light is in, it always moves in a straight line in that frame. Why is that? It doesn't seem like something to me that should necessarily be true. If some one runs forward and sends ...
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2answers
353 views

How fast can time flow? A question of time dilation

(I would describe myself as an illiterate physics enthusiast, so I hope you'll forgive me if my ignorance is borderline offensive.) If I've understood anything of the concept of time dilation, your ...
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0answers
31 views

Remote control wireless helicopter hovering in cruising aircraft, will it move the aircraft with stability [duplicate]

A Mini Toy Helicopter is left for hovering inside the cabin of an Aircraft, while the aircraft is cruising at the speed of 900 Km/h. Helicopter has no contact with the surface of the aircraft. The ...
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2answers
652 views

Is the newton's second law for a body experiencing a frictional force, relative to the surface in contact?

Let us consider a routine mechanics problem.A block of $ 10kg $ rests on another block of $ 2kg $ on a frictionless table.Let $ \mu = 0.2 $ be the coefficient of friction(let's neglect the difference ...