# Questions tagged [reference-frames]

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 denote particular coordinates chosen on the spacetime manifold.

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### How to explain path of light in different reference frame?

There is a standard diagram for deriving time dilation in special relativity. So say there's a ship in deep space with observer O' in inertial reference frame S' moving to the right with respect to an ...
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### Why specifically should a Foucault pendulum be long and massive?

For a pendulum to easily demonstrate the Foucault effect, it should have as long a cable as possible (this one is 52 feet) and a heavy symmetrical bob (this one is hollow brass, weighing about 240 ...
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### The work done by Tension and Normal is not always zero

Some forces are perpendicular to the displacement of a body placed in a "vehicle", if the vehicle at rest. But, the motion of "vehicle" causes the body to move in some direction ...
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### Why didn't he consider the centripetal force in the left hand side of the equation?

Note:- Figure is the screen shot from https://www.youtube.com/watch?v=ofgusnhQ07Q Doubt regarding the application of Newton's second law in the radial direction. Left hand side of the equation ...
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### What is this one parameter subgroup of the Lie group Diff(R)?

I am interested in the time reparametrization. t is replaced by $\tau$. See how the infinite dimentional lie group Diff(R) is introcuced. t is a real number of R. and $\tau$ a function on R. To have ...
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### Why is the centripetal force non-conservative while the centrifugal isn't?

Aren't both perpendicular to $R$? Also, doing $\nabla \times \vec{F}$ using $\hat{r}$, $\hat{t}$, $\hat{k}$ as versors (with $t$ tangent to $r$ and $k$ perpendicular to the surface) i get $0$ with ...
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### Type of friction on circular road [duplicate]

Why is the friction static while driving a car (at constant speed) on a horizontal circular road? Please explain in details.
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### How does brain perceive time dilation due to gravity?

Einstein's General Relativity says gravity warps spacetime. Consider a hypothetical scenario: A person travels into space from Earth. He landed on a different planet in some far off galaxy where time ...
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### If the fictitious forces (ex: centrifugal) help the observer feel stationary, why does the person feel a force in the opposite direction?

I searched and viewed similar questions on here but still am very confused. Is it because of the observer's inertia? If so, shouldn't we feel a 'push' towards the point where we are actually ...
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### Change in Kinetic Energy in two different reference frames (one of them non-inertial) [duplicate]

Let's say an object of mass 10 kg is fired from a cart of mass 90 kg. The object and the cart, of total mass 100 kg were initially moving together with a speed of 10 m/s. Then, the object is fired by ...
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### Can there be a theoretical synchronised ‘now’ moment at all points across the universe?

Einstein’s relativity rejects the notion of a universal ‘now’ moment. It underlines how the concept of ‘now’ is compromised due to time passing at differing rates in differing frames of reference, ...
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### Is it correct to say that; an object moving at a constant velocity is moving in a straight line?

I think it is correct because: v = displacement/time, if the direction changes, the displacement changes so the velocity is not constant anymore. Please correct me if I'm wrong
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### Find angular momentum using center of mass frame

Suppose I have a square-shaped plate getting hit by a ball as shown in the picture below (notice how the force vector applied by the ball is not parallel to the $r$ vector). Let's set the origin to be ...
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### Vectorize altazimuth coordinates?

Is there a way to convert a pair of altazimuth coordinates (one for each of two observers) into vectors so as to compute the difference between them (i.e. both altazimuth coordinates point at the same ...
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### Normal force acting not on the center [closed]

I have a question about normal force: How do you find it's magnitude when there is just on contact point betwenn bodies and not directly below the center of the mass? For example, how would one ...
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### Local Lorentz Reference Frame

What is a local Lorentz reference frame? How exactly does it differ from an inertial reference frame or a locally inertial reference frame? Are there synonyms of this expression?
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### Special Relativity - When and where did the rod start moving? [closed]

In the below figure, A rigid rod is at rest. End point $A$ and $B$ are at $x=0$ and $x=1$ respectively. At $T=0$, in an instant $A$ starts moving at velocity $+v$. i.e. coordinate of this event i.e. ...
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### Special Relativity - Need a physical argument

Consider the following experimental setup: 2 parallel lines close to each other. The direction of +ve x-axis is same for both the lines. There is a standard 1-meter rod on each of these lines. Let ...
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### Multidimensional center-of-mass and relative coordinates

For a two-dimensional cartesian coordinate system $(x,y)$ describing two particles of unit mass, one frequently encounters a transformation into center-of-mass and relative coordinates $(R,r)$ defined ...
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### Is there a proof that a sum of torques equals a net torque?

I apologize for asking a question similar to this before, where in particular I asked for a derivation of Fr = Iα from the presuppositions of F = MA and the conservation of angular momentum. However, ...
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### Transformation to rotating frame

I want to apply a transformation to the rotating frame of a two level system such that a state in the transformed frame is $|\hat{\phi} \rangle = U |\phi \rangle$, where U is the generator of ...
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### Mach's conjecture and the twin paradox in an empty universe

What would happen if we revisit the twin paradox in an empty universe? Indeed, according to Mach's principle, when a spaceship accelerates in our universe filled with too many stars and other objects, ...
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### Reference Point and Change in Potential Energy

Okay so I am VERY confused. Everything online is telling me that I can choose any reference point for potential being zero and still get a consistent result for potential difference HOWEVER I have ...
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### Derivation of the acceleration of an observer through direct use of the Lorentz transformation

In his paper "Uniformly accelerated reference frames in Special Relativity", E. Desloge describes the acceleration of an accelerated observer through what he refers to as a direct ...
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### Validity of a differing frame of reference than that used in Landau and Lifshitz's solution [closed]

For the following problem, first problem in chapter 2 (page 16) of Landau and Lifshitz's Classical Mechanics text: I am trying to see whether the picture I drew when originally solving the problem ...
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### How is the Earth an approximate inertial frame? How can one validate this?

I have always been confused by the idea of Earth being an approximately inertial frame as the speed at which it rotates is so high. Everywhere vaguely it is referred through calculation it can be seen ...
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### How to prove that a particle or a body moves in a circular trajectory with respect to centre of mass? [closed]

In some situations in mechanics, observing motion of bodies with respect to centre of mass often gives useful insights to visualize the situation and obtain many results.In some cases the motion of ...
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### How to understand pseudo forces in the Lagrangian formalism?

Consider a free particle in a rotating reference frame. Let us say the coordinates of the particle are given by $(x',y')$ in this frame. For an observer on this frame, (I suspect) the Lagrangian is ...
Considering that the luminiferous ether exists around earth, if all electrodynamics experiments are done in earth frame how do we know that the speed of light $c$ we calculated from it is not $c$ with ...