# 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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### Moment of Inertia of cube about the axis along one of its diagonal [closed]

How can we find Moment of Inertia of a solid as well as a hollow cube about the axis passing from one of its diagonal? I tried really hard solving it, please guide me to the solution.
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### Transforming normals into a specific coordinate system [migrated]

Let's say I have normals defined from points of latitude and longitude on a sphere (represents the satellite object). The coordinate system we want to transform these normals into is z points to ...
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### How do I know if a motion is 1 dimensional or 2 dimensional?

If an object is moving in a straight line with an angle with x axis (it may be vertical or horizontal) , is it 1 dimensional or 2 dimensional? The question was asked by my teacher and he himself gave ...
1 vote
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### Relative acceleration, mechanics

Consider a system of two objects with mass $M, m$ as presented below. Assume that there's no friction between both the surface, M and between M, m. I'm asked to find the acceleration of m, M using ...
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### Resolution of Ehrenfest paradox using only special relativity

It was my understanding the solution to the Ehrenfest paradox requires non-Euclidean geometry. Observers riding the rotating disk measure small distances according to the Langevin-Landau-Lifschitz ...
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### Is angular velocity about any point in a rigid body always the same?

Suppose I have a disc that is in translational as well as rotational motion. Question 1: If we talk about the angular velocity of the disc, about which point is this angular velocity measured? (I mean ...
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### Motion of person in bus in inertial and non-inertial frame

A person is sitting in a bus that is moving with constant velocity, due to friction their is no relative velocity between bus and the person. Suddenly the bus starts to move with a constant ...
1 vote
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### Can the motion of a rigid body be decomposed into translational and rotational motion about ANY point?

My textbook mentions Chasles' theorem which asserts that " it is always possible to represent an arbitrary displacement of a rigid body by a translation of its center of mass plus a rotation ...
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### What is the correct way to think of position?

How accurate would it be to think of position (along some axis) as the component of radius vector. Example: $$\textbf{r} = x \hat{\textbf{i}} + y \hat{\textbf{j}}$$ And if that is correct, we could ...
1 vote
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### Why do objects in circular motion move outwards instead of inwards? [duplicate]

I know that whenever an object undergoes circular motion then it has a centripetal acceleration towards the centre and a tangential acceleration. Due to these accelerations I had thought that ab ...
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### Justifying that the gold nucleus is at rest in a Rutherford experiment

This is an example on the Rutherford Experiment from Young and Freedman's University Physics. In the last paragraph of the solution the book states that it is valid to assume that the gold nucleus ...
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### Do you always experience the gravitational influence of other mass as you see them in your frame?

You see a galaxy far away. That galaxy is attracting you with a certain amount of gravity. I'm wondering if the gravity influence of the galaxy on you, as measured by you, always ends up being what ...
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### Tug of war between observers in frames with different rate of time

You have a very dense hollow sphere of matter. Observer A is inside the sphere inside a rocket. Observer B is in an identical rocket outside the sphere where the ring's gravity is negligible. They are ...
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### Non-inertial frames in quantum mechanics

In classical physics, non-inertial frames necessitate adjustments to Newton's laws due to acceleration and rotation, yet in general relativity, Einstein successfully incorporates such frames. Why does ...
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### Do off-centre forces create additional energy?

When a force is applied to a rigid body at its centre of mass, it accelerates. Over time, that acceleration becomes velocity, which gives the body a certain amount of kinetic energy. However, if that ...
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### What is the relationship between gravitation, centripetal and centrifugal force on the Earth?

I'm trying to analyze a situation wherein a ship is moving across the surface of the earth. I am trying to analyze this situation in a reference frame that is rotating with the earth (NED frame). I am ...
1 vote
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### Relative speed in unpolarized cross-section

In section 5.1 of Peskin and Schroeder, we are presented the computation of the amplitude for the $e^+e^-\to \mu^+\mu^-$ reaction and then the computation of the unpolarized cross section. After ...
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### What is the physical significance of Moment of Ineria about a point vs Moment of Inertia about an axis. In which types of problems we use the former?

I know the mathematical formula of both about an axis and a point. But where exactly we use moment of Inertia about a point. For 2d objects like discs rotating in its own plane, the moment of Inertia ...
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### A pop pop boat heated with a self-contained heat source and run in a superfluid appears to show a preferred frame [closed]

Suppose there is a pop pop boat but the water is replaced by a superfluid and the candle is replaced by a radioactive heat source The pop pop boat should accelerate forwards; Now let’s imaging a ...
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### Pseudo Orbital motion only due to Coriolis force

Planet, say of mass M and radius R is rotating with some angular velocity ω and a object of mass m (initially on the surface and rotating with the planet) was launched with velocity v vertically ...
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### Why, when we calculate forces in banking of roads, we do not take the centripetal force but the centrifugal force?

I know the we only apply centrifugal force when in non inertial frame of the object but in banking of road questions, why cant we make equations from inertial frame rather than non inertial frame? ...
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### When you are in a gravitational field, do object far away get physically closer to you as you get closer to the mass?

An observer A is close to a black hole and an observer B one light year away. They are both remaining at constant radial distance from the black hole. A is at 2 Rs away from the center of the black ...
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### Does two clocks moving toward each other at the same speed, then decelerating at the same rate coming to a stop create a paradox? [duplicate]

Note I have asked this question previously but upon reflection I realized I was not specific enough, leading to confusion in the answers. I have decided to ask the question again instead of editing my ...
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### Drawing a free-body diagram for a body residing on Lagrange point $L_4$ and $L_5$

I am aware of the fact that Lagrange points $L_4$ and $L_5$ are stable equilibrium points however I cannot understand why they are equilibrium points in the first place. Consider a Earth-Moon system ...
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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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### Forces in tangentially-accelerating circular motion

Let's say a car is moving along a semicircular path. It moves with constant speed for the first half. Then, it accelerates with constant tangential acceleration for the second half of the semicircle. ...
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### Where did the rotational energy come from? [duplicate]

Consider a floating static pencil in flat space with no gravity. If we apply an impact force F for a short time t to the pencil vertically, pencil must get a CM momentum Ft, and corresponding kinetic ...
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### What is angular momentum and torque due to the center of mass? Give me an example

when I was studying angular momentum and fixed axis rotation in kleppner book, I had a topic on motion involving translation and rotation. On that topic, they said that components of angular momentum ...
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### Radiation from accelerated charged in a co-accelerated reference frame

I was wondering if I sit on a accelerated charge particle I will not observe radiation since the particle is at rest according to me. But my friend observes it from outside and he will see radiation ...
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### Torque applying on both rotating and non-rotating object

In rod reference frame: origin is center of the rod, pitching axis is coinciding with the rod external torque is applied in rolling axis If a torque is applied to non-rotating object(the rod and the ...
1 vote
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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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### Newton's second law when the force does not pass through the center of mass [duplicate]

If I have a stick with even mass distribution, and for scenario (1) I give it an instantaneous force at the tip, how can I determine the speed of the center of mass? Is it by Newton's second law? ...
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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 ...
1 vote
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### Radiation from a stationary charge relative to earth for a free falling observer

It is my understanding that if we have a charge at rest on earth, a free falling observer will see it radiating, as the charge is in an accelerated frame of reference. This observer can in principle ...
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### Two questions on the instantaneous centre of rotation

In one of the "Additional solved problems" in chapter 6 of Analytical Mechanics, an Introduction (Fasano, Marmi, 2006) the following is stated: Let $C(t)$ be the instantaneous centre of ...
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### How does $g$-force work?

I am attempting to correct a previous, unclear question. I was doing an assignment on rollercoasters and it stated that $g$-force was a major part of rollercoaster safety. It also related $g$-force to ...
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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 ...
1 vote
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### What is the point of torque?

I am having a hard time understanding torque. What is the physical significance of torque? Why is it defined as $\mathbf{r} \times \mathbf{F}$? I know that torque helps us to calculate the angular ...