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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Why isn't a pseudo force considered for a block on an accelerating block?

I was going through an example where there is a system of blocks where two blocks A and B of mass $m$ are attached through a pully on another block C of mass $M$. The friction between the blocks is $\...
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Two-Body Problem [closed]

Say two celestial bodies are in a circular orbit around each other, how do I find their speed ratios if I'm provided with their mass ratios,say $m_1 : m_2$?
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Centripetal force on the surface of earth

Initially, I was looking for how centripetal force is produced on the surface of the rotating earth for a mass kept at any latitude. I went through the following threads - Which force provides the ...
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2 answers
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Why does pseudoforce act in opposite directions when we move from ground frame to the centre of mass?

In the solution of above question, the authors of the book I'm reading solved it by changing the reference frame from the ground frame to the frame of the center of mass of the masses. But we know ...
2 votes
1 answer
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Find COM velocity with respect to laboratory reference frame [closed]

I'm trying to solve the following homework question. Suppose that in the laboratory frame of reference we have $2$ particles. Particle "$a$" is at rest with total energy $E_a$, while ...
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2 answers
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Deaccelerating to the Speed of Light

The trajectory of an observer with a uniform proper acceleration $a$ (Rindler) in an inertial frame $(t,z)$ can be described by the hyperbola \begin{equation} \left(z+\frac{\gamma_{0}}{a}\right)^{...
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How to apply a screw motion for this case?

I'm completely new to the notion of a screw motion. As far as I know, it carries out the the rotation and translation simultaneously in comparison with the homogenous transformation matrix that ...
9 votes
3 answers
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Physics of the dead man's (or jellyfish) float

This question arised during a discussion with a friend, when I discovered that he cannot do the dead man's (or jellyfish) float, i.e. float horizontally on the water, not even when he was a child. ...
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How is the trajectory of a star found relative to the Sun?

So i know we can get radial velocity by measuring blue shift and then we can use the distance to the star and its proper motion to get its tangential velocity. In the case of Bernards star, its ...
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Moment of Inertia of Cylinder through horizontal axis [closed]

Regarding moment of inertia (MOI) of a solid cyclinder, I want to calculate the MOI through the horizontal axis (central diameter) which is the 2nd diagram in the figure: I want to do so by using the ...
1 vote
2 answers
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Centrifugal force has always been a nightmare to me

Suppose I'm in a rotating space station (that is somewhere in free space) and there is no other force. Now how am I supposed to fall to the circumference of the station if nothing pulls me? I will ...
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Sabine Hossenfelder says time dilation is due to acceleration in the twin's paradox. Is this true?

Sabine Hossenfelder says time dilation is due to acceleration in the twin's paradox. Is this true? At 12 minutes into this video https://www.youtube.com/watch?v=ZdrZf4lQTSg, Hossenfelder states, &...
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Two different observations by different frame of references? How are both observations true and exists simultaneously?

Consider the scenario wherein an electron undergoes acceleration in free space, eventually emitting radiation. Two distinct observers are involved: one accelerates alongside the electron, while the ...
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Shape and even connectedness of accelerating components in SR is frame dependent?

In some inertial frame consider a disk of radius 1 lightyear at rest. Then along the edge of the disk there are some people in spacesuits at rest hovering right above the disk (which has negligible ...
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Euler force for pendulum

Hello I have a question related to the Euler force. Why is this force never considered for a simple pendulum? As far as I understand, Euler force is given by (assume I would consider the 2d pendulum ...
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Does pressure at a point in a fluid flow change with respect to different frames?

Let there be a cylindrical pipe through which a fluid is flowing. Between 2 points at the same height: $$p_1+\frac12\rho(v_1)^2 = p_2+\frac12\rho(v_2)^2$$ Thus, $$p_1 - p_2 = \frac12\rho[(v_2)^2 - (...
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Non-orthogonal frames

Fix $4$-dimensional Minkowskian spacetime $(\mathbb{R}^4, \eta)$. As in my previous posts, a reference frame is then simply a choice of basis vectors $\{e_{\mu}\}\subset \mathbb{R}^4$ such that $\{e_0\...
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Does the first postulate of special relativity imply constant speed of light?

The two postulates of special relativity are The laws of physics take the same form in all inertial frames of reference. The speed of light in free space has the same value $c$ in all inertial frames ...
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In a moving car / bus when the car suddenly accelerates do we go back due to inertia or is it due to pseudo force?

When an car moves forward we go back and we have always read that the reason was that our legs stay in contact with the ground and our body goes back since it was at rest and with sudden motion it ...
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Finding angular velocity of a frame given an angular velocity in another frame

I was trying to understand the Earth's frame of reference. You stick a coordinate system on the surface of the Earth and due to the Earth's rotation the coordinate rotates with respect to the axis of ...
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Confusion about angular momentum of rigid bodies

I know that angular momentum of a rigid body has a parallel and perpendicular component to the angular velocity. I also know that the perpendicular component is due to constraints that keep the axis ...
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Multibody Dynamics - Modeling joints - Spherical verses Gimbal

I am working through a rigid multibody dynamics problem using this paper as a reference: Newton-Euler Dynamic Equations of Motion for a Multi-body Spacecraft. I am trying to understand the differences ...
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2 answers
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Earth as inertial reference frame: finite radius effects

In a recent question I tried to clarify under what conditions the Earth can be considered as an inertial reference frame. The opinions, summarized in my own answer to the cited question, are that ...
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Is the jerk caused by pseudo force when bus come to stop suddenly?

If a bus stops moving suddenly, we tend to fall forwards due to the inertia of motion of our upper body. But can we explain the sudden jerk in the forward direction due to some pseudo force? (Because ...
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Is time period of a pendulum in an accelerating elevator dependent on the weight of the bob?

If a simple pendulum is in an elevator accelerating upwards, Its $T$ decreases. But why is that? The only thing that changes is the apparent weight so does $T$ depend on the weight of the bob but ...
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Reference frames in Minkowskian spacetime

I want to make sure that I understand the concept of reference frame in Minkowskian spacetime. Suppose that $(\mathbb{R}^4, \eta)$ is $4$-dimensional Minkowskian spacetime. Then what we call "...
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Time observed in a clock when you start moving relative to it

There's an example given by Brian Green, https://www.youtube.com/watch?v=XFV2feKDK9E&t=17675s To be precise the example start at 04:56:00. In brief it is as follows: There are two persons Gracie ...
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If time moves slower the faster you go. Doesn't that mean that the gravity experienced will be lower too?

Disclaimer: I still don't understand the theory of general relativity. I'm completely ignorant. I was watching the movie Interstellar yesterday and saw their interpretation of time dilation, I also ...
29 votes
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How does a laser from Earth manage to hit the Moon with precision?

A question I've been asked is how a laser, fired from earth, would hit the moon without "leading it" (or hit it with precision). When firing a laser at the moon, it takes about 3 seconds to ...
1 vote
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Why isn't there such a thing as "internal momentum"?

The three most well-known conserved quantities in classical physics are energy, momentum, and angular momentum. Suppose we have a system with no external forces acting on it. We can talk about the ...
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What is the velocity of centre of mass in centre of mass frame?

Velocity of centre of mass in centre of mass frame is considered zero. But how are the two contradictory statements written in the book?
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Angular velocity about an arbitrary point

Consider a rigid body rotating with angular velocity $\vec{\omega}$. Now, we know that this $\vec{\omega}$ is an intrinsic property for the rigid body, in the sense that: Each point on the rigid body ...
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Does the length of an object change after acceleration in Special Relativity? [duplicate]

In special relativity, an object (a box, perhaps) travelling at 0.5c relative to us, if it thinks it's 1lightsecond long in its own reference frame, will look 0.866 lightseconds long to us. My ...
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Does Kepler's 3rd law of planetary motion violate the first postulate of special relativity?

Consider a distant observer traveling at .866 c relative to the solar system along the line that is co-linear with the sun's axis of rotation. According to his/her wristwatch the observer measures the ...
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Big Ben Paradox? [closed]

Consider a distant observer traveling at 0.867 c ( $\gamma=2$ ) relative to the solar system along the line that is collinear with the sun's axis of rotation. As the clockwork solar system spins ...
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1 answer
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Pseudo forces in accelerating reference frames

I was going through HC Verma's lecture on this and he mathematically derived the pseudo force in reference frames that are going under translational acceleration with respect to an inertial reference ...
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Will the water stay in a falling glass?

Everyday experience tells me that a glass filled with water will fall and smash on the ground as a whole object as it started (water and glass together), given that it falls face up and it does not ...
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Where does the energy come from to cause Coriolis effect?

I read that the rotation of Earth on it's axis caused the deflection of wind along the equator but the air and the Earth are moving together so where do the energy comes from to produce coriolis ...
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Observers in the same frame of reference

Let’s suppose that $\gamma, \gamma^{*} : I\rightarrow U\subset M$ are worldlines of two observers and $\{e_{\mu}\}^4_{\mu = 1}\in TU$ is a frame field. Then we know that for the observers to use the ...
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Why the orientation of an object on Earth rotates with the rotation of earth? Is this due to friction?

The moon always faces toward Earth. That's because the moon rotates itself while revolving around the earth. This might be the result of tidal force for a long time, but what about objects on Earth? ...
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Change of observer vs change of reference frame

I read this statement "A change in observer affects the points in space-time and not the coordinates of points. However, a change of reference frame changes the coordinates of points (and not the ...
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Observer as a future-directed, timelike curve on $M$

Let's suppose that $M$ is spacetime manifold. An observer is then defined as a future-directed, timelike curve $\gamma : I\rightarrow M$, together with a frame field $\{e_{\mu}(\tau)\}^4_{\mu = 1}\in ...
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Moving faster than light, find out velocity in inertial reference frame, or am i stupid? [duplicate]

Say that you are in a really big box that is moving at a constant velocity of 99% speed of light. If you run from the back of the box to the front at 99% the speed of light are you going faster than ...
3 votes
4 answers
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Gravitational Time Dilation and the Apparent Speed of Light

It has been proven that time far away from Earth is faster than time on the surface of Earth, due to gravitational time dilation. (GPS satellites take gravitational time dilation to account.) Would ...
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Lorentz boost that result in $\vec{E}$ parallel to $\vec{B}$

This is a question related to the an answer for this question regarding boosts which make $\vec{E}$ parallel to $\vec{B}$ in the boosted frame. I do understand the approach and the cases 1a and 1b. ...
5 votes
1 answer
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Newton's Bucket, Artificial Gravity, Absolute Rotation, and Mach's Principle

I have been trying to understand how we can talk about absolute rotation in general relativity. I get that it is an area of active debate with some adherents of Mach's Principle and others believing ...
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Classification of connection coefficients

We always can define the connection coefficient using such a formula: $$D_{ \mu} e_\nu(x)= \frac{\partial e_\nu(x)}{\partial x^\mu}-\Gamma^\rho_{\mu\nu}(x)e_\rho(x)=0$$ Here is a problem, the ...
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5 answers
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Is relativity of simultaneity an "observer issue"?

There are some threads about this, but some answers seem to disagree. First, this is what Einstein said on this matter: The light rays emitted by the flashes of lightning A and B would reach him ...
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Reference frames in relativity

Here is how Sachs and Wu do it in General Relativity for Mathematicians (1977). They start by defining a spacetime to be a connected 4-dimensional, oriented, and time-oriented Lorentzian manifold $(M,...
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On the distinction between frame of reference and observer

A Stack Exchange answer illustrates reference frame and observer as follows: A frame of reference means a co-ordinate system and an observer is someone using that co-ordinate system. For example I ...

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