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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Using time-dilation to our advantage?

Assume we have a super-computer working on an important problem, and we estimate it will take about $200$ years to solve. But, we need the answer in $20$ years. Could we ship the super-computer into ...
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44 views

Understanding Mach Principle

I cannot get the essence of Mach's Principle. In my undestanding, it states that the cause of the so-called "fictitious forces" which arise in non-uniform motion has to be found in the relative ...
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Does a magnet moving in a uniform electric field experience torque?

Assume a uniform electric field of $E_y$ along $y$ in the lab frame of reference $(x,y,z,t)$. A simple magnet bar is set in motion at $v$ along $x$ in this electric field so that the alignment of the ...
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42 views

Inverted Pendulum on a Cart Problem and Moment of Inertia

In the classic inverted pendulum on a cart problem there is a pendulum represented with a point mass at a fixed distance from a horizontally free cart. A slight modification is to use a rod instead ...
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52 views

Designating a coordinate system with multiple objects

So I am slightly new to physics, but am thoroughly enjoying the contextual thinking changes that physics brings about. My question is regarding coordinate system designation, on a group of individual ...
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29 views

Pseudo force for the box on wedge

Can anyone please tell as im confused like why pseudo force acts on a block when we accelerate the wedge towards opposite direction, in my knowledge its coz of fram , which is wedge itself , but what ...
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1answer
34 views

Weight in an elevator accelerating downwards with $2g$

I am aware of the classical apparent weight physics but I am confused about what happens when elevator falls with greater acceleration than g say two g or something.if I calculate the normal force it ...
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30 views

Artificial Gravity - Practical Experiments [on hold]

I'm not a scientist but I am curious as to actual artificial gravity experiments conducted in a zero-gravity environment. I can't seem to find any but I've just read through some well thought out ...
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47 views

The confusing Wikipedia article about Euler's equation of rotation

Here is an article from Wikipedia on Euler's equations. In the derivation of those equations, I find the torque $\mathbf M$ quite confusing. There are two torques there -- $\mathbf M$ and $\mathbf M_{...
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151 views

Why doesn't accelerating reference frames in QFT lead to horrible paradoxes?

Background So I remember that in Special Relativity while one can define acceleration things can go horribly wrong has happened historically (I'm sure there many other paradoxes). The real reason of ...
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32 views

Under the constraint that the 4-acceleration is constant in each instantaneous rest frame

Suppose a particle is in a global inertial frame and moving in the $x$ direction, starting from rest at $t=0$. Under the assumption that the 4-acceleration of the particle in its each instantaneous ...
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Frame-referenced time derivatives

I have reviewed and am familiar with the similar questions asked and answered previously on this forum, the various Wikipedia references and the derivation used by Kane and Levinson [2]. The ...
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76 views

Stationary aether with respect to Earth? [on hold]

Michelson & Morley's experiment implies that either the aether does not exist OR that the Earth is stationary with respect to aether. Now, one consequence of relativity is that we CAN physically ...
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Does a “gimbal hinge” have the same degrees of freedom as a ball-and-socket joint?

I am trying to design a joint to 3D print and considering different models. One model was a traditional spherical ball-and-socket joint. However, due to production issues, I am considering other ...
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1answer
26 views

Transforming inertia tensor from corner of a beam to the centre [on hold]

So I have a beam with mass M and sides a (x-direction) ,b (y-direction) and c (z-direction). I have figured out that the inertia tensor in the corner is $$I = M\begin{bmatrix} \frac{b^2 + c^2}{3} &...
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Why is $g$-force pointed downward during upward acceleration?

After a recent physics project in school, where we worked with rollercoasters (it was rather simple questions, about where you would fell the heaviest, lightest etc.) I was reading about g-forces on ...
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Projection of a vector onto another vector [on hold]

We say that the projection of a 2D vector along the $x$-axis is the norm of that vector times the cosine of the angle the vector makes with the x-axis. And likewise, the projection of a 2D vector ...
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Why do vehicles executing circular motion tend to over turn outwards and not inwards?

I understand that when systems are executing circular motion the resultant force acting on the system (centripetal force) acts towards the centre of the circle. However, if a vehicle should over turn ...
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1answer
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Linear momentum of a system remains conserved, but with respect to which frame of reference?

I have studied that linear momentum of a system remains conserved. But i can't figure out with which reference of frame it is conserved. Is it conserved with respect to system reference frame or in a ...
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76 views

Establishing an inertial frame of reference by observing a closed mechanical system?

Suppose one has a closed system of $N$ material points with known masses, $m_1 ,..., m_N$ . Also given are their coordinates as functions of time, ${\bf r}_1(t), ..., {\bf r}_N(t)$, in some laboratory ...
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What is definition of motion? [closed]

I know this question is too basic and silly. But if you really contemplate about the definition of motion which say that it is "change" in position over time then you will find a loop. "Change" is ...
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124 views

How does Newtonian Gravity explain the difference between the experience of 1G and zero gravity? Or can it?

Humans can feel the difference between being on earth's ground, and being in space (zero gravity) This can be explained by general relativity, where in (1), force is applied by the ground upwards, ...
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1answer
50 views

Why people feel outward force in rotor ride?

I know centrifugal is fictious force but what is possible reason for the force that people feel in rotor ride outwards. The reason for people not falling down is friction force which is in vertical ...
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Measurement of time span of a free fall into a black hole

Observations of events seen from different reference systems always confuse me: A good example is the free fall of a particle into a -- for simplicity -- non-rotating black hole. In the reference ...
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In a binary system, why $m_1 a_1 = m_2 a_2$?

In the center of mass coordinate, $m_1 r_1 = m_2 r_2$, which is straightforward. Yet in this detailed deviation of radial velocity page 27, it says that the $r_1$, which is the magnitude of the vector ...
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Will there be any radial velocity in absence of centripetal force and angular acceleration?

In the question it is stated that there is a ring which can move along a smooth rod. The rod itself is rotating in a horizontal plane with one fixed end with uniform angular velocity. Initially the ...
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Weight at Poles and Equator Wrong?

Recently, I was thinking about the acceleration due to gravity on Earth when I remembered that I was taught that you weigh less at the equator because a fictitious centrifugal force "throws" you off ...
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Does the equivalence principle imply that light must move slower when moving away from a massive object?

Thought experiment: Elevator going up at an extreme acceleration, pulse of light bouncing up, and down between mirrors on the floor, and the ceiling. Won't it take light longer to travel from the ...
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Comoving system of expanding or collapsing dust-like particles

I have a question on comoving and synchronized reference systems. I read the corresponding section (97) on Landau/Lifshitz's second book field theory on it. In particular it is said that space ...
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What does the term “Coriolis Effect” refer to?

I am familiar with Coriolis Effect referring to the effect that deflects eastward a projectile moving north on the rotating Earth. That is, as something applicable to north/south motion only. There ...
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Is the ADM Mass for a boosted black hole $M$ or $\gamma M$?

If you were to take the metric for a Schwarzschild black hole and "boost" it, such that it were traveling with velocity $v$, would the ADM mass, corresponding to a time translation, be $M$ or $\gamma ...
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66 views

Friction as a cause of motion

Can anyone help me to understand why friction force is not opposing the resultant centripetal force acting on the box and instead being the cause of its existence?
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1answer
87 views

Reference frames symmetry in Special Relativity

I have a couple of questions related to reference frames in STR. Let's consider a rocket that is inertially moving towards a star with a relative velocity 0.9c. I'd like to look at this example ...
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Blinking flashlight infalling into the black hole, as frequency → ∞

This is my third question from a series were I progressively refine my thought experiment. The others are: Can something (again) ever fall through the event horizon? Time of flashlight crossing the ...
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What justifies the use of centre of mass?

Centre of mass gives the location where the 'weighted average' of all the elemental mass of a body acts. Considering that weighted average is one of the many statistical tools used to find the spread ...
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Should all objects have equal acceleration to be considered a system?

Should all objects have equal acceleration to be considered a system?
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1answer
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Spacetime dilation Moon vs Earth [closed]

As I understand it, time will pass slightly faster on the surface of the moon compared to the earth This is due to the difference in velocity and gravitational fields of the two planets. How could I ...
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1answer
25 views

Dependence of Compton effect on frame of reference

I want to know whether Compton effect depends on the frame of reference or not. The wavelength change is a thing, but does not depend on frame of reference since it depends only on $\theta$. But if I ...
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Relationship between torque and force [duplicate]

While studying Newtonian mechanics, we are concerned with only three laws of motion that describes force and it's relations. But when we switch to rotational dynamics, we are presented with torque and ...
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Equivalent fictitious forces on rigid body

I know that when I study the motion of a rigid body from a non-inertial frame, I have to take into account the fictitious forces. However, it is not straightforward to find the resultant force (and ...
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2answers
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How can this hinge point be at rest? [closed]

The given situation is shown in figure, my doubt is that point O must be accelerating but it is not possible. Where am I going wrong in my method? Please help
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Transforming a tensor from Crystal to Laboratory frame of Reference

I want to transform the stiffness tensor of a rhombohedral crystal from crystallographic frame of reference to laboratory fame of reference, how to do it ? For crystal structures having orthogonal ...
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3answers
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Finding Direction of Angular Velocity

Suppose I have a 3D rigid object on which some external forces act at various points located on it. The resulting motion would, in general, be the translation of center of mass plus rotation about the ...
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Acceleration in general relativity

Let's say that, from my point of view, another observer is accelerating. Now, from his point of view, he is standing still: all he feels is an overall fictitious force of gravity, which is just a ...
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Why is force invariant under a Galilean transformation? [duplicate]

I've had a look around online, but I haven't been able to find something which answers this in a way I understand. Essentially, I'm trying to figure out why force is invariant under a Galilean ...
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96 views

Reference frames versus coordinate systems

I have the following distinction clear in my mind: Reference frame → state of motion of the observer Coordinate system → set of numbers used to map the space points within a reference frame So for ...
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1answer
67 views

Navier Stokes equations for rotating falling object

I am considering the problem of a rigid body moving in a fluid, free to translate and rotate (say, for example, a falling body in a fluid). Some of the approaches I found in some articles involve a ...
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2answers
91 views

Can Newton's laws be reformulated with reference to “relative” frames

I was reading: https://arxiv.org/abs/1205.2326 page 2 which gives thorough discussion of the three laws. In particular there is a refutation of the (paraphrased) 2nd law implies the first because ...
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What is exactly meant by “time is relative”?

Let's say we are traveling to the galaxy X - which will take 4 years for us to reach. However, it will take 100 years for the observer on the earth. How come is it possible? How 4 years can be ...
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3answers
196 views

Total force on upper block in two block system

If a block $m$ is placed on another block $M$ and a force $F$ is applied on bolck $M$. Then how many forces are acting on block $m$.(Friction is non zero) The image is taken from this site. Is ...