# 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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### Is torque possible without any pivot point?

In the picture, AB is a rod. The resultant of force P=10N and Q=5N works at point C which is P+Q. When I apply both forces simultaneously, the rod will rotate and accelerate. 5N from both end will ...
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### Coriolis force effect on freely falling body

If an object is let to fall freely from a tower of 100 meters, then it will get deflected towards East. Now clarify that if an object is allowed to fall freely from a rocket at 100 meters height. Does ...
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### Integration approach for calculating the moment of inertia of a square "loop"

I must confess, most of the time when I was calculating moments of inertia I was using tables and the parallel axis theorem. Now I'm trying to compute the moment of inertia by integration. The problem ...
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### Derivation of differential in relativistic physics

I am struggling with derivation of differential $d^3 p$ in relativistic mechanics. In Quantum Electrodynamics: Volume 4 by V. B. Berestetskii, E.M. Lifshitz and L. P. Pitaevskii, Chapter VII, authors ...
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### Does gravity cause the Earth's equatorial bulge?

The way I understand centrifugal force, I don't see how Earth's daily rotation alone would cause equatorial bulges to form. The usual explanation is that the centrifugal force increases with distance ...
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### Why is momentum not conserved when one body is fixed? [closed]

Why is momentum not conserved when one body is fixed? Why can't I apply C.O.M with a ball hits a wall, I mean I don't get the correct answer doing so?
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### If one "fell" from the center of a spinning space station, which was creating "gravity" by using centripetal force, what forces would one feel?

Let's imagine a grand hamster wheel in space. The wheel is very large and is constructed of the same reasonably inelastic material. It has three main features: The first is a solid disk rotating at ...
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### Rigid body doing rotation [duplicate]

Is it possible that a rigid body is rotating but net torque about any point in the body is $0$? The reason behind asking this is if we apply two unequal like parallel forces at two ends of a uniform ...
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### Relating Minkowski and Schwarzschild

In the article https://arxiv.org/abs/gr-qc/0309072 the author finds a link between Minkowski coordinates of a freely falling observer and Schwarzschild coordinates. He makes use of a Galilean ...
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### Defining acceleration in gravity-free space

Without information from outside a closed spaceship, an astronaut cannot distinguish A from B. A) In gravity-free space, the floor accelerates upwards at $a=g$ and hits a dropped watch. B) On earth's ...
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### Could a civilisation use “inverse time dilation” to live longer?

If an observer were to live near a black hole of sufficient mass, half an hour could elapse for them, but the outside universe could progress hundreds of years. This is due to time dilation. We know ...
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### Is net gravitational force directed to the center of mass?

I have a question regarding the answer from this topic Is the force of gravity always directed towards the center of mass? One of the answers states the following problem. "Three spherically ...
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### Twin Paradox (SR): How can we express the comparative length of arbitrary world-lines mathematically?

The simplest and most intuitive way I have found so far for explaining which twin ages less in the Twin Paradox, is that it's the twin who's world-line is the longest (if it's the longest in one ...
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### Reference frame

What does "In the reference frame of" exactly mean? For example, imagine a man is standing on a rocket moving at a speed $0.8c$. The man is stationary in the reference frame of the rocket. ...
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### Why is a reference frame moving with constant velocity with respect to an inertial frame also inertial?

We define an inertial frame, as a frame of reference where: Newton's 1st law holds. It is then stated that a reference frame moving with constant velocity with respect to an inertial frame is also ...
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### Helicopter blade aerodynamic forces in rotating frame of reference

It is well known that helicopters use the concept of centrifugal stiffening. That is, there is a centrifugal force that acts against the motion of the blade due to the thrust and thereby reducing ...
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### Using position of a free particle to measure time

Hartle, gravity An observer in an inertial frame can discover a parameter t with respect to which the positions of all free particles are changing at constant rates. This is time Then goes on to say ...
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### Rotating reference frame - Taylor Problem 1.27 [closed]

I'm having trouble understanding how to think about Problem 1.27 in Taylor's Classical Mechanics. I want to be able to solve similar problems qualitatively when it comes to changing reference frame so ...
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### Kinetic energy of two particles [closed]

I am not getting the expression of kinetic energy of two particles from ground frame in terms of the velocity of the center of mass, please give the expression and explain how to arrive into the ...
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### Temperature & frames of reference [duplicate]

Is the temperature (as defined by rms velocity of particles) higher for a system that is moving relative to the observer? So for instance if a box of particles is moving relative to the observer, ...
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### Pseudo torque for angular acceleration

Suppose $F_{net}$ on a body is 0 (still it has torque). i.e. it is performing pure rotational motion. Now I want to consider torque about any point that is not center of mass. Should I include ...
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### Times of free-fall in different inertial reference frames in classical physics

Imagine that a ball is thrown directly down at a rate of 20 m/s from a height of 100m in Earth's gravitational field. If we were to calculate the time it takes to fall we could use the kinematic ...
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### What frame of refernce to select in statistical mechanics?

Suppose we have a solid particle suspended inside a fluid such as an ideal gas, as shown in the following picture: Our system is the solid particle and the environment is the gas (which acts as a ...
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### Question on work done and frame of reference [closed]

While solving a question which is..... If an object is kept in a non inertial frame moving up with acceleration 'A' find the work done by pseudo force on the object taking a person standing on ground ...
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### How realistic is the science behind the Buzz Lightyear movie?

How realistic is the science behind the Lightyear movie? Buzz Lightyear flies into space, and accelerates up to 70% of the speed of light. When he lands, suddenly four years have past back home, while ...
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### When a spring is held from one end and some mass is put on the other end and it's rotated then why does it get stretched? [duplicate]

While going through a question I found that if we hold a spring from one end and rotate it then it gets stretched. One explanation may be that centrifugal force is acting on it...but if I want to ...
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### Motion of a projectile in a rotated coordinate system, and converting equations to the new system

Context: This in essence is explained on Mathematics of Classical & Quantum Physics by Byron & Fuller. This is from Section $1.4$, pages $12-13$. So, suppose we launch a projectile (mass $m$) ...
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### 6 DoF rigid body equations and tensor of inertia

I am writing here as I have a doubt on the tensor of inertia found in the Euler's equations. As far as I have understood, people usually solve the Euler's equations in "body coordinates" as: ...
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### If I make the entire universe rotate, will the bucket appear to be rotating?

Newton's bucket experiment is a way to tell if you are in a rotating frame. If you have a bucket of water which is spinning the water surface will form a concave shape. Likewise, if the bucket is not ...
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### Why exactly is Newton’s Second Law valid only in non-accelerating frames?

A reference frame where Newton’s second law is valid is called an inertial frame of reference. Force is absolute, so is mass(for sufficiently small speeds), so if a frame is to measure an ...
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### What is the meaning of the statement"centre of mass of a body remains in equilibrium"?

Explain the meaning of statement"centre of mass of body remains in equilibrium if total the total external force on the body is zero".what is exactly meant by the term equilibrium here?
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### In spacetime what is the time $t$ on the $ct$ axis?

In spacetime, I understand that we multiply time by the speed of light to deal with homogeneous distances over the four axis, space and time. But what does $t$ refers to precisely? Where is $t$ ...
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