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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2-body spring system confined to a plane [closed]

I'm trying to work out the Lagrangian for the following system: "a mass-less spring has an un-stretched length $b$ and spring constant $k$, and is used to connect two particles of mass $m_1$ and $...
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Why can I use different formulas for moment of inertia to model this simple rod system?

Suppose I have a rod of uniform mass as follows. enter image description here I'm confused how to pick the moment of inertia for this rod. One possibility would be to use the formula corresponding to ...
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Why is it incorrect to say that KE of COM is not the same as total KE? Does this also apply to PE?

I sort of understand why KE(com) is not the same as KE(total)... correct me if I'm wrong.. its like when a symmetric body is performing rotational motion even though the COM is not moving it still has ...
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What is the maximum deflection angle of a pendulum in a car, when the car, initially at rest, accelerates suddenly? [closed]

I was doing Kleppner-D.-Kolenkow-R.J. and I came across the following problem:- A pendulum is tied vertically to a car at rest, the car suddenly accelerates at a rate A. Find the maximum angle of ...
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The Leaning Tower of Pisa [closed]

Can we calculate the time when the Tower will lean to the extreme point that its center of mass will be out of its balance...?
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Can velocity of an object be different as seen from inertial frame and center of mass frame of reference? [closed]

Is it necessary for an object to have same velocity in Center of mass frame and well as inertial frame of reference under the action of a constant force? I was solving one question and I got different ...
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Simple harmonic motion in a non-inertial frame [closed]

For simple harmonic motion in a non-inertial frame is amplitude same on both sides? As in one direction pseudo force supports acceleration due to spring and in another direction it opposes ...
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Why do we use relative velocity when describing flow rate of exhaust gases leaving a nozzle?

Reference Why do we take the relative velocity instead of the absolute analysis in analysis of flow rates? More particularly when we write down momentum, isn't the 'velocity' term which we multiply ...
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Why is torque measured about the centre of mass? [duplicate]

The original question I was asking was Why does a body not rotate if force is applied on the centre of mass?. The obvious answer is that the torque ${\bf r}\times{\bf F}$ is zero as ${\bf f}$ is zero ...
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Why does a body not rotate if force is applied on the centre of mass?

The definition of centre of mass on Wikipedia is given as This is the point to which a force may be applied to cause a linear acceleration without an angular acceleration. How can I prove that such ...
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If an elevator and its shaft were weighted, would they weight less as the elevator goes up?

I have a question that would probably be easy for you. If we could put an elevator together with its shaft on a scale and have its motor pull it up, lets say for 50 floors. will the reading on the ...
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Whats the center of gravity/mass of this figure? [closed]

I've tried figuring this out, but I can't seem to find a way to find it, since I dont have any other information to calculate something. The only thing I could find out was that its possible to use ...
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Can't understand derivation of relativistic mass in Kleppner

In Kleppner and Kolenkow chapter 13, they derive the expression of relativistic mass by considering a symmetric glancing elastic collision. It was analyzed from two reference frames. One in which the ...
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What does not accelerate?

According to special (& galilean) relativity, there is no difference between constant movement or not moving at all but it is possible to detect acceleration and rotation is acceleration. The ...
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Centripetal force for a rotor or centrifugal force?

From basic physics any object experiencing a uniform circular motion, experiences a centripetal force. However in electric machines at high speed the rotor would be deformed outward as if a ...
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Finding roll/pitch/yaw for rotating local vector to another vector

Although I looked into similar questions regarding this subject, I could not make the link to my situation so correct me if the answer is already on StackExchange... My starting point is a global xyz-...
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Inside a car with light bouncing vertically between two mirrors, now if the car starts moving will the light also move with the car?

Consider two person one is travelling in a space ship and one is on the moon, now in the space ship there is a device where a light beam or photon is bouncing between two mirrors now when the space ...
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Relative time dilation at relativistic speeds

As I understand it (feel free to correct me if I'm wrong here), the core tenet of Einstein's theory of special relativity is the idea that if you were on a sealed train moving at any constant velocity ...
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Why do some Lagrangians have both a rotational and a non-rotational part of their kinetic energy?

I was going through a derivation and I came across a term in the kinetic energy of the Lagrangian that had both a velocity term and a rotational term. So I looked it up and I found these two links ...
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Mobile inclined plane

If i have a block falling down a mobile inclined plane, would it work if I solve the problem as usual in the plane's frame of reference and use the relative acceleration formula $\vec a_{b|p}=\vec a_b-...
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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 ...
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Maxwell Equations in Earth frame

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 ...
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A Hidden Principle in Relativity

Thought experiments are very common in Special and General Relativity (SR,GR). Usually a thought experiment is structured as follows: We present a setup in a frame of reference We apply principles of ...

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