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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 does the vector in the banked turn problem not point to the center?

I am currently working on the banked turn problem. I know that once you set up all the equations properly you end up with $$v^2=gR\tan{\theta}$$ I drew a little diagramm of the forces: I know that ...
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tell me what is the relationship between lorentz and Galilean transformation [on hold]

where came the Gamma tell me the relationship between lorentz and Galilean transformation
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1answer
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Deriving center of mass coordinates of a uniform semicircular disc without using an area integral

In how many ways can I derive centre of mass coordinates for a uniform semicircular disc? One way my friend told me was to use the areal mass density (which gave correct coordinates), but I attempted ...
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Fictitious force [on hold]

Especially confused by example :on a particle at rest wrt a lift which is descending with an acceleration =g ,the particle is weightless and thus REMAIN SUSPENDED IN THE AIR ,-a Condition of ...
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Angle between velocities: Elastic collision question in 3D

I have been given a scenario of a proton (particle 1) travelling with u1 = 3î+ 4j–6k and a Helium-3 nucleus (particle 2) travelling with u2= 3î+ 4j–2k. The two particles collide and the proton (...
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1answer
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Violation of law of conservation of linear momentum

A bike is moving with some velocity.There is some frictional force between the wheels of bike and ground.Now you apply brakes and the bike stops after some time.Brakes apply a force say F on wheels ...
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3answers
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How many moment of inertia about center of mass exist?

So imagine we have a rigid body and we want to find the moment of inertia about center of mass . Doesnt exist infinite axis that pass trough center of mass therefore infinte moment of inertia? Do they ...
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2answers
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Do the inertial mass and gravitational mass of an object depend on the frame of reference?

My understanding is that the "relativistic mass" of an object means any of the following three quantities (which are all identical): The "mass-energy", as defined by the formula $m = E / c^2$. The ...
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1answer
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Confusion about angular momentum of earth moon system [on hold]

My assumed definition of angular momentum is the sum over $i$ of $L_i =r_i\times{\omega_i}\times{r_i}$ for each particle about some origin. We have two spheres rotating about the centre of orbit. For ...
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3answers
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Which observer measures proper time in the twins paradox?

I know that proper time is defined as the time which the clock moving relative to that observe shows. That is, a clock attached to observer A will always be As proper time. I also understand that this ...
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1answer
52 views

What does a basis rotation correspond to physically for linear position-momemtum?

For polarization and angular momentum, rotating the basis corresponds to a very straightforward physical transformation, namely, the physical rotation of an experimental apparatus about an axis in ...
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1answer
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Can Pseudo force be used for the monkey B [on hold]

Monkey $B$ is holding on to the tail of monkey $A$, which is climbing up a rope. The masses of the monkeys $A$ and $B$ are $5 \; \text{kg}$ and $2 \; \text{kg}$ respectively. If $A$ can tolerate a ...
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6answers
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How does rotational “artificial gravity” differ from normal gravity?

I am not a physicist, just a curious mind. I was reading a novel by Iain Banks where it was mentioned, that shifting from artificial rotational "gravity" (in space, on a rotating space craft) to real ...
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2answers
41 views

Reason of no reaction force on a person in a lift falling freely under gravity

a person in a lift falling freely under gravity feels weightlessness because the force of reaction between the person and the plane with which he is in contact vanishes. why force of reaction vanishes?...
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2answers
88 views

Example contradicting the first postulate of special relativity [closed]

Is there any example of a non-inertial frame of reference where all the physics laws differ?
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Equation for surface of a rotating liquid [duplicate]

The surface of rotating coffee in a mug after it has been stirred takes on a characteristic shape, depending on angular velocity, viscosity of the liquid, etc and I've been interested in knowing what ...
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1answer
59 views

Can a free electron accelerating in a gravitational field absorb photons? [duplicate]

An 'free' electron accelerated in an electromagnetic field can both absorb and emit a photon. What about an election accelerating in a gravitational field? Edit: Some users have suggested that the ...
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2answers
82 views

How to choose the perpendicular axis?

This site https://en.wikipedia.org/wiki/Perpendicular_axis_theorem says: Define perpendicular axes $x$, $y$, and $z$ (which meet at origin $O$) so that the body lies in the $xy$-plane, and the $z$-...
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1answer
45 views

Gravitational potential energy with both bodies moving [closed]

When deducing the formula for the gravitational potential energy of one body in relation to the gravitational force of another body, my teacher assumed that one body was standing still. I tried ...
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0answers
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is it possible to add moments of inertia with two different centers of mass using the parallel axis theorem? [on hold]

In my physics lab, we did an experiment where we had a disk on a free spinning axis and hanging from a pully of negligible mass we had a weight. this weight was wrapped around the shaft connected to ...
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1answer
43 views

Relation between centre of mass and axis of rotation [duplicate]

Why is that in absence of a manually fixed point, a body shows its turning effect on application of a torque with its axis of rotation through its centre of mass? my attempt: is there a definition of ...
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1answer
65 views

Practical explanation for Coriolis force

Just to get an intuitive feel about the force. I came up with a imaginary situation of a long aisle on a huge rotating disk, like a groove along the diameter of a disk. Now as this disk starts ...
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0answers
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A person is standing in an elevator, can the elevator and the person have downward acceleration $a> g$? [duplicate]

will the person stick to the ceiling of the elevator? what will happen if only the person have acceleration a>g? what will happen if only the elevator have acceleration a>g?
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1answer
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Differentiating linear and non-linear motion

If there is a Person sitting in a vehicle which is moving, how could he find out whether it's in motion and if he figures that out, how does he find out whether it's accelerated or non-accelerated?
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1answer
56 views

Are Maxwell's equations valid in a rotating frame?

Maxwell's equations are covariant under Lorentz transformations. Are they covariant under going to a rotating frame and if not how do they look?
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6answers
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Why we use vectors?

When we say that the position of an object is +5m on x axis why we need to use vectors? I mean could we don't use vectors and just say +5m on x or y or z axis instead of writing 5*unit vector either $...
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1answer
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Super luminal speed paradox? [duplicate]

Chosing a reference frame in which the Earth is at rest and doesn't rotate is related, but different. Here I'm asking if there is a paradox and if my attempt at resolution has merit, and if not ...
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5answers
757 views

Why does relativistic velocity subtraction produce a larger relative velocity than classically?

I got a question for homework saying that there were two rockets on a parallel track, heading towards earth. Rocket A was in front of Rocket B. Rocket A was travelling at a velocity of $0.75c$ from ...
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2answers
83 views

Would Earth clocks tick faster than a clock on a GPS satellite if we disregard gravitational time dilation?

If we disregard the time dilation caused by a GPS satellite being in lower gravity, would earth's clocks tick faster or slower from the satellite's perspective? Edit. From earth, we understand that a ...
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1answer
67 views

Why dont we account the rotation of the earth in aircrafts? Is it just because of the air or is there more to it? [duplicate]

Today there was a seminar regarding modern air crafts held in my school. At the starting of the lecture, we were asked a question which I found quite intriguing. "Why does a helicopter or a fixed ...
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3answers
778 views

Mass-energy in relativity

If an object's mass, when measured in its frame of reference (even if that frame is moving), is invariant, then why does it require more and more energy to accelerate it? I can understand this when ...
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3answers
40 views

Coriolis force decomposition of angular velocity [closed]

I can’t for the life of me understand how the $\omega$ in this is decomposed to $$\vec{\omega}= \omega (-\sin(\theta),0, \cos(\theta))$$ Any help would be greatly appreciated!
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Gravity: Inverse square law over extreme distance [duplicate]

I'm just an enthusiast, with a gap in my knowledge of physics. Please don't hesitate to be brutal, I would really like to know where I'm wrong. Thank you! Rockets gain speed if they launch East ...
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2answers
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Would a Schwarzschild black hole “appear” to be a sphere in all reference frames?

I am wondering if all observers would agree that a non-rotating, non-charged black hole is spherical (i.e. there is no reference frame where one would measure it to be oblong in one dimension). I ...
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2answers
47 views

What is Coriolis force? [duplicate]

I have read that the Coriolis force is a psuedo force in a rotating system. But what is the origin and direction of this force? I searched in Wikipedia but it didn't help me much. So I need a ...
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1answer
60 views

Movement of a helium filled vs lower-density-gas filled balloon inside an accelerated car

It's well known that a helium balloon inside of a car moves forward when the car accelerates, and backward when it slows down. What would happen, though, if a lower density gas was used instead of ...
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2answers
64 views

If we filmed what happens on an extremely time dilated planet in realtime

Say there was a planet that experienced insane time dilation due to orbiting a black hole, say 1hr on there is 7 years on Earth (something like that), and that it was close enough to Earth for light ...
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1answer
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Attraction of matter in curved spacetime

Is there still going be a force between them (converging space which makes the two bodies meet together at a point)if both of them are absolute rest with respect any frame of reference.
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2answers
65 views

Does the position in universe of a person not matter regarding death?

I might be completely wrong, but watching a documentary about time, the Einstein theory of relativity came up. From their explanation, time "slows down" depending where you are in the universe. I ...
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1answer
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Accelerometer readings

Good Night. I have a implemented a accelerometer, a gyro and a Magnetometer in raspberry pi. I want measure the linear acceleration (acceleration without gravity) but i don't know how to do it or if ...
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0answers
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What is the deep “physical” intiutive meaning of torque? [duplicate]

There is a rule that $$lever\;arm(1)\times force(1)=lever\;arm(2)\times force (2)$$ This is because the torque equivalence principle. I am only talking about vertical forces (so cross products' mods ...
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1answer
120 views

Can the horizon of a black hole move?

Because of time dilation we cannot observe a black hole forming in a finite amount of time. For the same reason I suppose we also cannot observe the horizon moving: everything happening on the horizon ...
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1answer
53 views

Why radial acceleration is expressed as the negative of centripetal acceleration?

My book says that:$$a_r=-a_c=-\frac{v^2}{r}$$Why radial acceleration is expressed as the negative of centripetal acceleration? Shouldn't they have the same direction and magnitude?
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How to find inertial frame of reference? [duplicate]

How do you find an inertial frame of reference or what you can regard as it? How do you evaluate the difference between a given frame and an inertial frame?
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3answers
97 views

Artificial gravity in movie Interstellar

Cooper Station is somewhat like a hollow cylinder. Buildings are attached on the surface. It has artificial gravity to hold on buildings and people. If, a person needs to fly in this spot Will the ...
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2answers
64 views

Does the 'Center of mass' make sense in a $0g$ environment?

Center of mass is as if where the entire mass of a body is located, here on earth there is this intuition of the object balancing at a pivot point, is there a better intuition for this applicable ...
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4answers
828 views

How is momentum conserved in this example? [duplicate]

Suppose a sticky substance is thrown at wall. The initial momentum of the wall and substance system is only due to velocity of the substance but the final momentum is 0. Why is momentum not conserved?
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2answers
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Conceptual problem in rotation

A particle is rotating in a conical pendulum with help of a string of length $\ell$. The speed of the particle is constant and angle theta is also constant with time. Why isn't the net torque on the ...
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3answers
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How to distinguish force from accelerating frame of reference?

So in this question Is this the reason why acceleration is said absolute?, author asks is his reasoning about absolute acceleration right, and he concludes that we can measure absolute acceleration of ...
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1answer
50 views

Cosmic scale factor $a(t)$ in a general frame of reference

In considering an FLRW type universe, the scale factor $a$ is generally indicated as being strictly a function of time $a(t)$. Isn't this only true for a comoving frame? In some other reference ...