# 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 calculate the latitude/longitude by the RPM in the basin?

By seeing how the water swirls and whirlpools when going down the drain in a washbasin is it possible to tell something about the longitude or latitude where the individual is?
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### Two particles connected by an inelastic string [closed]

Problem Statement: Two particles of equal mass at a distance $l$ are tied together using an inelastic string of length $l$. Determine the minimum velocity $v_0$ which must be given to one of the ...
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### Angular velocity and acceleration in a vehicle ($x,y,z$ axis) [closed]

I would like to understand what you understand by Angular velocity and Angular acceleration on x,y and z axis for a vehicle please. I do not mean with respect to the wheel but of the vehicle ( Eg: Yaw,...
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### How to tell if a force on a rigid body does any work?

In problems in Newtonian mechanics I often need to understand whether there is energy conservation. When only conservative forces (such as gravity) are present, it's obvious. But what about when we ...
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### Physical pendulum: axis of rotation

Why is the angular momentum always considered to be parallel to the angular velocity in physical pendulum problems, as if the axis of rotation were a principal axis always? I don't see the ...
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### Properties of the Center of Mass

My students are currently going through the rigid rotor and hydrogen atom unit in their quantum physical chemistry course and I found myself at a loss on how to justify what seems a natural way to ...
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### Normal force in a hypothetical rotating space station with artificial gravity

In order to simulate gravity on hypothetical space stations, one approach involves rotating the space station so that a centripetal force is present. Occupants within the space station's frame of ...
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### Why when spinning over non-principal axis, it will change the axis of rotation?

Let's look at a disk which is rotating around non-principal axis. I know the explanation when looking in a rotating frame, the centrifugal force on the edges of a the disk create a torque that wants ...
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### When we are on the ground do we still accelerate with 9.8 m/s²? [duplicate]

Do we accelerate with 9.8 m/s² when we are on the ground , if so why we do not fall inside the eart . How is the net force is zero , how many real forces acts on the body ?
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### How to know the position of an object when calculating the center of mass, without using integrals? [closed]

For example, if I have a 1/4 piece of pizza, what is the position?
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### Would objects really be at rest relative to each other in orbit?

I am currently reading Gravitation by Misner, Thorne, and Wheeler (MTW). In the very first chapter on weightlessness they make the following claim: “Contemplate the interior of a spaceship and a key, ...
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### On the equivalence principle

In its simplest version, it states that the effects of a uniform gravitational field are indistinguishable from those of a uniform linear acceleration of the frame of reference The thing is, you can ...
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### Equilibrium when torque about the COM is zero?

I'm reading a book on physics in which there is a line that I do not understand. I quote it here: If a body is placed on a horizontal surface, the torque of the contact forces about the centre of ...
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### Doubt in rotational work-energy theorem

This is a question on work-energy theorem in rotational motion with its answer for the first part of the question. Here, I have a doubt about why is the book taking the translational work done by the ...
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### Combined KE for an object rotating about a point other than COM

I know the KE equation for combined translational and rotational motion, which is $K=Mv^2/2+I\omega^2/2$, and here $v$ is the velocity of COM and $I$ is the moment of inertia about the COM. But what ...
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### If we know forces and torques applied to one point of a rigid body, how do we calculate them for another point of a rigid body

Suppose we have a rigid body with known length $r$ that is fixed at point $A$. We also have a force-torque sensor at point $A$. The body is fixed such that it doesn't move. We apply forces to point $B$...
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### Absolute rest in special relativity

I am new to special relativity and with still little experience in Physics. I would like to know why a privileged frame of reference that determines absolute rest is not possible. I want you to ...
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### Showing that trajectories deviate from straight line in accelerated reference frames

Explanation of light bending in an accelerated frame in a relativistic setting We know the transformation that takes us from one inertial frame to another. In special relativity, the transformation is ...
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### Fictitious forces and the Unruh effect

Here is a (practically infeasible) method to determine whether you are in a non-inertial frame of reference: Look around you, and calculate all of the forces acting on you. The piece of lint on the ...
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### Effect of rotation of earth on levitating objects [duplicate]

If a drone is designed, such that it applies a constant force equal to it's weight ($mg$) on air, to maintain a constant elevation (say of 10 meters). What will happen when considerable time has ...
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### Energy in different coordinates in central force motion

With reference to central force, we see that K.E has 2 terms in 2D cartesian cordinate but just 1 term in polar coordinates and potential energy has 1 term in cartesian but 2 terms in polar. Basically ...
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### Confusion In rotational dynamics

Since angular acceleration is absolute on entire body (correct me if i am wrong), can we choose an arbitrary axis to calculate the torque and then calculate the angular acceleration? (Regardless of ...
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### (Time-)Orientability in the Language of Fiber Bundles

I'm currently studying spin geometry through Hamilton's book Mathematical Gauge Theory. At a given point, Hamilton considers a pseudo-Riemannian manifold, which I'll take to be Lorentzian in $d=3+1$ ...
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### How to determine when angular momentum is conserved and when torque is zero?

In physics problems, I struggle to incorporate the ideas of angular conservation and torque. For example, how do I know when torque is zero in a problem like this: can I even apply angular momentum ...
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### Angular momentum of the Moon (or any body orbiting another orbiting body)

Let the Moon have angular velocity $\omega$ around the Earth. The Earth itself revolves with velocity $V$ around the Sun. The radius vectors are $r_i$ from Sun to a point on the Moon, $r_i'$ from ...
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### Finding the Cartesian position of the lunar ascending node

What I'm trying to derive The Cartesian position of the lunar ascending node relative to the true equator and equinox of date reference frame. My issue is I'm getting a bit tripped up with reference ...
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### Is the net force on centre of mass always equal to the external force applied on one block of a two block system connected by a spring?

I came across a question which says- "Two blocks of equal mass m are connected by an unstretched spring and the system is kept at rest on a frictionless horizontal surface. A constant force F is ...
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### Coriolis acceleration for wind at some latitude and longitude on the surface

I was trying to do an alternative general expression for Coriolis acceleration to the one explained on Wikipedia which I find not too intuitive and too simple calculation wise. Supposing the $z$ axis ...
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### Doppler shift phenomenon for non-inertia frames

The Doppler shift phenomenon is well understood when the source and observer are in relative constant motion. However, I'm curious to know how the Doppler shift phenomenon is modified when they (i.e., ...
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### How does moving the launch area forward impact the area of impact?

If we launch an object in some way, and it lands at distance x, if we move forward the launching mechanism y units forward or backwards, will the landing distance also move forwards or backwards y ...
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### Rigid body and angular motion

I am studying rigid body motion in Engineering Mechanics Dynamics by J.L. Meriam, 8th Edition. After he explains about rigid body motion in page 316 at the top of that page the following paragraph: &...
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### Does quantum mechanics respect the principle of relativity?

The principle of relativity says that all observers see the same laws of physics. It is, to my knowledge, the underlying principle behind General Relativity; put alternatively, General Relativity is ...
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### Why do we use the Earth inertial frame when solving problems involving non-inertial reference frames? [duplicate]

I was solving a question in my textbook that was under the heading "Solving Problems Related to Objects in a Non-inertial Frame of Reference" The problem reads "A teacher suspends a ...
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### Can a force acting on a single point of a resting, freely movable body cause it to spin without causing translational movement?

In his answer to another post user Albertus Magnus describes the situation of a bullet hitting a rod in free space on its tip in a "purely tangential" way causing the rod to spin in a purely ...
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### Why are we able to observe seemingly "inertial objects" in our day to day life? (apparent paradox from Newton's second law)

On the table and near to the computer where I write this question on, is my phone, and in my observation, it is completely still. Now, by Newton´s first law, since it is inertial, there is no net ...
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### Is either Geocentrism/Heliocentrism more "correct" than the other? [duplicate]

Yesterday there was a casual discussion of the findings of the scientist Galileo in my German class. It was mentioned that his breakthrough was that, he found a convincing explanation that earth ...
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### Violation of Newton's second law if the mass if changing?

I learned some thing called Galilean principle of relativity which says that two inertial frames are equivalent and the laws of physics are the same in both inertial frames. However here comes the ...
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### What defines weightlessness feeling during a fall?

So I know that we feel weightless during a free fall as there's no normal force acting on us or my other way of thinking is that considering a lift and both lift and I fall with $g$ then no normal ...
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### Confusion about breaking apart vectors

Hi I've probably got a very basic question but I'm really confused about this. If I have a vector that starts at the origin and points to say (3,-3) so the 4th quadrant, and I am wanting to split ...
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### Relativity and variable time - there is an alternate formulation where time is always linear. Has it been studied?

Lorentz, Einstein et. al. assume time is the variable which changes in a gravity well or as speed approaches $c$. That's the commonly accepted model. For nearly 50 years I've wondered if anyone else ...
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### Scalar multiple of inertial frame

Consider an inertial frame of reference $S$. Now take a second frame $S'$, defined as follows: If a point $P$ has coordinates $$(t,x_1,x_2,x_3)$$ in $S$, then it has coordinates $$(t,2x_1,2x_2,2x_3)$$ ...
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### 2d elastic scattering with an impact parameter

Hello guys I have homework that has tasked me with connecting the effect of the scattering parameter to the energy transfer in a 2d elastic collision of two arbitrary spheres with one of them standing ...
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### How was the reference system taken here?

in this Atwood machine digram y1 is the amount that m1 gets displaced and y2 is the amount that m2 gets displaced the lecturer then wrote $y1=-y2$ , I am trying to understand what did he do to get ...
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### Is the square modulus of the cross product between three-vectors a Lorentz invariant?

Is the square modulus of the cross product between two vectors $\vec{v_1}$ and $\vec{v_2}$ ,i.e., $(\vec{v_1}\times\vec{v_2})^2$, a Lorentz invariant or not? The result of a cross product between two ...
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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 \left(z+\frac{\gamma_{0}}{a}\right)^{...
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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 ...
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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 ...
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 - (...
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\...