New answers tagged reference-frames
2
votes
Why does torque increase with radius or distance from the centre?
As with everything in physics, it's about modelling the real world, based on experiments.
We all experienced that, e.g. for turning a screw, the same force applied to a longer lever has a better ...
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0
votes
Why does torque increase with radius or distance from the centre?
Angular momentum is defined as
$$\mathbf L = \mathbf r\times\mathbf p$$
This can be changed in 2 ways: by changing $\mathbf r$ and by changing $\mathbf p$ (or a combination of both):
$$\frac{\mathrm d\...
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0
votes
Why does torque increase with radius or distance from the centre?
Well, torque does not necessarily increase with radius since you didn't hold the force constant since $\tau=\vec r \times \vec F$
The reason why we are considering torque is because of the angular ...
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0
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Are the mass, diameter and age of the Universe frame dependent?
Yes, diameter and age are frame-dependent, due to length-contraction and time-dilation. To a "moving" observer, both are reduced, although the diameter depends on which direction you measure ...
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5
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How is special relativity explained by general relativity?
Let me expand on the comment by WillO.
In order to be able to formulate newtonian mechanics we grant Pythagoras' relation:
$$ r^2 = x^2 + y^2 + z^2 \tag{1} $$
Interestingly, back when Isaac Newton ...
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0
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Why it is stated, that there is no inertial frame of reference?
Absolute interial frames of reference are mathematical model, and do not exist in real world
In modern physics the term “absolute” refers to a quantity from a reference frame that is physically ...
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-1
votes
Which force provides the centripetal acceleration that makes objects on earth's surface rotate about Earth's axis of rotation?
cbr paradox:
...water level is not horizontal everywhere, because if it were then the Earth would either be a stationary sphere or flat...
The rise of the water is (on average and at 45 degrees ...
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1
vote
Question about distribution of mass
Does there exist a point P such that, given any line L through P, L bisects the mass of S?
Not necessarily. Counterexample - place three equal point masses $m$ at three corners of an equilateral ...
- 47.4k
3
votes
Connection between pseudometric and Einstein elevator
I do not see how the metric tensor relates to accelerated reference frames.
The relationship is:
The metric tensor for an arbitrailty accelerating reference frame in flat spacetime has (pseudo)-...
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5
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Connection between pseudometric and Einstein elevator
lalala asked: I do not see how the metric tensor relates to accelerated reference frames.
The purpose of the elevator experiment is to showcase that a uniform gravitational field can be switched off ...
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0
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Why does the work done depend on the frame of reference?
A force perpendicular to velocity doesn't do any work. Work is the change in kinetic energy, so
$$\frac{d}{dt}\left(\frac{1}{2}m \vec{v}\cdot\vec{v}\right) = m\vec{v}\cdot\vec{a} = \vec{v}\cdot\vec{F}...
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A mass in a rotating tube
I think that there is a problem with most replies.
If you look at the equations of motion expressed in polar coordinates you get indeed as many have written
$\ddot{r}=r\dot{\theta}^2$ and $F=mr\ddot{\...
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1
vote
Does a photon in vacuum have a rest frame?
I am going to approach the matter otherwise. I do not dispute the arguments made in answers in [1] or answers in [2]. However, based on my learning style, these arguments do not work for me. This is ...
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5
votes
Accepted
Are the mass, diameter and age of the Universe frame dependent?
Spacetime geometry is, some details aside, very similar to ordinary geometry. Length and time are as relative in spacetime geometry as length is relative in ordinary geometry. And length isn't ...
- 25.3k
0
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Time Dilation According to Stationary Object
Einstein's view of special relativity can lead to many paradoxes like this , the correct view of special relativity is the Minkwoski metric.It says that the spacetime interval is the same for all ...
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0
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Accepted
Time Dilation According to Stationary Object
Usually in physics we talk about events as being instantaneous, so if you have an event in the everyday sense that takes 2 seconds we would treat that as an interval between a pair of events, namely a ...
- 25k
0
votes
Body on a frictionless rotatating surface
Suppose that there was friction and the body on the rotating surface was not moving relative to the rotating surface.
In the inertial (lab) frame there is a force $\vec F = -mr\dot \theta^2\,\hat r$ ...
- 93.6k
2
votes
Body on a frictionless rotatating surface
How do we analyse this rotation motion from the frame attached to the rotating surface ?
There are two alternative methods:
Use Newton's laws to derive the equation of motion in the non-rotating ...
- 47.4k
1
vote
Accepted
Vanishing of angular momentum with scale
… the whole universe doesn't possess an angular momentum. It can't: If it had one, the centre would be a special point (which isn't allowed) and the velocities of the outer regions would easily become ...
- 15.4k
5
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Teacher told us we're not allowed to write negative vectors, is this correct or not?
Your teacher is instructing you to define the vectors in terms of their components,
\begin{align}
\vec F_1 &= \hat x \cdot 20\,\mathrm N
\\
\vec F_2 &= \hat x \cdot (-10\,\mathrm N)
\end{align}...
rob♦
- 86.3k
0
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Teacher told us we're not allowed to write negative vectors, is this correct or not?
You are both right. You should learn what your teacher is talking about. You will need both ways of thinking.
You are thinking of forces like numbers on a number line. Those to the right are + and ...
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1
vote
Teacher told us we're not allowed to write negative vectors, is this correct or not?
Presumably the teacher is trying to build a good habit of separating the vector from the representation.
Vectors and numbers are different types of things. One can represent vectors well using numbers ...
- 30.9k
0
votes
Accepted
Choice of origin and change in angular momentum
When you choose the fixed point which coincides with the point of hit as the origin of our coordinate system, the angular momentum of the rod is
$$\vec{L}= M (\vec{R}\times \vec{V}_{cm}) + I_{cm}\vec{\...
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0
votes
Accepted
Rotating of a system of mass
You can understand this question intuitively like this:
The motion of the centre of mass is only dependent on external force on the entire system. Therefore if there is no external force, the centre ...
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4
votes
Accepted
Equivalence principle and gravitons
Before diving into answer this question, first let's acknowledge as discussed in the comments that the notion of "gravitons" is not needed to describe any gravitational observation to date. ...
- 46.5k
1
vote
Doubts about the parallel-axis theorem
Consider a body orbiting at some distance around a fixed point and spinning about its center of mass (such that both rotations are in the same plane).
The orbital angular momentum will be that of a ...
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0
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Reference frame doubts about isotropy
Even though Rennie and Moretti's answers cover most of the issues connected to this question, I would like to address it more directly, eliminating possible misunderstandings from the beginning.
First,...
4
votes
Accepted
Can non-inertial/fictitious forces be understood as covariant derivatives?
Yes it is possible already in classical physics.
In classical physics, spacetime is a 4-dimensional affine space $\mathbb{A}^4$ equipped with a surjective affine map $T: \mathbb{A}^4 \to \mathbb{R}$ ...
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1
vote
Does Galilean relativity constitute a dynamical symmetry or an isometry?
The Galilean group is a double isometry. It leaves invariant the following two quadratic forms:
$$dt^2, \quad \left(\frac{∂}{∂x}\right)^2 + \left(\frac{∂}{∂y}\right)^2 + \left(\frac{∂}{∂z}\right)^2,$$
...
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0
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Can non-inertial/fictitious forces be understood as covariant derivatives?
Of course, the Euler-Langrange mechanism to derive equations of motion implies terms, quadratic in the velocities by the kinetic energy term $$\frac{d}{dt}\nabla_{\dot x(t)}(\dot x\cdot g(x(t)) \cdot \...
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3
votes
How does the definition of a rigid body imply constant distance from the center of mass?
Let us cosider the case of $N$ material points $\vec{x}_i$, with masses $m_i$, $i=1,2,\ldots, N$.
Let us denote by $\vec{G}$ the position of the center of mass and by $M$ the total mass.
$$M(\vec{G} - ...
- 69.9k
0
votes
Cause-effect definition of fictitious forces
Newton's first law states that "A body remains at rest, or in motion at a constant speed in a straight line, unless acted upon by a force."
Maybe you could expand on this using a similar ...
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6
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Cause-effect definition of fictitious forces
In Newtonian mechanics, there are special reference frames where bodies, if sufficiently far from any other body in the universe, move with constant velocity (including the case of zero velocity).
...
- 69.9k
0
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Cause-effect definition of fictitious forces
Fictious forces do not exist in reality,as the name suggests, They are "Fictious", When we write equations in non-inertial reference frame, We add an extra 'Mathematical term having ...
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2
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Cause-effect definition of fictitious forces
My recommendation is to start with emphasizing the central role of inertia.
I will first discuss the concept of Inertia, and from there I will move to the expression 'fictitious force'.
In daily life: ...
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0
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Is the movement of earth absolute or does it just depend on the frame of reference
Rotation is absolute. It may be that rotation is relative, but relative to all the gravitational interactions with the rest of the universe, in which case the absolute character of rotation is an ...
- 12.7k
1
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Cause-effect definition of fictitious forces
I was giving them an explanation using Reference Frames (real forces are visible in every frame while fictitious ones are not), but they weren't convinced
Your explanation is correct. Them not being ...
- 94.5k
12
votes
Cause-effect definition of fictitious forces
I find engineers to be fixated on coordinates, which makes sense: they calculate and build stuff. They make CAD drawings, too. Coordinates matter.
A key part of physics is: physics does not care about ...
- 31.4k
5
votes
Cause-effect definition of fictitious forces
Yes, the student’s comment does make sense in most circumstances, given our most basic causal intuitions, but it’s impossible to see why without stepping back and figuring out where these intuitions ...
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10
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Cause-effect definition of fictitious forces
The idea of adding an extra "force" in some situations because it enables the use of Newton's laws.
Imaging a book on a frictionless horizontal table.
You are sitting at the table and ...
- 93.6k
24
votes
Accepted
Cause-effect definition of fictitious forces
I have found that assigning causality to forces and accelerations is tricky. It's intuitive to think that forces cause accelerations, and 99% of the time it won't get you into trouble. But its ...
- 45.6k
4
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Cause-effect definition of fictitious forces
I have never taught General Physics for engineers, but I applaud the mindset of the student that asked the question. He/she has a valid point: if there is no way to practically distinguish two ...
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1
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How do quantum probabilities transform under Lorentz transformations?
For the free QFT, the boost of a time dependent solution of the Schrodinger eqn is given by :
$$|p_1, p_2...\rangle e^{-i\omega _{p_1} t -i \omega _{p_2} t....}\rightarrow |\Lambda p_1 , \Lambda p_2......
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