# Tag Info

### 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 ...

### 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}...

### 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 ...
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 ...
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. ...
Accepted

### What experiment should be conducted?

Given the two scenarios: The bus is moving at steady state speed on a flat ground and suddenly accelerates forward The bus is moving at a steady state speed on a flat ground and suddenly encounters ...

### 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)-...

### 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 ...

### 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 ...
1 vote
Accepted

### Trajectories of Non-Uniform Accelerations in Special Relativity

The natural way is to specify proper acceleration with respect to proper time. This uniquely specifies your trajectory, given that you give its initial spacetime event and corresponding normalised 4-...
1 vote

### Centre of masses of 2 bodies together

Yes. Because the definition of center of mass is defined as $$\vec r_{cm} = \frac{\sum_i m_i \vec r_i}{\sum_i m_i}$$ If the masses were all equal, this would be the "average" position of the ...
1 vote

### Centre of masses of 2 bodies together

It is correct. What you're actually saying is: $$\frac{ab+cd}{a+c}=\frac{a\frac{ab}{a}+c\frac{cd}{d}}{a+b}$$ Where a is mass of the rod, b is the coordinate of its center of mass, c is mass of the ...
1 vote

### Conceptual meaning of frame of reference for kinetic energy

This is a pretty strange concept, I don't blame you for feeling confused! To use your example of two objects approaching each other at 8 and 10m/s respectively, you might be able to see that if you ...
1 vote

### What experiment should be conducted?

Measure the acceleration with an accelerometer (or measure the effective g by measuring the period a pendulum), and the direction in which a pendulum at rest hangs from the ceiling. With those two ...
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 ...
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 ...
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 ...
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 ...
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  or answers in . However, based on my learning style, these arguments do not work for me. This is ...

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