# All Questions

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### How does an optical chopper work?

How is an optical chopper eliminating drift? Or in more concrete words: Why is a chopped signal eliminating drift (DC -> AC)?
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### Creation and annihilation operators for fermions from anticommutator

In a question, I was given that $a^{\dagger}a + a a^{\dagger} =1$ and asked to show what a|n> and $a^{\dagger}$|n> would be, given that H|n>=($a^{\dagger}a$ + 1/2). I am getting the wrong ...
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### What determines the focal point of a mirror or a lens?

Definition:The focal point of a lens or mirror is the point in space where parallel light rays meet after passing through the lens or bouncing off the mirror. A "perfect" lens or mirror ...
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### Variation of entropy in a transference of heat

When we put two bodies with different temperatures in contact, there will be a heat transference between them. The variation of the entropy of the system is well known (this is computed in most ...
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### How to work out tension required for static equilibrium in a trebuchet? [closed]

I have the above system The dotted line at 45degrees of the horizontal is a rope with tension T The total mass of the system is 6035kg with the centre of mass located 0.82m from the pivot. The length ...
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### Does aerogel “blow up” in a vacuum?

Discussions in comments below How can aerogel be lighter than air? really make an answer to this important. The conclusions there are that aerogel could in some cases be "lighter than air" ...
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### If two opposite charges are placed next to each other and a field line leaves from one, at what angle does it reach the other one?

Two charges $+q_1$ and $-q_2$ are placed at A and B respectively, A line of force emerges from $q_1$ at angle $\alpha$ with line AB. At what angle will it terminate at $-q_2$? My approach: Since no ...
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### How to determine the pivot point in a system?

I am just introduced to highschool physics and was wondering about the following thought-experiment. Suppose in the following setup m1 and m2 have the same mass. This system is floating in outer ...
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### How does one divide by a vector when calculating pressure?

Sir $P=F/A$ And since $F$ and $A$ both are vectors but $P$ is scalar. So doesn't it violates that "Division is NOT defined for vectors"?
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### Question on four velocity condition for timelike observers

I am a bit rusty on GR, I have the condition $u_{\mu}u^{\mu} = -1$, in some notes I am given that we can obtain: $$-1 = u^{2}_{t} [g_{tt} +2 \Omega g_{t \phi} + \Omega^{2} g_{\phi\phi}] \tag{1}$$ ...
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### What makes the electron, as an excitation in a field, discrete?

In standard quantum mechanics, the wave function have discrete energy-values due to a potential. However, my very limited understanding of QFT is that electrons are excitation in the Dirac field, and ...
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### Ricci Tensor component, index lowering doubt

$v^j$$v^k$$[R(e_i,e_j)]e_k$.$e_i$ gives $v^j$$v^k$$R^l$$_k$$_i$$_j e_l.e_i Then v^j$$v^k$$R^l$$_k$$_i$$_j$ $g$$_l$$_i$ Finally, $v^j$$v^k$$R$$_i$$_k$$_i$$_j$ Which upon summation on i gives $v^j$...
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### How does the period of an oscillating mass-spring system comply with the relativistic time dilation as viewed by a moving observer?

Consider a simple mass-spring system. The spring is attached to the ceiling in the lab frame of reference and a block of mass $m'$ is suspended from the spring. The mass starts to oscillate with ...
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### Connection between Hamiltonian and density matrix

I found this equation in my notes from a seminar on BCS theory $$H_{m,n} = \frac{dE}{d\rho_{n,m}}$$ where $H_{m,n}$ are elements of the Hamiltonian in matrix representation, $\rho_{n,m}$ are ...
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### Why does mass-spring system tend or prefer to oscillate with natural frequency? Can you explain it theoretically rather the derived version of it?

Why the mass spring system prefers to oscillate at natural frequency, why does it rather oscillate at other frequencies? Every object in the universe is always vibrating, is it true? if it is the ...
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### Haag's comment on the relation between fields and particles

I am very confused by the statement made in Haag's, Local Quantum Physics: Fields, Particles, Algebras (page 46): ... the idea that to each particle there is a corresponding field and to each field a ...
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### Static or dynamic?

Long time ago, I found a question in the book Introduction to Classical Mechanics-David Morin. The question was Sliding sideways on a plane: A block is placed on a plane inclined at an angle $\theta$....
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### What are the dimensions of the inflation slow-roll parameters?

The inflation slow-roll parameters are: $\epsilon = \frac{M_{pl}^2}{2}(\frac{V'}{V})^2$ $\eta=M_{pl}^2 \frac{V''}{V}$ What are the dimensions of $\epsilon$ and $\eta$? What about $V$ and its ...
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### What does negative time mean or what does it show?

In this question when I solve for time I get $t=-1$ & $t=5$. What does it actually mean at $t=-1$?
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### The work done by some weirdly behaving forces in some wierd trajectories

Sometimes we meet some peculiar situations, like some forces act on a body placed in a vehicle at rest. The forces appear as if they never do any work on the body. But, when the vehicle accelerates, ...
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### Ocean waves breaking slowly at the shore during periods of turbulence

During storms, I have often watched ocean waves breaking at the shore as if they were in slow motion. I've wondered if the slow curling of the waves during these times was somehow due to their ...
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### Understanding constraint for not losing contact

What is the exact constraint for two bodies remain in contact? Consider the case of a rod constrained to move downward on an inclined wedge. It is known that the velocity of the wedge and the velocity ...
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### Assumptions in a Moving Fluid

I saw this paper recently "On Integrals of the Hydrodynamic Equations That Correspond to Vortex Motions" by Hermann Von Helmholtz. It can be found here. In this research paper he discovered ...
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### What happens to the moving ball that collides with wall and the wall disappears at the instant all kinetic energy of ball becomes potential energy?

Consider a wall and a ball that is traveling towards the wall with uniform velocity. Assuming no friction of any kind in this situation. When ball collides with the ball, the ball gets squished as its ...
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### Why are black holes so special?

When reading about black holes, I have also read about things relating to black holes like holographic principles, parallel universes, black hole connecting multiple universes, etc. But black holes ...
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### On-site symmetries can be gauged, but is a gaugeable symmetry necessarily on-site?

I've always liked lattice QFT because it's mathematically unambiguous and non-perturbative, but it does have two drawbacks: (1) the lattice is artificial, and (2) some features are messy. One of those ...
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### Which particles does the Higgs Field give mass to?

I have found contradictory information about this. Does the Higgs field give mass only to the $W^+$, $W^-$, and $Z^0$ bosons or does it give mass to other particles as well?
Say we have $n$ engines sitting on a rigid body. Each engine has position $R_i$ and points in a certain direction and generates a force in that direction, $F_i$. The magnitude of that force ($k_i$) ...