All Questions

0
votes
0answers
19 views

Simple argument for Melia's linearly expanding Universe [on hold]

Here is a simpler version of my argument for Fulvio Melia's linearly expanding Universe. The Universe is assumed to be isotropic around every point P. Therefore, in Newtonian terms, the net ...
2
votes
2answers
54 views

How is mass of neutron compared with proton when it is inside the nucleus?

The mass of free neutron is around 0.1% more than the mass of proton. When it is bound inside the nucleus, neutron mass is less compared to its mass in free state. The question is how much mass it ...
3
votes
1answer
30 views

Why are displacements added algebraically during interference of waves , why not vectorially?

In the diagram (A) , the individual waves are crossing at an angle to each other , so the dispacemnt of the particle at their intersection must have an angle between them , and that must involve ...
1
vote
1answer
41 views

Not parabolic falling

Suppose a small mass m is launched from height h with horizontal initial speed v0; if the ground is horizontal, it's easy to calculate the equation of movement (parabolic), because the coordinates of ...
0
votes
0answers
28 views

Quantum Measurement in Different Basis [duplicate]

How can you make measurements on a qubit that is in a different basis than your measurement qubit? (i.e. You want to measure a single qubit in the $|+\rangle$/$|-\rangle$ basis but you can only make ...
0
votes
0answers
17 views

24a battery on a an engine that needs 14a only? [on hold]

Can you actually use an 24a battery on an engine that was made for a 14a battery using resistors ?
0
votes
2answers
43 views

Name of the matrix that appears in matrix form of Hamilton's equations of motion

Consider a harmonic oscillator described by the second order differential equation $$\ddot{\phi} + \omega_0^2 \phi = 0 \, .$$ Defining $v \equiv \dot \phi$ we get two simultaneous equations \begin{...
0
votes
1answer
37 views

Why do semiconductor lasers become weaker after extended use (not due to battery)?

Why do semiconductor lasers become less bright as they are used for longer periods of time (assuming their battery is full)? Does it have something to do with the PN junction reaching some kind of ...
0
votes
2answers
39 views

Finding 3 unknowns in 2D-collision problem [on hold]

While solving a 2D-collision problem, I obtained the following 3 equations: $$0.866\,|\vec v_{Af}|+|\vec v_{Bf}| \cos \theta_{Bf}=6$$ $$0.5\,|\vec v_{Af}|+|\vec v_{Bf}| \sin \theta_{Bf}=0$$ $$|\vec v_{...
2
votes
2answers
67 views

Does time stop inside a black hole? How about at it's singularity?

I'm a layperson who just read Brief Answers to Big Question and in the book, Hawking proposes that: A typical black hole is a star so massive that it has collapsed in on itself. It’s so massive ...
6
votes
1answer
65 views

Experimental tests of Schrodinger evolution, position distribution, in square well and other simple systems?

Have the energy eigenfunctions in position space ever been experimentally tested for the simplest system undergraduates encounter when learning quantum mechanics, the square well? If not, what is the ...
2
votes
2answers
26 views

Why is a delta resonance decay not a radioactive decay

A delta resonance decays as given in http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/delta.html . I wonder, why is it not a radioactive decay? In principle, most/all decays should be radioactive ...
1
vote
1answer
73 views

Is 1 liter always equal to 1 cubic decimeter, independently of temperature, pressure, etc?

I recently found this conversion table for the unit conversion $\rm mmol/m^3 \ \leftrightarrow\ \rm mmol/L$ (millimoles per cubic meter to millimoles per liter) My physics is very rusty, but just to ...
0
votes
0answers
46 views

It is said that an object cannot exceed the speed of light, because high speed makes the object closer to infinite mass, but speed relative to what?

That is, the speed of an object, when getting closer to speed of light, makes the mass of the object closer to infinite, but isn’t speed relative? So a man on earth feels a metal ball on the table has ...
3
votes
3answers
72 views

Confusion on symmetry and basis transformation

Let {$|a_n\rangle$} and {$|b_n\rangle$} be two basis related by: $|b_n\rangle = \hat{U}|a_n\rangle \forall n$. From my understanding then the unitary operator $\hat{U}$ only transforms the basis {$|...
2
votes
0answers
42 views

Huygens’s principle [on hold]

On the application of Huygens’s principle with an incident wavefront on the mirror, we see that wavelets are emitted at each point as shown. My question is how can the direction of propagation of the ...
1
vote
0answers
19 views

Query about the Michelson interferometer

Say I am cycling past a Michelson interferometer set up. Someone at rest with the experiment will see both beams travelling a distance $L_0$ and hence conclude that $T_{AB}=T_{AC}=\frac{2L_0}{c}$. ...
1
vote
1answer
34 views

How to (hypothetically) calculate $\tau$, the mean free time?

Referring to the Drude Model, I've seen a lot of excellent questions on whether $\tau$ should be thought of as the "average time between collisions" or the "average time until the next collision", and ...
0
votes
0answers
18 views

Total mass-energy of a system in GR [duplicate]

the Gravitation book confirm that "for closed universe the total mass-energy and angular momentum is undefined and undefinable" why many cosmologists claim that the total energy of a closed universe ...
0
votes
1answer
13 views

Difference between haidager fringes and Newton rings?

What's the difference between a Haidinger fringe and Newton's Ring? I know visually, the central fringe of a Newton ring is dark, whereas the central Haidinger fringe is bright, but what's the ...
0
votes
0answers
31 views

Spherical capacitor with dielectrics [on hold]

Consider the following system: which consists of a conducting sphere with free charge 𝑄, a dielectric shell with permittivity $\epsilon_1$, another dielectric shell with permittivity $\epsilon_2$ ...
0
votes
1answer
20 views

Lorentz force within a flux tube

I know the formula for the Lorenz force exerted on a point charge $q$ moving with velocity $\vec{v}$ is given by $\vec{F} = q\vec{v} \times \vec{B}$. Now consider the flux tube with cross section $S$ ...
-2
votes
0answers
11 views

Lenz's Law/Right Hand Grip Rule for Diamond Shaped Loop [duplicate]

A diamond-shaped loop of wire is pulled at a constant velocity through a region where the magnetic field is directed into the paper in the left half and is zero in the right half, as shown in the ...
2
votes
0answers
32 views

Is there a scalar field that gives rise to the energy-momentum tensor of a perfect fluid?

If I understand correctly, Sean Carroll's Spacetime and Gravity says that the energy-momentum tensor for a perfect fluid $$T^{\mu\nu} = (\rho + p)U^\mu U^\nu + pg^{\mu\nu}$$ can be obtained as the ...
-1
votes
1answer
19 views

How many degrees Celsius/Fahrenheit can be reached in 1 square meter area using magnifying glasses?

The magnifying glasses are each 10 meters in diameter, their collected light is concentrated via mirrors into a 1 square meter spot. Is it possible to get a temperature higher than 2000 degrees ...
3
votes
0answers
61 views

We just need a solar panelled SPAIN to power the world? [on hold]

A stiffer, more formal (form, schmform!) way to ask this is: Is a solar panel field the size of Spain enough to cover the worlds energy needs? (This assumes that everything in the world runs on ...
0
votes
1answer
24 views

Monte Carlo importance sampling in statistical physics finite approximation question

I am trying to understand a formula found in "Topics in Current Physics: Monte Carlo Methods in Statistical Physics Springer-Verlag" attributed to L. D. Fosdick methods comp physics 1963. I can't get ...
0
votes
0answers
20 views

Doubt in application of $GDT$ in electrostatics

Consider a volume charge distribution with continuous density $\rho({\bf r'})$. The electric field at ${\bf r}$ is: $${\bf E}({\bf r})=k\int_V \frac{\rho({\bf r'})}{R^2}\hat{\bf R}\, \mathrm dV$$ ...
-1
votes
1answer
30 views

In the SHM equation $F= -kx$, $k =mw^2$ why not use $mf^2$ where $f$ is frequency $w$ here comes out to be $1/s$ not $\text{rad}/s$?

The reason I am stating this is because on calculating the units of w(omega) I found is equal to s^-1 not regular the rad/s. Proof: ...
2
votes
2answers
141 views

A formula for delta function in quantum mechanics

I met a formula for delta function in a QM book ( not in English). The formula is used in the scattering theory. Its form is $$\lim_{\alpha\rightarrow \infty}\exp[i\alpha x]=2i\delta(x), ~~(x\geq 0).$...
-3
votes
0answers
37 views

Why quantum physicists saying quantum physics is very difficult to understand? [on hold]

I read the statements like "Anyone who thinks he understands quantum mechanics, doesn't understand quantum mechanics". I like to know why it is hard to understand.
0
votes
2answers
40 views

Trouble deriving the Lorentz transforms

After separately arriving at the two equations $x'=\gamma(v^2)(x-vt)$ and $t'=\phi(v^2)(t-\frac{vx}{c^2}$). Where $x'$ and $t'$ are the coordinates a moving observer ascribes to events. I had to show ...
1
vote
0answers
40 views

About device in which potential drops in direction opposite to current

A small intro: We have mainly three types of electronic-devices with their characteristic property: Resistor with Resistance($R$) Capacitor with Capacitance($C$) Inductor with ...
1
vote
0answers
22 views

Can we also Legendre transform the Lagrangian using $\frac{\partial L}{\partial q}$ instead of $\frac{\partial L}{\partial \dot q}$? [duplicate]

We calculate the Hamiltonian as the Legendre transform of the Lagrangian $$H(q,p,t) = p \dot q - L(q,\dot q,t), $$ where $p$ is the slope function $$p \equiv \frac{\partial L}{\partial \dot q} .$$ ...
-2
votes
0answers
33 views

Elasticity Young’s Modulus problem [on hold]

I don’t know whether my answer is correct ir not. The problem is: A steel cable $3.00 \text{ cm}^3$ in cross sectional area has a mass of $2.40 \text{ kg}$ per meter of length. If $500 \text{ m}$ ...
0
votes
0answers
26 views

What process is it when the polytropic index $n$ is equal to $- \infty$?

From $$Pv^n = \mathrm{constant}$$ When $n = - \infty$ What kind of process is it.... Eg... When $n= \infty$, It is constant volume.... Therefore when $n =- \infty$... What is constant?
-2
votes
1answer
37 views

Invariant mass in special relativity [on hold]

I'm following a special relativity course and I'm trying to understand how the invariant mass works. In particular I don't get how the following passages work. We have a collision between two ...
2
votes
2answers
30 views

Finding the distance to a distant star using standard candles

The thing is that i understand how we can measure the luminosity of a close star through absolute and apparent magnitude, because we can easily find the distance through trigonometric parallax. What i ...
-1
votes
0answers
38 views

Elasticity problem have insufficient information for it to be solved [on hold]

My teacher gave us this problem and I can’t solve it no matter how hard I tried. The information given by the problem is maybe lacking some information for it to be solved. Here is the problem: ...
1
vote
1answer
50 views

Is $H$ proportional to $B$ or not?

I was reading an article saying in the cylindrical coordinates a uniform magnetic intensity $H$ along the z-axis produces a magnetic field $B$ along the z-axes given by $$B\left(r\right)=\frac{...
0
votes
0answers
19 views

Ancient Southwesterners recorded same SN shapes as modern remnant images--why?

Ancient Southwestern images of the historical supernovae oftentimes count the number of days in daylight (esp. SNs 1006 and 1054), sometimes count the total --or total days of the first year, and ...
-1
votes
0answers
16 views

Diffraction at the everyday scale [on hold]

I know that the reason the wave like nature of matter isnt observerable at the everyday scale is because of how small planck's constant is. But if it were larger, say of the order of $10^2$ such that ...
2
votes
5answers
512 views

Why does energy conservation give me the wrong answer in this inelastic collision problem?

Suppose there are two objects. The first object is stationary and has the next parameters: v1=0 m/s, m1= 4kg. The second object is heading towards the first one with speed v2= 8m/s and mass m2= 60 kg. ...
3
votes
2answers
126 views

Why does electron $(-)$ keep rotating round the nucleus $(+)$ even they are attracted? [duplicate]

I'm a secondary school pupil, my physics textbook discussed atomic physics without deep details because we are young boys, to explain electron motion we first learn about rotational motion. The main ...
-2
votes
2answers
89 views

What is exact definition of potential energy if all forms of energy except kinetic comes under potential energy? [duplicate]

I need to know the exact meaning of potential energy. I AM confused what it actually means.They say all types of energy comes under 2 types kinetic and potential so what is potential energy then? My ...
2
votes
0answers
36 views

How is “linewidth” defined for a damped driven oscillator?

Suppose we have a damped driven harmonic oscillator governed by the following equation of motion: $$\ddot \phi(t) + 2 \beta \dot \phi(t) + \omega_0^2 \phi(t) = J(t) \, .$$ In the case that $J(t) = A \...
1
vote
3answers
34 views

Why can we ignore the interaction between heat reservior and our system in canonical ensemble?

When we derive the expression of canonical ensemble, we write the Hamiltonian of the whole system which contains our system and a heat reservior in the form, $$ H_{\text{whole}} = H_1(\boldsymbol x) + ...
4
votes
2answers
62 views

Recovering symmetry in coupled oscillators

Consider a pair of LC oscillators, one with capacitance $C_1$ and inductance $L_1$ and the other with capacitance $C_2$ and inductance $L_2$. Suppose they're connected through a capacitor $C_g$. We ...
0
votes
1answer
32 views

Faraday's Law, circuit, current change

The answer is A. Why doesn't the current change? The circuit experiences a change in magnetic flux because the area increases. This change in magnetic flux will induce an emf in the circuit. Wouldn't ...
0
votes
1answer
17 views

Equation for $N$-body problem using Jacobi Coordinate

For reference on Jacobi Coordinate used for solving 2-Body problem, I referred Wikipedia Jacobi Coordinate, and on looking at those equation I can't get the meaning of the symbol q in the equation for ...

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