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1answer
185 views

Finding angular momentum about the center of mass?

If we have a couple of particles of an equal, unknown mass: $$r_{+} = (c + e^{-Bt} \cos({\theta}))\textbf{x} + (d + e^{-Bt} \sin({\theta}))\textbf{y}$$ $$r_{-} = (c - e^{-Bt} ...
0
votes
1answer
95 views

Help deriving the general linear wave equation $d^2y/dx^2=(1/v^2)d^2y/dt^2$ [closed]

How do I derive the General Linear Wave Equation $$d^2y/dx^2=(1/v^2)d^2y/dt^2?$$ My teacher differentiated the general wave function $f(x + vt)+g(x - vt)$ twice with respect to both variables to get ...
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0answers
61 views

Why such hypersurface orthogonal vector leading to $g_{0i}=0$ for $i=1,2,3$?

Suppose that the hypersurface orthogonal co-vector $W$ us perpendicular to the family of hypersurface defined by a function $\varphi$ with $\varphi=constant$. If we choose a coordinate in which ...
1
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1answer
94 views

What volume of helium at standard ambient pressure and temperature is required to lift one kilogram of mass? [closed]

I used the Ideal Gas Law PV = nRT where P is the pressure of the gas P = 1.033 kgf/cm squared V is the unknown volume of the gas n is the amount of substance of ...
1
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1answer
124 views

Ampère's Law for an infinitely long straight wire

In order to find the magnetic field generate by an infinitely long straight wire of radius R, I have to use local Ampère's Law : $\boxed{ \oint_L B\cdot dl = \mu_0 \sum_j I_j}$ If $r > R$ then $ ...
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1answer
53 views

What is a realistic launch speed of a grappling hook gun? [closed]

In The Dark Knight, at the end of the movie Batman throws the Joker off of a building and then shoots his grapple hook gun to catch him. Is 68m/s a reasonable speed for a grapple gun to shoot?
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0answers
135 views

Angular momentum of 2d harmonic oscillator

So, I have the problem of determining the spectrum of H and L, in terms of creation and annihilation operators of angular momentum... The problem goes along with what is happening on this page. ...
4
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1answer
251 views

What the heck is negative effective mass?

I am reading this book:Solid State Electronic Devices by Ben G Streetman and Sanjay Kumar Banerjee. I have some doubts in the article 3.2.2 Effective mass. In this the aythors say that ...
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0answers
79 views

Having trouble with a homework problem involving rotation [closed]

This is for a past homework assignment so it's already been solved. We wrap a light, nonstretching cable around a 9.00kg solid cylinder with diameter of 34.0cm . The cylinder rotates with ...
1
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1answer
146 views

correct formula for Mass Defect / Binding Energy? [closed]

I'm a web developer and I have to change an online course. The course teaches Advanced Nuclear Theory. In the 'Mass Defect and Binding Energy' chapter, it has this formula: $ \Delta M = Z(m_p) + ...
1
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2answers
618 views

What is the Reduced Density Matrix?

The difference between pure and mixed states is the difference in their density matrix structure. For density matrix $\rho$ of mixed state the trace of $\rho^{2}$ should be less than 1. For pure ...
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1answer
78 views

Determine the distance from time-velocity graph [duplicate]

A small car that we made in class was travelling and its time was recorded at 1,2,3, and 4 meters. The points in the form (seconds, meters) are as follows: ...
0
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1answer
53 views

Translation invariance in crystals

Context: solid state physics - dynamics of atoms in crystals. Coupling constants are defined as: \begin{align*} \frac{\partial^2\Phi}{\partial r_{nai}\partial r_{mbj}} = \Phi_{nai}^{mbj} ...
2
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1answer
98 views

Lagrangian to Hamiltonian

I'm having some problems with an assignment where I have to state the Hamiltonian from the kinetic energy $T$ and potential energy $U$. These are as follows: ...
0
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1answer
72 views

Capacitor with dielectric between the plates

Let's assume we have a capacitor of capacitance $C$ and potential difference $U$. After charging it we disconnect it. Then we put a dielectric between the plates. I know that capacitance will increase ...
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1answer
161 views

Atwood machine with spring

I'm just beginning to learn about Lagrangian mechanics, and I am asked to find the kinetic energy of this Atwood machine (See figure). I am told, that the kinetic energy should be: ...
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1answer
219 views

Commutator with Pauli spin matrices and the momentum operator

How is $\left[\vec\sigma \cdot \vec p, \vec \sigma \right]$ proportional to $\vec \sigma\times \vec p$, where $\sigma$ are the Pauli spin matrices and $p$ is the momentum operator?
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1answer
193 views

free falling object with air resistance?

we already know that if we plot speed vs time of free falling object it will be y=gx graph because we know the acceleration is gravity. If it have air-resistance acceleration will be change right?. ...
2
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0answers
63 views

Limits of integration for the radial wave function of the Hydrogen atom in the WKB approximation

I am working a problem where we have to find the energy eigenvalues for the radial wave function of the hydrogen atom for $\ell=0$ using the WKB approximation. I am sure that I set up the integral ...
1
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1answer
46 views

Taylor expansion term equals zero?

I have to Taylor expand an effective potential $U_{eff}$, which is given by: $$U_{eff}(r)=-\frac{Gm_{1}m_{2}}{r}+\frac{l^{2}}{2\mu r^{2}}$$ I then expand it and get: ...
4
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0answers
55 views

Wall stress of a hexagonal pressure vessel

Problem: I want to calculate the stress in the walls of a hexagonal pressure vessel but I can't manage to get coherent results. For long vessels, cylinders are supposed to have the lowest hoop stress ...
0
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1answer
73 views

Addition of Angular Momentum

I am tring to find the eigenvectors of a two spin system, with $j_1=3/2$ and $j_2=1/2$. To start, $$m_1 =-3/2,-1/2,1/2,3/2$$ $$m_2=-1/2,1/2$$ For $j_1$, there are 4 possible states, and 2 possible ...
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1answer
83 views

Current Carrying Coil - Torque [closed]

i'm stuck on how to answer the last part of this question and would really appreciate some pointers. A rectangular coil (sides $a = 15 \, \mathrm{cm}$ and $b = 20 \, \mathrm{cm}$, $m= 15 \, ...
1
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1answer
180 views

Write down equations for the electric field and magnetic fields components of a linearly polarized plane wave [closed]

A linearly polarized plane wave at 100 MHz is propagating in the $z$ direction. The electric field vector makes an angle of 30° with the $x$-axis. Its peaks amplitude is measured to be ...
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3answers
65 views

electric current in parallel circuit [closed]

If I have a parallel circuit with two resistors ($R_1=48 \Omega$ and $R_2=16 \Omega$) - and $R_1$ has a $0.1 A$ current running through, is it possible to calculate the current through $R_2$ without ...
3
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2answers
126 views

Why don't the derivatives of the blackbody spectrum over frequency and wavelength match?

The question is: The radiation emitted by a black body can be represented either by the energy distribution over the wavelength or by that over the frequency. In the first case the maximum energy ...
1
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2answers
74 views

Using an electric winch to compress a spring and launch an object

I'm having trouble grokking the relationship between a winch's pull/torque and a spring's potential energy. I would like to compress a spring using an electric winch and figure out how far it will be ...
0
votes
1answer
87 views

Physics of Skiing: Ideal Carving Equation

I am conducting a research paper on the physics of skiing, specifically how ski parameters affect the ski's ideal carve. I have come across this paper (abstract link to arXiv paper), which is ...
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votes
1answer
47 views

Linear speed of falling pencil [closed]

Find the final linear speed of pencil. Pencil would move because of gravity. Let's assume that it is balancing on the tip. We do not take friction into consideration. – Given $$d=0.15m$$ Similar ...
1
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0answers
22 views

Finding the initial vertical and horizontal velocity [closed]

Basically I need to get an initial vertical velocity to make the ball reach a specific height, but also I need to get a horizontal verlocity so that the ball will travel the distance. For an example ...
1
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4answers
171 views

Can you lift yourself standing on a wooden block using pulley mounted on block? [duplicate]

I am new to this site. Please help me figure out this problem. Is it possible to lift oneself like shown in schematic? Waiting for good suggestions. Chirag
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1answer
91 views

Null Geodesics in flat 2+1 dimensional Minkowski space

For a given line element in flat 2+1 dimensional Minkowski space $$ g = ds^{2} = − dz \otimes dz + dx \otimes dx + dy \otimes dy .$$ The null geodesics are supposedly given by: $$ x = lu + l' $$ ...
3
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0answers
90 views

Computing the Einstein tensor for a spherically symmetrical metric using the tetrad formalism

I am having some trouble understanding how to use the tetrad formalism. I will start with what I have so far, my question will be after that. I begin with the metric $$ \text{d}s^2 = e^{2a} \text{ ...
1
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0answers
194 views

Power dissipated through resistor in RC circuit with charged Capacitor [closed]

so here's the problem: A simple RC circuit where the capacitor has been charged, and two resistors are in parallel configuration. How does one find the power (as function of time) through any of the ...
1
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3answers
97 views

Current through two inductors after a long time

I'm having trouble with visualizing the following problem, which is asking me for the final, steady current in both inductors $L_{1}$ and $L_{2}$. I was thinking that after a long time, essentially ...
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0answers
90 views

Difference between expectation values of $L^2$, $L_z$ and measuring $L^2$, $L_z$

I was given with this hydrogen radial wavefunction $$ R_{21} =\left(\sqrt{\frac{1}{3}}Y^0_1 + \sqrt{\frac{2}{3}}Y^1_1\right) $$ and was asked to find a) What are the expectation values of the ...
1
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1answer
146 views

Spring Damper System : Recoil Reduction

I am designing a man-"portable" 30mm sniper rifle. Although it may be classified as a anti-materiel cannon. I have the mass of the round in total, and the mass of the projectile leaving the barrel, ...
0
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1answer
144 views

Expectation value of energy from the position state of hydrogen atom [closed]

I was given with the position state of hydrogen atom: $$ R_{21} =\left(\sqrt{\frac{1}{3}}Y^0_1 + \sqrt{\frac{2}{3}}Y^1_1\right) $$ I am getting confused about getting the expectation value of ...
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0answers
44 views

Spherical Harmonic projection on axis

I am trying to solve for the Spherical harmonics $Y^m_{l=1}$ with a second axis at an angle $\alpha$ with respect to the z axis. Then this can be used to find the probability that a particle with ...
0
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2answers
141 views

A bird problem and trains [closed]

So that the problem: There are 2 trains at speed of 30km/h in the same trail and opposite directions and moving for a collision at 60km of distance of each other. One bird in front of one of the ...
3
votes
1answer
183 views

Harmonic Oscillator potential, proof that Gaussians remain Gaussians?

I read in several papers that for a Harmonic Oscillator Hamiltonian in the time dependent Schrödinger equation a Gaussian wave packet remains Gaussian. Unfortunately I could not find any proof for ...
0
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1answer
54 views

Construction of Angular Momentum eigenvectors

I have a problem that asks (verbatim) Carryout the construction of the eigenvectors of total angular momentum and its z component for $j_1$=3/2 and $j_2$=1/2 I am not completely sure where to ...
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2answers
125 views

Total internal reflection problem — could the textbook be wrong?

This regards the following problem: A ray of light is traveling in glass and strikes a glass/liquid interface. The angle of incidence is $58.0^\circ$ and the index of refraction of the glass is $n = ...
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votes
1answer
64 views

Can someone explain the solution (provided) of this conical pendulum work problem [closed]

In the image, it looks like the tangential direction is always 45 degrees away from the string, not 90 degrees. Is it not the circular path that the solution is talking about?
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2answers
125 views

Photoelectric effect: current vs wavelength

In an experiment where the type of metal,intensity of light and potential difference across a battery is kept constant at 2V the results show that an increase in wavelength, obviously in turn ...
1
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1answer
101 views

Is speed squared constant?

q. A heavy particle is projected at speed $U$ at an angle $\alpha$ to the horizontal. The particle is subject to air resistance which is experimentally found to vary proportionally to the square ...
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2answers
78 views

Conservation of Mass, Momentum and Energy - Airburst Rounds

I have a airburst round with set amount of fragments with a certain shape inside my bullet. After a certain distance has travelled, a charge explodes in the back of the round disintegrating the shell ...
1
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1answer
246 views

3D ballistic trajectory with quadratic drag. Calculating position and velocity at time $t$

A particle starts at the origin and has an initial velocity represented by a 3D vector. The particle experiences gravity and air resistance with quadratic drag (based on velocity^2). What I've been ...
0
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1answer
99 views

Magnetic Shielding?

Can magnetic fields be "blocked"? For example, in the game, TitanFall, a robot stop bullets with ( presumably ) a magnetic shield. I wish to calculate the magnetic force required to stop a bullet ...
2
votes
1answer
32 views

Kinetic Energy of a Symmetric Dumb-Bell

The problem is as follows: Consider the system consisting of the symmetric dumb-bell (two particles each of mass $m$ connected by a light, inextensible rod of length $l$) moving in the plane. Use as ...