# Tagged Questions

Applies to questions of primarily educational value - not only questions that arise from actual homework assignments, but any question where it is preferable to guide the asker to the answer rather than giving it away outright. Please READ THE GUIDANCE IN META before asking homework questions.

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### “Redshifting” of forces in stationary space - times

Here's the problem statement: Let $(M,g_{ab})$ be a stationary spacetime with timelike killing field $\xi ^{a}$. Let $V^{2} = -\xi _{a}\xi ^{a}$ ($V$ is called the redshift factor). (a) Show that the ...
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### Gamma Matrices in Dimensional Regularization

Prove that $tr\left(\gamma_\mu\gamma_\nu\gamma_\rho\gamma_\sigma\gamma_5\right)=0$ when the spacetime dimension is not 4. What I have tried: We know that $\gamma_\alpha\gamma^\alpha=d\mathbb{1}$, so ...
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### The problem in Sredniki's textbook: How do I calculate loop corrections for $\phi\phi\to\phi\phi$ with this Lagrangian?

The problem in Sredniki's textbook 10.5 : For a free scalar field $\psi$, the Lagrangian is $$\cal{L}= -\frac{1}{2}\partial^\mu\psi\partial_\mu\psi-\frac{1}{2}m^2\psi^2$$ Here we use the metric ...
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### Is it possible to determine particles' velocities from their crossings on a ring?

2 particles are constrained to move in a ring. Both particles begin moving at $t=0$ from $p=0$, each particle moving in the opposite direction of the other. It is known that they are moving at ...
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### Poles for a particle scattered in a delta potential

I am working on problem a professor gave me to get an idea for the research he does, and have hit a point where I'm having a difficult time seeing where I need to go from where I'm at. I would also ...
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### Is there a graphical representation of the Lorentz transformation equations?

I always loved theoretical physics as a kid and when I came upon this site while seeking computer advice via superuser I had to stick my silly little head into an oasis of intelligence. I have often ...
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### An Electric Potential Glued to a Cubic Insulator to Replicate a Point Charge: Charge Distribution

I have been going back over this problem with a friend for the better part of a day: A potential is glued to a cube insulator so that outside of the insulator the field is the same as a point ...
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### Homopolar motor and Lorentz force

My second grader thought making a homopolar motor for her science experiment would be fun. And, it was. Now I am trying to explain how it works and the Lorentz force. Please help me by giving me a ...
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Well this got me stumbled, because I've been wondering what the "question" is. One of the example examns got the following question (strangely there are no supplied solution books): A $0.2 {\rm ... 1answer 266 views ### Semiconductor problem: how much Boron is required to establish a certain amount of charge carrier density The problem description is as follows: Boron is used to dope 1 kg of germanium (Ge). How much boron (B) is required to establish a charge carrier density of 3.091 x 10^17 / cm^3. One mole of ... 1answer 56 views ### Looking for a simple proof of symmetry of linear susceptibility tensor Getting a diagonal susceptibility tensor: Define the linear susceptibility tensor as$\chi_{ij}$:$P_i = \epsilon_0\chi_{ij}E_j$, using standard notation for the electric field and the polarization. ... 1answer 140 views ### Sum of forces with liquid in rotation It's not homework (I'm teacher). I would like to compute sum of forces on this study : The shape is symmetrical like that I'm sure the center of gravity is in the center of the shape. I compute ... 0answers 327 views ### Equation of motion of charged particle under effect of magnetic field A charged particle with specific charge q/m diverges from the origin [to the first quadrant] by angle X with x axis and with velocity v. To bring the particle back to the x axis, a magnetic field B is ... 0answers 58 views ### Switching from an accelerated frame of reference to a locally inertial reference system Using the equivalence principle, show that the interval for an accelerated observer ($\textbf{g}$uniform and constant) has the form $$ds^2|_{\text{first order in ... 0answers 158 views ### Wave equations for two intervals at Potential step Lets say we have a potential step as in the picture: In the region I there is a free particle with a wavefunction \psi_I while in the region II the wave function will be \psi_{II}. Let me ... 0answers 38 views ### Is it easier to determine the number of states with raising/lowering operators or using scattering? A particle is bound by$$V(x) = \begin{cases}\infty,& x <0 \\ \frac{-32\hbar^2}{ma}, & x\le a \\ 0, & x \le a\end{cases}$$a) how many states are there? i'm attempting ... 0answers 1k views ### Why a person with a further near point experience a larger magnification with a magnifier Two people, Micah and Lyra, with different near points are equally close to an object. Both inspect the object through the same magnifier by holding the lens close to the eye. Micah's near ... 0answers 291 views ### Maximum Shear on a Beam - beam with fixed support on one end and hinge on other end A beam \displaystyle 3m long with fixed support on one end and hinge on the other end is subjected to a uniform load of 10\ kN/m. What is the maximum shear of this beam? The solution is this one: ... 0answers 285 views ### What is the current induced in a straight wire when a magnetic field is turned from on to off? Consider a straight wire which runs along the y-axis. A B-field exists which is uniform and only in the x-direction:$$\vec{B}=(B(t),0,0)$$The B-field at time t_0 is 'switched off' in ... 0answers 399 views ### Nuclear reaction: creation of deuterium A proton p collides with a neutron(at rest) n at relatively low-energies and creates a 'deuterium-core' d:$$p+n->d+\gamma$$Find the wavelength for the photon as a function of the proton's ... 0answers 120 views ### Finding final charge distribution on capacitors on grounding I am trying to solve the following problem on capacitors: Here's the circuit I have to find the total energy loss when all three switches are closed, and subsequent equilibrium attained. I had ... 0answers 41 views ### How do I minimize the the stress created by fore-aft forces? My friends and I are designing a bench press spotter. Essentially, when the user needs help lifting the barbell due to muscle fatigue, an arm on each side of the user raises to provide assistance. ... 0answers 380 views ### Whats the contact area for elastic contact between a cylinder and a plane surface? I am trying to determine the contact area between a cylinder and a plane surface of two different materials so that the plane lies tangent to the cylinder. There is elastic contact between the two ... 0answers 122 views ### Number of microstates of discretized paths Let us consider a square grid, which has been rotated by 45deg. On this grid we deﬁne a path, the directed polymer, which starts at the origin (t = 0) and extends in the positive t-direction ... 0answers 305 views ### How to solve fixed-fixed beam with finite difference method? What equations to use on this system to form a matrix A with dimensions [n,n] and load vector q with dimension [n] ? I am trying to get vertical displacement w.$$w = A^{-1}\times q$$... 0answers 320 views ### How to find angular velocity of a point inner a circumference Let's consider a cicumference that have the center in the origin of axes and rotates around x-axes. Let's stick a bar in a point$A$of this circumference and at the end of the bar let's stick a mass ... 0answers 352 views ### How do you determine the torque caused by the mass of a lever? Suppose we have two objects sitting on two side of a lever, and the lever also has a mass, and those objects have masses. Then how we can balance$\sum τ$? This is what I have done: ... 0answers 94 views ### Using thermodynamics and Kinematics together to solve a parachuter problem? I need to find a parachutist's displacement after a given height (nearly 37000m) and at a given latitude. I have his mass, area, parachute area, drop height, parachute deployment height, data about ... 0answers 358 views ### What is the pressure in a vertical pipe that has moving fluid through it? I have a system like this When the valve is closed, pressure along any point along the thin tube may be found thru pgh. 1. What about when the valve is opened? 2. Does the top of the jar need to be ... 0answers 530 views ### Question about the refraction of light in a swimming pool Here is a homework question that I'm having a hard time understanding: Out of pure intellectual curiosity you have donned a snorkeling face mask and allowed yourself to sink to the bottom of the ... 0answers 269 views ### Reflectance and phase shift vs incident angle and wavelength Plot the refectance and phase shift on reflection$\varphi$for p and s polarized light versus incident angle$\theta$for various wavelength$\lambda_1=.., \lambda_2=..,..$in the simple case of ... 0answers 152 views ### I'm having trouble understanding the dimensions of this falling body problem I'm working with this: A 50 kg mass is shot from a cannon straight up with an initial velocity of 10m/s off a bridge that is 100 meters above the ground. If air resistance is given by 5v ... 0answers 153 views ### Describing the movement of the object in a particular situation in Lagrangian way Suppose there is a object M, (sliding motion) moving by the initial speed$v$and the initial location$x_0$. Otherwise noted, friction is assumed to be nonexistent. It then meets a circular mold ... 0answers 87 views ### deriving differential equation of a cart moved by a motor This is homework and I'm having some trouble getting started. How do I go from whats given to the form that they ask for? Normally in something like this I would try to balance torque, but I'm not ... 0answers 100 views ### Mass loss rate of planetary nebulae The “interacting wind” model of planetary nebulae is based on the idea that the white dwarf phase of stellar evolution is preceded by a red giant phase. A fast wind from the hot white dwarf overtakes ... 0answers 185 views ### Reduction in velocity in pipe I have two pipes divided from main pipe. If I add one more pipe, what would be the amount of velocity in each pipes. I know the velocity of main pipe, then diameter and radius of each pipe. Edit: 1) ... 0answers 509 views ### Error Analysis, Change in momentum, Kinetic Energy If momentum of an object is increased by 10%,then its kinetic energy will be increased by? Well, I don't need the solution logically by calculating the change in Kinetic Energy etc, but my question ... 0answers 81 views ### Is it necessary to use all solutions when calculating an expectation value in a spin state? I'm given an spinor$\Psi$which is solution of the Free Dirac equation, such that is an eigenfunction of$\hat{\vec{p}}$and has positive energy. Then I'm asked to calculate the expectation value of ... 0answers 79 views ### How do I write the energy of a constant, uniform 2D charge distribution? Let's consider a 2D electromagnetic field defined in a square domain$[0,\Lambda]^2$, with periodic boundary conditions, with a constant charge distribution, uniform all over the aforementioned ... 0answers 85 views ### Can a wave propagate in an elastic fluid in the absence of volume forces? A motion (wave)$\mathbf{x}: \mathcal{B}_0 \times [t_0,t_1] \to \mathcal{E}:$such that$q-o = \mathbf{x}(p,t)=(p-o)+\mathbf{a}_0 cos(\mathbf{k}_0\cdot(p-o) - \omega_0 t)$can propagate in an elastic ... 0answers 172 views ### How do I tackle this problem WITHOUT focusing the whole system as a “single mass”? Problem/Solution ! ! In the third FBD, they made the whole system as one object. What happens if I don't want to do that? What if I jsut apply Newton's second Law to the mass M? I tried it out but ... 0answers 135 views ### 2 streams going into an engine and 2 come out. Find the enthalpy? Problem: Steam supply to an engine is made of two streams that mix before entering the engine. Stream 1 flows at a$.01\frac{kg}{s}$with an enthalpy of$2952\frac{kJ}{kg}$and a velocity of ... 0answers 462 views ### Ground state energies with fermions of same spin? Consider two non-interacting Fermions (half-integer spin) confined in a 'box'. Construct the anti-symmetric wavefunctions and compare the corresponding ground-state energies of the two systems; ... 0answers 13 views ### Spherical charge distribution along a wire of length$L\$ in which its center coincides with the origin of the system

I've already tried to find the expression but I don't find the expected result for the quantity of charge when I use the expression I found. For example, I checked it by integrating the expression ...