# 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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### Calculating the $J$ value for atomic terms, having a lot of trouble with this. Already attempted

I am trying to understand this, and want to be very very clear. This is a homework question but I already attempted to answer it, so please don't put this question on hold. The question What ...
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### Free fall question: Bill shoots his gun [closed]

Bill borrows his dad's gun and when he gets outside he fires it straight up into the air. If the bullet left the gun at 400 m/s: How long does it take the bullet to reach its highest point. I know ...
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### Is there a systematic way to derive constraint equations?

There's this problem in Goldstein's (Classical Mechanics) derivations section: 5. Two wheels of radius $a$ are mounted on the ends of a common axle of length $b$ such that the wheels rotate ...
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### How can we show that a map is a completely positive map? [migrated]

I am doing a homework problem where I have to find whether the map $$\rho ~\rightarrow~ {\rm tr}(\rho) I - \rho$$ is completely positive. If the map is not completely positive, a counter-example ...
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### Evaluating position vector between 2 hydrogen states

I am trying to find the quantity: $$\langle1,0,0|\vec r|2,0,0\rangle$$ Where $|n,l,m\rangle$ are the hydrogen states. For this, can I just integrate r from? 0 to infinity? Or do I have to break it ...
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### A question about Hamiltonian phase flow

Show that if a one-parameter group of difeomorphisms of a symplectic manifold preserves the symplectic structure then it is a locally hamiltonian phase flow. Note that A locally hamiltonian ...
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### A block falling from a height on a block suspended by spring [closed]

The block suspended by the spring is hanging freely and its mass is M. The small block of mass m is dropped on the bigger block from height h. After the small block is dropped 》》》 I want help in ...
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### Integrating for speed [closed]

Trying to determine the speed of a falling body with respect to traveled distance and initial speed. I've been provided with the following equation for acceleration as a function of distance and the ...
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### Null Geodesics in Einstein Universe

I am currently taking a course in General Relativity, and I've hit a bit of a roadblock with a homework assignment. We are given the metric for Einstein's universe to be (forgive me, this is meant to ...
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### Hydrogen ionization energy

Proton and electron have a distance equal to the Bohr radius apart in the hydrogen atom. Knowing this, what's the ionization energy of the atom? So we know $U = -kq^2/a_B$, but this is potential ...
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### Finding release angle and final velocity of projectile when the target has a different coordinations [closed]

I am trying to write some codes for auto-snowball mechanism which seems like a regular projectile but instead of hitting the targets on the ground it is going to be able to hit the target in the air ...
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### Angular Momentum Expectation in Magnetic Field

I am trying to find the time dependent expectation value for J ($\langle J(t) \rangle$) for a spin 3/2 particle in a uniform magnetic field (in the z direction). My method is as follows: ...
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### Kirchoff's rules and RC circuits [closed]

You can view the image bigger: http://i.stack.imgur.com/4moUP.png I'm studying RC circuits and I can't understand the part when we use Kirchoff's rule. When working with batteries, when we're ...
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### Eigenstates of coupled Angular Momentum

SO I have a hamiltonian: $$H=\alpha J_1\cdot J_2$$ And I am asked to find the eigenstates and eigenvalues of this Hamiltonian in terms of products of the eigenstates of the z components of the ...
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### Blocks stacked on an incline connected by rope around pulley

Two objects A and B, of masses 5 kg and 20 kg respectively, are connected by a massless string passing over a frictionless pulley at the top of an inclined plane, as shown in the figure. The ...
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### How to calculate mass from compound lever? [closed]

A class 1 lever and a class 2 lever are connected in series, on the resistance end of the class one lever is a 500g weight. On the 2nd class lever sits a weight of an unknown mass. From the end of the ...
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In my textbook, it is given: 1 atomic mass unit equals $1/12$th mass of one carbon 12-atom. Since mass of $6.02 * 10^{26}$ atoms of carbon-12 is $12\space\text{kg}$. Thus, $$1 \text{ a.m.u ... 1answer 41 views ### If you have N 1ohm resistors, how many distinct equivalent resistances can you create? If you have N 1ohm resistors, how many distinct equivalent resistances can you create? Assume that only parallel and series and mixture of them is allowed and no bridging between two parallel ... 0answers 26 views ### Ratio of position error of orbiter to the size of the orbit This is an assignment question for online AP physics. As if it isn't already tough. This question is killing me. I even asked my physics teacher from last year and my calculus teacher. It stumped ... 1answer 80 views ### 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 ... 1answer 91 views ### 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 ... 2answers 66 views ### How much force is required to compress air? How much force (Newtons) is required to compress normal air in a chamber to 2 atm? For example, if I had a sealed piston pump, how much force would need to be exerted in order for the air to be ... 2answers 60 views ### Ice throw from industrial wind turbine I was watching a documentary where the residents didn't want to have a wind turbine erected nearby. One of the comments from the company was that an ice throw would have very little importance since ... 1answer 45 views ### Confused with how to approach this pulley problem (solved) [closed] Question: In the following figure block A has mass m and block B has mass 6.00. The coefficient of kinetic friction between block A and the tabletop is 0.40 The mass of the rope connecting the blocks ... 0answers 32 views ### Fourier series and diffraction gratings [closed] I'm having trouble with this question and I don't know where to start. A plane wave illuminates a periodic grating which consists of a large number of narrow apertures of width b spaced equally a ... 1answer 56 views ### Lagrangian for a moving spring device How can I write the proper Lagrangian for such as system as the one shown in picture? Am confused about what is the suitable way to designate the coordinate. 1answer 90 views ### Diagonal Hamiltonian of 3 Spin 1/2 Particles I have three Spin 1/2 Particles and a Hamiltonian given by$$H=A(S_1\cdot S_2)+B(S_2\cdot S_3+S_1\cdot S_3)$$In order to find the energy spectrum, I want to diagonalize H in terms of ... 1answer 25 views ### Do I need to take both particles' momentum into account in photoelectric emission? [closed] An aluminum dust particle of mass m=2.2*10^{-18} grams is floating in space ( initial velocity is zero). The particle emits electron under influence of a photon whose frequency is 8*10^{17} ... 0answers 38 views ### Showing that the maximum possible uncertainty for any observable is half the difference between its maximum and minimum eigenvalues Show that the maximum possible uncertainty for any observable is \frac{1}{2}|x_2 - x_1| where x_1 and x_2 are the extreme eigenvalues of X (Maximize \Sigma_i p_ix_i^2 - (\Sigma_i p_ix_i)^2) ... 1answer 41 views ### Equation for magnetic field due to a current in straight wire [closed] How can I derive the expression for the magnetic field due to a long straight current carrying wire using the Biot-Savart law? The final expression should be: ... 0answers 26 views ### charged oil drop in electric field [closed] I am currently doing coursework for my A2 physics course. I am dropping a charged oil drop in a horizontal electric field and I was wondering how to calculate how much the droplet would deviate given ... 1answer 51 views ### Max altitude of rocket [closed] A rocket is launched perpendicularly with an initial acceleration of 10 m/s2, if the fuel runs out after 1 minute, what will be the maximum height reached by the rocket? 1answer 31 views ### Acceleration Power [closed] Suppose we have a trolley currently at rest at point A, but the motor switches on, so that it accelerates and reaches point B, at point B, the velocity is 5m/s, Distance of A-B is 10m, mass of the ... 2answers 119 views ### Angular Momentum Operator in Quantum Field Theory I'm following along with David Tong's QFT course and am trying to derive the result shown in question 6 on his 2nd problem sheet, but am running into problems when applying it to the free real scalar ... 1answer 91 views ### Free particle propagator - Evaluating Integral In path integral formalism, when evaluating the free particle propagator, we obtain the functional integral of the form,$$ K_0 = \lim_{n\rightarrow\infty} \bigg( \frac{m}{2\pi ...
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I know there are a lot of variables that affect how quickly this would occur. Any information on the topic would be highly appreciated. For sake of argument, lets say 1 square foot chamber, lets say ...
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### Three integrals in Peskin's Textbook

Peskin's QFT textbook 1.page 14 $$\int_0 ^\infty \mathrm{d}p\ p \sin px \ e^{-it\sqrt{p^2 +m^2}}$$ when $x^2\gg t^2$, how do I apply the method of stationary phase to get the book's answer. ...
### Why $A\sin(2\pi ft) =\frac{A}{2j}(e^{j2\pi ft}-e^{-j2\pi ft})$ but not $\frac{A}{2}(e^{j2\pi ft}-e^{-j2\pi ft})$? [migrated]
v_{\mathrm{in}}(t)=A\sin(2\pi ft) =\frac{A}{2j}\left(e^{j2\pi ft}-e^{-j2\pi ft}\right) \\ |H(f)|=|H(-f)|;\angle H(f) = -\angle H(f) \\ v_{\mathrm{out}}=H(f)v_{\mathrm{in}}=\frac{A}{2j}H(f)e^{j2\pi ...
So this is the question given in my text book: A particle of mass m is at rest at the origin at time $t = 0$. It is subjected to a force $F (t) = F_0e^{–bt}$ in the $x$ direction. Its speed ...