# 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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### 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 68 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 30 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 152 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 139 views ### An Electric Potential Glued to a Cube-Shaped 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-shaped insulator so that outside of the insulator the field is the same as a point ... 2answers 157 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 72 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 104 views ### Lagrangian for a moving spring device [closed] 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 147 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 27 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 42 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 87 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 36 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 111 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 36 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 189 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 151 views ### Free particle propagator - Evaluating Integral [closed] 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. ...
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### When is the speed specified for an object experiencing an exponential force?

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 ...
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### What forces are involved in this situation?

A old massless rope of 12 meters attached to a ceiling can sustain a maximum tension force of 1200N before breaking. An 85 kg person climbs up the rope. What is the minimum possible time in which ...
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### Sudden Approximation for Beta Decay of Tritium Atom

I am working out this problem right now, and I'm confused by the answers I'm getting. Problem: A tritium nucleus (Z = 1) in a tritium atom undergoes beta decay, i.e., a neutron in the nucleus emits an ...
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### Deriving group velocity formula

A formula for the group velocity of waves is: $u=k*dv/dk + v$ But then, since $k=2π/λ$, this equation can be rewritten as: $u=v-dv/dλ*λ$ But how? My attempt: $k*dv/dk$ = $((2π/λ)*dv/d(2π/λ)$ ...
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### Have I calculated the air flow of this fan correctly?

To calculate air flow capacity of a fan in cubic feet per minute (cfm): multiply the average air speed you measured in feet/minute (fpm) by the area of the fan face in square feet. (Area of circle =þ ...
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### The angle to shoot moving object

I am obliged to count very simple problem (at least it seemed that it is simple, I hope it isn't to simple for this site). So i got observer who is standing $H$ below the object. The object is fired ...
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### Differentiate wave speed, don't understand

The speed $v$ of some wave is $ω/k$ and I want to differentiate this with respect to $k$. Apparently this equals: $dv/dk = d(ω/k)/dk-ω/k^2$ But I don't understand why. Isn't this just saying "the ...
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### Free fall from space [duplicate]

If you leave a ball weighing 10 kg at a height of 500 km above sea level (neglecting air friction). How can calculate how long the ball hits the ground and what will be its speed? I know that: On ...
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### Adiabatic Expansion with expanding control volume

I have a tube with a length "L" and diameter "d" that is open on 1 side . At a certain point ( say "x" ) from the closed end of the tube, I have a gas with a high pressure. At this point, "x", there ...
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### Why dimensionality of the Electric Charge varies with the spacetime dimensions?

The point is: We can find via dimensional analysis that the electric charge dimensionality varies with the dimension of space-time. $$[\text{charge}] = eV^{(3-D)/2}$$(You can see below the way I did ...
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### Infinite Resistive lattice problem

Ok so while there have been many discussions regarding the solution(say using the Fourier transform or the lattice Green's function approach) to the two-point resistance of an infinite-square ...
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### Conservation of energy quick question

Say we had a particle moving in a frictionless funnel and was projected horizontally. If we had some initial conditions for the energy E, then would these conditions be the same always? For ...
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### Ideal gas system with diathermic piston externally manipulated

I wanted to ask about the following problem: An ideal monatomic gas is separated into two volumes $V_{1}$ and $V_{2}$ through a diathermic piston, such that each volume containing $N$ atoms and the ...
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### Choosing proper centripedal force in Newtonian gravitational forces [closed]

I am trying to do an orbit simulation, I took Newton's law of Newtonian gravitational forces. It seems to work correct, but I can't really determine the velocity of an object needed to stay in free ...
I want to compute the magnetic field due to a homogeneously charged, rotating sphere with radius $R$, angular velocity $\vec{\omega} = \omega\hat{z}$ and total charge $Q$. I want to use Biot-Savart ...