# 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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### Integration of 3-momentum

During a lecture that I missed, I was trapped when the lecturer uses the relation $$dp_x~ dp_y ~dp_z ~=~d^3\mathbf{p} ~=~ 4\pi p^2 dp.$$ Can I know how is this relation derived please?
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### Vector Summation [closed]

When two vectors are sketched from a single point, the angle between them is θ. Show that the size of their vector summation is given in the expression: $\sqrt{A^2 + B^2 +2ABcosθ}$. Any ...
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### Basic Physics Question [closed]

I'm having a difficult time answering this question. I think I'm just converting the units wrong somewhere: You're the CEO of a courier company, and you decide to select an electric car for your ...
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### Scalar QFT Fock Space

I want to demostrate the following relation of the normal ordered product: $\Omega\equiv:\exp{\left(-\int d^3k~a^{\dagger}(k)a(k)\right)}:=|0\rangle\langle0|.$ I proved the commutation relation ...
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### How does volume of water in closed system relate to humidity at 25(C) degrees

If I have a closed system with a volume of 100L, at ~24(C) or ~75(F) degrees, what volume must be filled with water to reach a humidity of 90%? I imagined if you fill it 100% with water, humidity is ...
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### Loop Rule Parts Problem

[Loop Rule Diagram] The problem is thus: Apply the loop rule to abcdefghia and find the expression which adds to zero. I tried this, but I was not able to get it correct. I know that when crossing ...
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### Would this box on the floor rotate based on friction?

There is a floor that friction is proportional to its velocity (like $F=-kv$) and there is a box with its width as $l$ and its height as $h$. (you may assume that $l$ is longer than $h$). It is on the ...
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### Calculate average acceleration without time? [closed]

I have a question about a flea jumping, for which I need to show that the average acceleration is around 1000 $ms^{-2}$. I know that to calculate average acceleration you can use $\frac{dv}{dt}$, ...
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### How to establish relation between flow rate and height of the water column of the tank?

Suppose a water tank has 1" diameter drain at the bottom and is filled with water up to one meter height above the drain. What time it will take the tank to drain out completely. Now say, the tank is ...
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### Finding the tangential force experienced by a bob of mass m on a simple pendulum via the gradient/nabla operator)

The problem was posed as follows. Given a pendulum of length $L$ with a mass $m$ attached to it, which forms an angle $\theta$ from the y-axis to the direction of swinging. First we had to find the ...
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### Doubt in solving question related to photoelectric effect?

Question:- When a beam of $1.06eV$ photon of intensity = $2.0 W/m^2$, falls on a platinum surface of area $1.0*10^{-4}m^2$, and work function $5.6eV$, $0.53$% of incident photons ejected photo ...
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### Mathematical physics help appreciated [closed]

Is anyone able to help me answer the following questions? What is the method? a) Monochromatic light is incident on a slit of width 0.200 mm. On a screen 1.43 m beyond the slit, the first-order dark ...
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### Determing time to complete known distance with constant acceleration [closed]

Answer: $a=v^2/d$ is the formula I needed. This is a problem in a programming assignment, I haven't taken physics. I have a starting speed (0 m/s), an final speed (208.33m/s) and the distance it ...
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### Working with a Routhian for a specific system

I asked a more general question earlier about the Routhian, but I'm still having trouble working with it. Here's my specific case. Given the following Lagrangian: ...
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### Given wave function at $t=0$, what is the process of deriving time dependent wave equation? [closed]

Suppose $$\Psi (x, t=0)=Ae^{i\alpha _1}\psi _1(x)+Be^{i\alpha _2}\psi_2(x)+Ce^{i\alpha _3}\psi_3(x).$$ If $\psi _n$ are the energy eigenfunctions how would I derive $\Psi (x,t)$? I am having trouble ...
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### How to determine the positions of two points in a radial line by an intensity level dB?

The following is the question from my school. A source emits sound uniformly in all directions. A radial line is drawn from this source. On this line, determine the positions of two points, 1.00m ...
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### Antimatter collision - Energy Released

I think it's about time for me to ask this question, as I've been contemplating this for a while. $$E = mc^2.$$ This is Einstein's most famous equation. But what does it mean? On my own, I had to ...
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### Decrease in Intensity [closed]

A beam of particles pass through a target made of thin foil of a very small thickness $\Delta x$ having $N$ particles per unit volume. Let the collision cross section be $\sigma$ . If the intensity of ...
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### Current Carrying Wire Loop near Current Carrying Long Wire

A rectangular wire loop of (width $a$ and height $b$) carrying clockwise current $I_1$ is a distance $d$ below a horizontal infinitely long wire carrying a current $I_2$ to the right. What is the ...
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### Calculate for Multiple Resistors with a Bridge

Background A circuit is constructed with five resistors and a battery as shown. The battery voltage is V = 12 V. The values for the resistors are: R1 = 78 Ω, R2 = 150 Ω, R3 = 132 Ω, and R4 = 83 Ω. ...
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### How should I think about reciprocal lattice and Miller indices?

When I hear someone talking about a (100) plane or a (111) plane or an (hkl) in general, my first thought is, is the system cubic. The reason I think this is because I tend NOT to think of the planes ...
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### Projectile motion formulas

Assuming: The equations for the vertical and horizontal components of my initial velocity. Are: $v_{x,i} = v_i \cos\theta$ and $v_{y,i} = v_i \sin\theta$ And the displacement components are ...
I know base kets in the Heisenberg picture are $U^\dagger |{a}\rangle$ but if the base kets are the base of the hamiltonian, and the hamiltonian is independent of time, are all of the base kets ...