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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4
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1answer
101 views

Photon Escape Angle From Black Hole

Consider a photon source emitting photons near the surface of a Schwarzschild black hole. What angle, as a function of the source's radius from the event horizon, must the photons be emitted at such ...
1
vote
0answers
76 views

Calculate heat loss of aquifer with unknown shape

I am trying to calculate the heat loss of an aquifer. An aquifer is basically a hot-cold storage $80\: \mathrm{meters}$ under the ground. It is an open system and utilizes groundwater. By pumping for ...
0
votes
2answers
331 views

Why don't the components of these forces add up correctly? [closed]

In the arrangement as shown in the figure below, the ends P and Q of an inextensible string move downwards with uniform speed $u$. Pulleys A and B are fixed. Mass $M$ moves upward with a speed: ...
-1
votes
1answer
337 views

How do you calculate the Milky Way’s galactic year? [closed]

The Solar system moves at a speed of 220 km / s around the galaxy. It’s about 27,000 light years from the Galactic Centre. How long does it take for the solar system to orbit around the Milky Way? ...
2
votes
1answer
169 views

Boundary conditions for Laplace's equation

Given a grounded conducting sphere, $V=0$ and $radius = R$, centered at the origin with a pure electric dipole (dipole moment $\vec p$) situated at the origin and pointing along the positive $z$ axis, ...
1
vote
2answers
159 views

$Q$-value in nuclear reaction $^{9}Be\left(\gamma,n\right)^{8}Be$

I have two questions. First, I understand that in a nuclear reaction $$Q:=K_{after}-K_{before}\equiv E_{0,before}-E_{0,after} \qquad (1)$$ where $K$ is the total kinetic energy, and $E_0$ is the ...
2
votes
1answer
286 views

Young's Double slit experiment (Finding the value for slit separation) [closed]

The question illustrates mono-chromatic red light passing through 2 slits and forming an interference pattern on the screen. The question then asks to suggest a suitable value for the slit separation. ...
2
votes
0answers
59 views

error propagation and collision in ideal gas

When dealing with gas, a statistical approach is needed because For N particles, you have to solve 6N equations which cant be done analytically. To know our time step for numerical solving, you can ...
2
votes
2answers
191 views

Find E and B from vector potential

I have a vector potential given by: $\mathbf{A}(x,t) = \mathbf{e}_{y}\frac{1}{2} e^{-(x-ct)^{2}/{4a^{2}}}$ Now, the question is "Determine the E and B under the condition that the scalar potential ...
-1
votes
2answers
113 views

Help me check units [closed]

I need to check units in this equation, I know, that there's Pa in the numerator, but what about the denominator? $p_1$ - pressure in [Pa] $T_1$, $T_2$ - temperature in [K] $\rho_w$ - density $g$ - ...
0
votes
1answer
129 views

Question about weightlessness

Let us consider +Y as upward and -Y axis as downward. So the acceleration that works these objects is always $-g$. For the first picture, if the lift is going upward with acceleration $a$, then ...
0
votes
1answer
58 views

Something about collision [closed]

A sphere P of mass m, travelling with speed $u$, makes a head-on collision with a stationary sphere Q also of mass m. After the collision, the velocities of P and Q are $v_1$ and $v_2$ ...
4
votes
2answers
704 views

Deriving the Angular Momentum Commutator Relations by using $\epsilon_{ijk}$ Identities

I've been trying to derive the relation $$[\hat L_i,\hat L_j] = i\hbar\epsilon_{ijk} \hat L_k $$ without doing each permutation of ${x,y,z}$ individually, but I'm not really getting anywhere. ...
1
vote
0answers
93 views

Is Gauss' law useful to determine the electric field strength of a charge distribution? [closed]

Under what conditions is useful Gauss' law to determine the electric field strength of a charge distribution? Can someone help me with this question?
3
votes
2answers
553 views

Equal mass on either end of a spring

I'm taking physics this term and this is the first time taking physics in 3 years. Here is my homework question (of course those variables all have values but we'll use variables for now): Consider ...
1
vote
2answers
276 views

Solving a light ray worldline with the geodesic equation

I'm having trouble solving the geodesic equation for a light ray. $$ {d^2 x^\mu \over d\tau^2} + \Gamma^\mu_{\alpha\beta} {dx^\alpha \over d\tau} {dx^\beta \over d\tau} = 0 $$ I apologise, but I'm a ...
-2
votes
1answer
263 views

Question on friction [closed]

Please look at the figure carefully. The question is written, and one has to match the two columns 1 and 2 given under "choices and explanation". Please tell me if I am really wrong, i.e. would you ...
3
votes
2answers
1k views

How to draw a ray diagram from focal length, object and image heights?

Q. An object of height 8 cm is placed in front of a lens. It's inverted image of height 4.8 cm is formed on the screen. If the focal length of the lens is 12 cm then by drawing at scale calculate the ...
2
votes
2answers
374 views

How to calculate air resistance of penny dropped from Empire State Building?

If a penny is dropped from the Empire State Building, then its speed, without taking air resistance into consideration, is $\sqrt{\left(32\frac{\textrm{ft}}{\textrm{s}^2}\right)(1454\textrm{ ...
5
votes
1answer
638 views

How much negative charge do I accumulate by touching the earth?

The Earth carries a negative electric charge of roughly 500 thousand Coulombs (according to different sources I've seen). If I touch the Earth I should therefore pick up some of this electric charge ...
1
vote
0answers
152 views

Maxwell-Boltzmann distribution

The short story is, that I have to calculate some transport coefficients, but using the the MB distribution as my distribution function. What I currently need to solve is: ${{\mathcal{L}}^{\,\left( ...
3
votes
1answer
130 views

Magnetic field due to a conducting sphere in unpolarized light

From Jackson, problem 10.3: A solid uniform sphere of radius $R$ and conductivity $\sigma$ acts as a scatterer of a plane-wave beam of unpolarized radiation of frequency $\omega$, with $\omega R ...
0
votes
2answers
490 views

Using the Normalization Condition with Wavefunction

I'm very confused with this problem and I was looking for some guidance. $$\psi(x) = Ae^{ikx}e^{-x^2/2a^2}$$ Use the normalization condition to find A. So I understand that you use the normalization ...
1
vote
0answers
67 views

Finding the force of an object? [closed]

Sorry if my initial question was seen as a "homework question" but I don't really know how to go about asking this type of question. I have a feeling I have to use Newtons law but there is no ...
0
votes
1answer
79 views

Current in AC circuit

The problem is to find the current on the capacitor. $I''$ should be correct, but I don't know how to construct the formula for $I'$. I managed to get the value for $I_c$ using Thevenin and Norton ...
8
votes
3answers
336 views

Where does the extra equation come from to determine the forces from an object on a table?

I have a question about basic statics but somehow I cannot manage to find the answer on my own (btw, this is not a homework. It's been so many years since school for me...). The problem is very ...
1
vote
1answer
245 views

Inclined plane question [closed]

An object, mass $m$ is placed on an incline, angle $\theta$. System is at equilibrium. coefficients of static and kinetic frictions are $\mu_s$ and $\mu_k$ respectively. Then: 1) What is the Total ...
-1
votes
1answer
67 views

Calculate energy from an reaction [closed]

I'm trying to calculate the released energy from a reaction. The radioactive substance polonium decays according to this formula: $$^{210}_{84}\mbox{Po} \rightarrow \mbox{X}+^4_{2}\mbox{He} $$ At ...
0
votes
2answers
4k views

What happens when non-equal voltages are put in parallel? [duplicate]

If two batteries, say 2 volts and 5 volts, are connected in parallel, are there any problems? The higher voltage will then want to flow out, but also towards the lower 2 volt battery end, right?
1
vote
0answers
151 views

How to find outflow pressure/force of air through a pipe

I have been trying to find the correct method for determining the outflow force resulting from a certain amount of air being forced into a pipe of a certain dimension. Essentially i'm trying to ...
1
vote
0answers
55 views

Physics (measurement) [closed]

In october,1707 four British warships ran aground because of error in position,sparking the effort to produce a marine clock that would be accurate to locate a position with in 30 miles after sailing ...
2
votes
1answer
142 views

What is the distance traveled

the red dots represent a side view of path traveled, F is downward force and the tool used here is a pen placing parallel to the coin I have newly started to study mechanical physics. based on study, ...
0
votes
1answer
330 views

1D Kinematics - Relative Motion involving non-inertial frames of reference

I recently came across a question involving non-inertial frames of reference. I didn't quite understand the way it had been solved due to some conceptual confusion regarding certain deductions made to ...
1
vote
2answers
817 views

Collision between a photon and an electron

Looking through this AP Physics question, I was struck by how the 'collision' between a photon and electron looks so much like a macroscopic collision. Is this even physically possible? Look at the ...
2
votes
1answer
94 views

Time evolution of a quantum state

I have another point in QM that I would like clarified. Suppose $$\{|n\rangle\}$$ is a set of eigenstates of both the Hamiltonian $H$ and another operator $\hat O$ corresponding to an observable also. ...
3
votes
1answer
88 views

Why doesn't this equation for orbital motion change with position in the orbit?

The question and answer are on pg.8-10 of this PDF: At first, I went through it, thinking nothing of it. But then, I wondered: "What if we picked a final state in which the space junk was NOT at ...
4
votes
3answers
158 views

What is the difference between material fatigue test with 1 sec cycle and 1 hour cycle?

Assume I have two pieces of metal. I want to test the material fatigue (e.g. how many cycles can the material stand before it will break). Onto one of them I apply a tension force every second. Onto ...
4
votes
2answers
361 views

How to directly calculate the infinitesimal generator of SU(2)

We commonly investigate the properties of SU(2) on the basis of SO(3). However, I want to directly calculte the infinitesimal generator of SU(2) according to the definition $$X_{i}=\frac{\partial ...
3
votes
3answers
94 views

Showing Hubble constant is time-independent

I have the following question for homework: Show that the Hubble constant $H$ is time-independent in a universe in which the only contribution to energy density comes from vacuum energy. So in ...
4
votes
2answers
441 views

Equivalent RC circuit to a RRC circuit?

I'm in doubt about a situation that I've seen sometimes: imagine we have a resistor in parallel with a resistor and a capacitor in series. Since I don't know how to generate figures of circuits to ...
-2
votes
1answer
201 views

A double-sided lever and multiple forces [closed]

Assume a double-sided lever where the left arm has a length of a and the right arm has a length of b + c. Please refer to this ...
0
votes
0answers
232 views

How does the volume change affect an adiabatic process?

A gas consisting of rigid diatomic molecules was initially under standard conditions. Then the gas was compressed adiabatically, 5 times the original volume. Find the mean KE of rotating molecules ...
0
votes
2answers
91 views

Dividing values with units

I'm reading about the subject of heat in a basic physics book. If I am not mistaken the formula to work out how much energy is required to increase the temperature of water is ...
1
vote
0answers
212 views

Gradient involved commutator in $\phi^4$ theory

In a phi fourth theory, the Hamiltonian density is: $$\mathcal{H}=\frac{1}{2}\pi^2+\frac{1}{2}(\nabla \phi)^2+\frac{1}{2}m^2\phi^2+\frac{\lambda}{4!}\phi^4$$ Now I impose the usual equal time ...
0
votes
1answer
122 views

Kinetic energy when $F(t)$ graph is given

The very basic problem here is that the point where this graph ends is ambiguous. I mean, what is the corresponding $y$-value for the point whose $x$-valu is $4.5s$? Then I think we should find the ...
0
votes
1answer
1k views

Relative Velocity problem

I was solving many problems on relative velocity and observed that when the question asks, "what is the minimum time taken by the boat/man to cross the river.?" so the solution says that he ...
0
votes
1answer
360 views

Doubt in problem regarding resistance a of cylinder

I've been working on some exercises and I'm in doubt if my procedure with this one is correct. We have a hollow cylinder with internal radius $r_a$, external radius $r_b$, resistivity $\rho$ and ...
1
vote
2answers
238 views

Electric potential energy after nuclear fission

We have a uranium-236 nucleus that fissions into two equal fragments, and I'm supposed to find the electrical potential energy just as the two fragments split apart. No other information is provided. ...
1
vote
2answers
354 views

Kinetic energy of two charged balls at infinite distance between them

If I have two balls with masses and charges $m_1, q_1^{+}$, $m_2, q_2^{+}$, initially held at distance $d$, and then released, how can I know the kinetic energies of each of the balls at infinite ...
1
vote
0answers
70 views

Solar sail area going to Proxima Centari [closed]

I have a physics question that I need some help with: "Proxima Centauri is a star in the Alpha Centauri solar system, it’s the nearest star to our sun (4.24light−years) ...