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-like questions.

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1answer
27 views

Feynman exercise - container with steel balls

You received many steel balls of the same diameter d and container of known volume V. All dimensions of the container are much greater than balls diameter. How many balls can be placed in a container? ...
0
votes
1answer
24 views

Need help to prove the following equation

I am able to show the equation at (2) but I am not sure how to show the equation at (3). Any help is greatly appreciated
-1
votes
1answer
15 views

Conduction problem

A hot reservoir with a temperature of $782 K$ is $0.56 m$ away from a cold reservoir with a temperature of $331 K$. The two reservoirs are insulated from each other except for a rod of brass ($k = 109 ...
0
votes
0answers
30 views

Gravitational force between rod and mass $m$

Why do we integrate along the whole length while finding the gravitational force between a object of mass ($m$) and a rod of length $L$ and mass $M\,?$ Can't we simply use $F=\frac{GmM}{r^2} \, ?$
0
votes
1answer
23 views

Energy method to solving equations of motion? Why does this method work and what is it called?

Given the stated system in the photo we are suppose to prove simple harmonic motion when given an initial displacement $x$ I first considered the total energy of the system which we will call $H$ ...
-1
votes
0answers
20 views

How do I find the co-ordinates and speed of this particle on the x-y plane?

I got this question from an old physics exam paper. I'm trying to use the kinematics equations to solve this question but the x and y co-ordinates keep confusing me. Here's the question: A particle ...
1
vote
1answer
49 views

How do I remove the negative sign from this derivation?

A homework problem required me to show that the first equation below can be written in the form of the second equation. It was all fairly simple except for the negative sign. I'm not sure how this is ...
1
vote
0answers
14 views

Density parameter (cosmology) as a function of redshift

Hopefully I'm missing some very basic algebra for this question. In essence, I need to derive the following: $$ \Omega=\Omega_{1}\frac{(1+z)}{1+\Omega_1{z}} $$ Now, I proceeded from the Friedmann ...
0
votes
0answers
40 views

Feynman exercise - crystal lattice

Can you explain why there is no crystals with a shape of regular pentagon? (Usually wall of crystal have shape of triangles, squares or hexagons). Could it be a correct solution? Let's assume that ...
1
vote
4answers
63 views

How to explain to a child why can acceleration be positive or negative? [on hold]

I want to find out how can acceleration be negative? I have an answer and this answer is: $$a=\frac{v_f-v_i}{t_f-t_i} < 0$$ because $v_f < v_i$ where $v_f$ is final velocity and $v_i$ is ...
1
vote
0answers
24 views

How to solve this problem : A glass sphere of radius 1 m and refractive index 1.5 is silvered…? [on hold]

Problem: A glass sphere of radius 1 m and refractive index 1.5 is silvered at its back. A point object is kept at a distance of 1 m from the front face, as shown in the figure. Find the position of ...
0
votes
2answers
29 views

Equivalent spring constant of setup

A mass $M$ is suspended using two springs having spring constant $k_{1}$ & $k_{2}$ with distance from mass as $a$ & $b$ respectively. Find equivalent spring constant of system. So I ...
0
votes
0answers
39 views

Spinor on an eigenstate of $J_z$

The state of a particle with spin 1/2 is described (in the $S_z$ representation) by a spinor $[\Psi]=\left( \begin{array}{c} \phi_+ \\ \phi_- \\ \end{array} \right)$ Its known to be eigenstate of ...
-2
votes
1answer
36 views

How many electrons pass trough a wire? [on hold]

How many electrons pass trough a wire if I = 6,4 Amps in 15 seconds? Can you do it step by step?
-2
votes
0answers
30 views

Reducing a vector calculus relation [on hold]

Someone can help me? I want to reduce this. $$ (\nabla \times \mathbf{B}) \times \mathbf{B} + (\nabla \times \mathbf{B}) $$ Thanks
0
votes
0answers
16 views

specific heat capacity (temperature) [on hold]

A 60KG bushwalker is suffering from hypothermia, having an average temperature of 33.5degrees.when rescued she was wrapped in a warm blanket and given two 310ml cups of warm tea each at a temperature ...
2
votes
1answer
21 views

Diagonal of a thin rectangular foil, inertial principal axis?

I'd like to know if the diagonal of a thin foil is an inertial principal axis. I know that if an axis isn't a symmetry axis then it isn't a principal axis. In the rectangle the diagonal isn't a ...
1
vote
0answers
32 views

Proving Probability Current and Momentum relationship

I am trying to show that $\mathbf j = \frac{\hbar}{2mi}\left(\Psi^* \mathbf \nabla \Psi - \Psi \mathbf \nabla \Psi^{*} \right) \,, $ simplifies to $\mathbf j = \frac{\mathbf p}{2m} (|\Psi|^2) $. I ...
4
votes
6answers
487 views

How come in free fall you feel weightless even though gravity is pulling down on you? [duplicate]

How come in free fall you feel weightless even though gravity is pulling down on you? (ignore air resistance when answering this question).
-1
votes
1answer
39 views

Find the indicated quantities from the given data [on hold]

This is a vector application. A rope is hung at both ends from a horizontal beam, and a weight m is suspended from it as shown in the figure. The left part of the rope exerts a force G at P, while ...
1
vote
0answers
18 views

What frequencies is radation being emited in infinite potential well

The problem that I am working on is. Consider an electron in an infinitely deep one-dimensional potential well of width 1nm . This electron is prepared in a state that is an equal linear ...
1
vote
1answer
20 views

Transformation of self-dual and anti-self-dual tensors and irreducibility of representations

I am working out exercise 2.5 of Maggiore's book. Part of the exercise is the following: Verify that self-dual and anti-self-dual tensors are irreducible representations of (real) dimension three of ...
0
votes
1answer
132 views

Do I need to consider the spin of electrons when they are in infinite potential well

This is the problem I have And this is one of my books tell me what should I do And my question is: Do I need to consider spin in this case?(that is, I don't think the book is right...) I found ...
3
votes
0answers
35 views

Elastic collision / relative velocity problem [on hold]

I'm having some trouble with a homework exercise and I'd really appreciate some help! I've done the first part correctly (according to the solution sheet), but I can't seem to get the second part ...
-3
votes
0answers
36 views

De-broglie wavelength [on hold]

The de-broglie wavelength of an electron emitted from the ground state of an H-atom after the absorption of a photon equals 1/2 of the de-broglie wavelength when it is in orbit. The energy of the ...
1
vote
2answers
44 views

How does a point charge interact with a Gaussian surface?

A spherical Gaussian surface encloses a point charge $q$. The point charge is moved to to a point away from the center of the sphere. Does the electric field at a point on the surface change? ...
1
vote
1answer
33 views

Calculation Equivalent Resistance In A Circuit With resistances connected in a circle [on hold]

I am having trouble in finding the equivalent resistance between the terminals 'a' and 'b' in the following circuit: Please can anyone explain to me how to solve such problems. Thank You!
1
vote
0answers
26 views

How can you find the $u$ using time dilation if you only know $\Delta t_o$? [on hold]

I've come across two problems dealing with time dilation now that ask how fast should an astronaut go in order to travel EG: 400 LYears in 10 Years. What I can not wrap my head around is how this is ...
2
votes
1answer
31 views

A couple of questions regarding work to velocity exchange [on hold]

I was tasked to express the velocity of the cube of mass $m$ at point B using the constants given in the picture below. Friction could be ignored, and the cube could be presumed to reach B with the ...
-4
votes
0answers
27 views

Problem - Escalator [on hold]

I need help in resolving the following problem: On a recent weekend, Anna was rushing through a shopping centre doing her shopping she had occasion to use the escalator 3 times. The first time the ...
-2
votes
1answer
45 views

Bohr radius question [on hold]

I'm doing a homework problem where it asks to calculate the diameter of a hydrogen atom with $n=600$. I used the equation $r=\frac{n^2a_0}{Z}$ where $a_0=0.529e^{-10}m$. Solving for $r$ yields: ...
1
vote
0answers
31 views

Relativity Mapmaking in spacetime [on hold]

The problem as stated: I do not know how I am supposed to map this. The instructions don't make sense to me and the textbook is really un-helpful. This is the map that I have to draw this on. I ...
0
votes
1answer
26 views

Simple Harmonic Motion given velocity and acceleration

I am trying to understand how to relate velocity and acceleration of an object to it's amplitude, period, and frequency given only the following: An object of mass m=20kg moves with SHM along the ...
0
votes
0answers
17 views

finding work done by the buoyant force on a submerged object [on hold]

A flotation device is in the shape of a right cylinder, with a height of 0.500 m and a face area of 4.00 m2 on top and bottom, and its density is 0.320 times that of fresh water. It is initially held ...
1
vote
1answer
22 views

Why is a series resistor needed in a simple thermistor experiment? [on hold]

I have come across the following simple question, however I can't see any need for a fixed resistor in series. The only possible reason that I can think of is that the resistance of the thermistor ...
0
votes
0answers
26 views

Calculate the Bohr radius of a muonic atom [on hold]

I want to better understand this problem and how to calculate the Bohr radius. The equation I found in my book is $r_n=\frac{n^2 \hbar^2}{mkZe^2}=\frac{n^2a_0}{Z}$. However, I found a problem online ...
0
votes
1answer
26 views

Methods to solve a vapor pressure equilibrium question with water on the moon? [on hold]

"Imagine a tall column of water on the Moon, maintained at a temperature of 50°C and left to the open vacuum at the top. Up to what depth will the water continue to boil? The Moon's radius is 0.273 ...
0
votes
0answers
21 views

One from Landau's Minimum (Macroscopic Electrodynamics)

A dielectric sphere with the electric and magnetic susceptibilities ε1 and µ1 is rotating with angular frequency ω in a constant electric field E~ in a medium, characterized by the parameters ε2 and ...
0
votes
1answer
19 views

a question on work and kinetic energy [on hold]

A bullet has a mass of 0.02kg and is moving with a speed of 10m/s. It can penetrate 10cm of a given target before coming to rest. If the same target is only 6cm thick, what will be the speed and ...
0
votes
0answers
32 views

meson decay in Yukawa theory

I want to calculate decay width $\Gamma_{i\rightarrow f}$ (under assumption that we don't measure the polarization of particles in final state) in a decay process $$\phi \rightarrow \psi \bar{\psi}$$ ...
0
votes
0answers
22 views

inlet velocity calculation from mass flow rate [on hold]

I am struggling for following calculation, Could somebody please help me with this Mass flow rate and inlet velocity at 300 K and 40 mbar are 4.246 gm/minute and 32.8 cm/sec respectively, if the ...
1
vote
0answers
11 views

Capacitor charge and current as a function of type question [on hold]

Two capacitors, one resistor and an ideal battery are connected through switches s1,s2 and s3 as shown in figure. Initially capacitor of capacitance 4C is charged, while other is uncharged. The upper ...
0
votes
0answers
17 views

Simple Harmonic Osillator [on hold]

For simple harmonic oscillator, calculate a. The energy b. The mean position c. The mean momentum square d. Standard deviation of position e. Standard deviation of momentum f. Cheek uncertainty ...
2
votes
1answer
45 views

Electric Potential due to Sphere when cavity is at arbitrary position [on hold]

While doing practice question, I came across a question... A solid sphere of radius a has a cavity of radius b which has a uniformly charge distributed with density $-\rho$ and the remaining part ...
1
vote
0answers
11 views

Bullet - Jet problem, How to equate horizontal displacement. projectile motion [on hold]

A jet is moving hotizontal with speed of 720km/h at height 1960m . when it passes above point on ground at same time bullet is fired from that point with velocity 400m/s I want to know the angle from ...
0
votes
1answer
25 views

Find the magnitude of the resultant force which these engines exert on the rocket?

A rocket fires two engines simultaneously. One produces a thrust of 760N directly forward while the other gives a thrust of 484N at an angle 34.0 degrees above the forward direction. What is the ...
0
votes
0answers
25 views

Determinant of block matrix with off diagonals as vectors [migrated]

I've been given an $(n+1)\times(n+1)$ square matrix, which is written in the form of a block matrix with the following dimensions $ \begin{bmatrix} (1\times1) & (n\times1)\\ ...
0
votes
0answers
46 views

Why is there not always a force in the direction of motion?

This was a question on a physics competition. A girl throws a teddy bear straight up. Consider the motion of the bear only after it has left the girl’s hand but before it touches the ground, and ...
1
vote
1answer
42 views

$dU = \sum E_i dp_i + \sum dE_i p_i = TdS - PdV \implies \sum dE_i p_i = - PdV$? [on hold]

This is a formula my professor gave in class, but I'm entirely lost how he arrived at the conclusion (I understand the assumption). Could anyone help me?