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.
0
votes
1answer
36 views
Kinetic rotational energy of a bar hooked to a coil
I have solved an exercise and I'd like to know if my proceeding about finding kinetic energy is correct or not, because this is the first time that I "meet" a situation like this.
"A bar has mass $M$ ...
3
votes
1answer
45 views
A Doppler Effect problem with a moving medium
I tried solving the following question and started having multiple doubts:
Two cars A and B are moving towards each other with some speed $25$ m/s. Wind is blowing with speed $5$ m/s in the ...
2
votes
0answers
45 views
Calculate large and small frequency separation for the Sun [migrated]
I want to determine the big and small frequency seperation from timeseries data for the sun. An excerpt of the data (timeseries and power series) is plotted below.
The power series is calculated in ...
1
vote
1answer
63 views
Proton mass and Weizsäcker's formula
I'm so confused in the use of nuclear masses and atomic masses.
I have two questions.
From the book "Outline of Modern Physics" by Ronald, I understand that the semiempirical mass formula ...
1
vote
1answer
35 views
Adding heat to a system doesn't equal the work done by gas?
So, I answered a physics question for a class that goes as follows:
A gas in a cylinder is kept at a constant pressure of $250000\: \mathrm{Pa}$ while $300\: \mathrm{kJ}$ of heat are added to it, ...
-3
votes
0answers
24 views
Find the half-life of the material? [closed]
It took 1.5 hours for 12g of radioactive material to disintegrate until only 1.5g remained. What is the half-life of the material?
I know this homework question is probably very easy, but I am ...
0
votes
0answers
24 views
Wavelength of a Proton with a KE of 1eV? [closed]
I have a homework question:
What is the wavelength of a proton with a KE of 1ev? Why is this different from the value calculated for a 1-eV electron?
The answer to the question is that the ...
1
vote
0answers
21 views
Find the molecular size of an oleic molecule in 2-layer tetrahedral sphere nest?
I'm so terribly confused about this - for a lab I was supposed to find the size of a molecule. In it I dropped one drop of oleic acid into a tub of water coated with a fine powder. For the first ...
1
vote
1answer
47 views
Superconducting diamagnetic sphere in uniform magnetic field
What is the size of the magnetic dipole moment $\vec m$ of a superconducting diamagnetic sphere $radius=R$ in a uniform magnetic field $\vec B_0$? Since there is no free current, we can solve for ...
0
votes
1answer
23 views
Doubt about coordinates and point of equilibrium
I'm solving an exercise about small oscillations and I have a doubt about coordinates that I have to use.
This is the text of the exercise:
"A bar has mass M and lenght l. Its extremity A is hooked ...
0
votes
0answers
22 views
How to maximizing inductance of solenoid using Optimization method? [closed]
My teacher told me to maximizing inductance of solenoid using Optimization method, and I don't know how to find it if you know please answer me quickly as possible.
Hypothesis: length$=1m$, ...
3
votes
1answer
60 views
Electric field a distance $z$ above the center of a circular loop. The Hard way [closed]
Problem 2.5: Find the electric field a distance $z$ above the center of a circular loop of radius $r$ which carries a uniform line charge $\lambda$.
This problem is in refereced here (with ...
1
vote
2answers
66 views
Velocity of an object undergoing homogenous acceleration
So I was considering the following problem within the context of Special Relativity:
Given an object O, with initial velocity v, undergoing constant acceleration at a rate of a, I want to express the ...
-1
votes
0answers
51 views
Maths manipulations for thermodynamics introduction
I study thermodynamics and statistical physics introduction.
I see statistical physics is considered by states.
$ \Omega \left ( E \right ) $ below is the number of states whose energy lies between ...
2
votes
1answer
76 views
Harmonic oscillator - wavefunctions
I understand now how I can derive the lowest energy state $W_0 = \tfrac{1}{2}\hbar \omega$ of the quantum harmonic oscillator (HO) using the ladder operators. What is the easiest way to now derive ...
-6
votes
0answers
40 views
How much torque can a cyclist achieve? [closed]
How much torque can a cyclist maximally accomplish (while pedaling) with their weight if his mass is 72 kg and the distance of the pedal is 17 cm from the pivot axle?
I believe I understand the ...
0
votes
0answers
39 views
How to calculate back emf? [closed]
I'm trying to answer the question:
The back emf in a motor is 85 V when operating at 1800 rpm. What would be the back emf at 2500 rpm if the magnetic field is unchanged?
So I don't know what ...
4
votes
1answer
42 views
Charge in a layered ball
A ball (radius $R$) has three layers. For $0<r<a$ it is a conductor with free charge $+Q$. For $a<r<b$ it is a linear dielectric $\epsilon$ with free charge embedded in it with density ...
-2
votes
0answers
37 views
Capacitance of stable circuit [closed]
A circuit contains two resistors ($10\: \Omega$ and $20\: \Omega$) and
two capacitors ($12\: \mathrm{\mu F}$ and $6\: \mathrm{\mu F}$)
connected to a $6\: \mathrm{V}$ battery, as shown in the ...
2
votes
1answer
47 views
Summing moments, where have I gone wrong?
Because I don't have enough rep to post images, the question is question 3 part iii) from here: http://www.mei.org.uk/files/papers/m209ju_jk32.pdf
The surface at the contact point B is rough. The ...
0
votes
1answer
45 views
Finding cooling time using an impulse of heat energy
A chip has a thermal resistance of 0.007 K/J s and thermal capacitance
of 1.6 J/K. You give it an impulse of heat equal to 100 J. How long
does it take for the chip to cool to within 10% of the ...
1
vote
2answers
70 views
Slowdown rate of rotating body due to friction force [closed]
This isn't a homework question, but it might as well be. The problem I have been pondering is:
If a disc (or children's roundabout if you like), of radius r, mass m, is spun around it's center ...
4
votes
1answer
72 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
42 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
67 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:
...
0
votes
1answer
53 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
79 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
47 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
76 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
28 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
35 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 ...
-3
votes
0answers
34 views
Does a change in mass of a bob affect the time period? [closed]
If the mass of the bob is doubled, how does it affect the time period of the pendulum when attached with:
Elastic Spring.
Inelastic Spring.
Respectively?
-1
votes
2answers
69 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
0answers
39 views
Simple Calculus angular velocity problem [closed]
I have a Lagrange equation and I have no idea why the solution note I looked at have got
$d \over {dt}$$(mr\omega) = Q$,
$d \over {dt}$$(mr\ddot\omega) = Q$.
I had
$d \over {dt}$$(mr\dot\omega) = ...
0
votes
1answer
50 views
Potential due to charged particle $Q$ placed at a distance $L$ from uncharged conducting sphere [closed]
What is the potential due to charged particle $Q$ placed at a distance $L$ from uncharged conducting sphere?
Is it $\frac{kQ}{L }$ or $\frac{kQ}{L} + \frac{kq}{R}$, where $q$ is the charge induced on ...
0
votes
1answer
50 views
Question about weightlessness
Let us consider +Y as upward and -Y axis as downward. So the acceleration that works these objects is always $-g$.
For the first picture, if the lift is going upward with acceleration $a$, then ...
0
votes
1answer
47 views
Something about collision [closed]
A sphere P of mass m, travelling with speed $u$, makes a head-on collision with a stationary sphere Q also of mass m. After the collision, the velocities of P and Q are $v_1$ and $v_2$ ...
4
votes
2answers
105 views
Deriving the Angular Momentum Commutator Relations by using $\epsilon_{ijk}$ Identities
I've been trying to derive the relation
$$[\hat L_i,\hat L_j] = i\hbar\epsilon_{ijk} \hat L_k $$
without doing each permutation of ${x,y,z}$ individually, but I'm not really getting anywhere. ...
-1
votes
0answers
29 views
Displacement current in the Ampere equation [closed]
Does it explain the significance of displacement current in the Ampere equation?
1
vote
0answers
33 views
Is Gauss' law useful to determine the electric field strength of a charge distribution? [closed]
Under what conditions is useful Gauss' law to determine the electric field strength of a charge distribution? Can someone help me with this question?
3
votes
2answers
88 views
Equal mass on either end of a spring
I'm taking physics this term and this is the first time taking physics in 3 years.
Here is my homework question (of course those variables all have values but we'll use variables for now):
Consider ...
1
vote
2answers
77 views
Solving a light ray worldline with the geodesic equation
I'm having trouble solving the geodesic equation for a light ray.
$$ {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 ...
0
votes
0answers
24 views
What is the optimal weight ratio of a two-stage rocket?
I've been researching this for some time now and I got stuck.
The best resource I've found is this book. The "optimising a multistage rocket" part talks about a three stage rocket but it seems ...
-2
votes
0answers
49 views
electrostatic repulsion that is troubling me [closed]
Consider a homogeneous ring of radius $R$ made of a thin wire of cross section $A (R^2\gg A)$. A Charge $Q_1=10\mu \mathrm{C}$, uniformly distributed over the ring, broke it apart due to electrostatic ...
-2
votes
1answer
61 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
60 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 ...
1
vote
1answer
41 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{ ...
0
votes
0answers
23 views
Spring constant problem [closed]
Say, a spring AB is hung from the ceiling, a block of mass m is attached to B. Another spring CD making angle A with the vertical is attached to side B. What will happen next? Can the system stay in ...
4
votes
1answer
89 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
42 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( ...
