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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0
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1answer
11 views

Calculating stopping distance of a car on a slope

I'm having trouble setting up a homework problem properly. We are given the following: A given car can brake to a stop from a speed of 60.0 mi/h in a distance of 123 ft on a level roadway. What ...
-2
votes
0answers
23 views

Motion question [on hold]

I have been struggling to understand how to do this question so some guidance would be great. A racing car of mass 941kg accelerates from 28km/h to 102km/h over a distance of 297m. Frictional forces ...
-1
votes
0answers
14 views

How to find frequency from wave equation [on hold]

I have wave equation for vibrating string with damping $M\ddot{x}+C\dot{x}+Kx=0.$ I used this formulae for finding (natural)frequency $\sqrt{\frac{K}{M}-(\frac{C}{2M})^2},$ is it ok?
1
vote
0answers
13 views

Need some help in finding Poynting vector

I am asked to Calculate the Poynting vector of the transmitted wave but I am not sure how. I am able to show the electric field of the transmitted wave as shown above in the picture.
1
vote
1answer
11 views

Does the stretching of a spring depend on its rest lenght?

I don't understand if the stretching of a spring depends on its rest lenght besides its force constant $k$. I'll make an example to show my doubt. Consider a vertical spring and a mass $m$ ...
-2
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0answers
17 views

Torque Problem (tension and thrust)

A crane boom which weighs 200 lb and which ia 10 ft long is supported so that it makes an angle 50° with the supporting wall and so that the rope makes an angle of 70° with the wall. If load of 700 lb ...
-2
votes
0answers
12 views

Torque (Distance × MomentArm)

A bar 2.0 m long makes an angle of 30° with the horizontal. A vertical force of 40N is applied 0.4 m from the upper end. Calculate the torque due to this force about each end.
0
votes
1answer
7 views

Solar radiation conversion check

I'm analysing some solar radiation data, which is the calculated as the daily sum of PAR in MJ per m$^2$ per day. Assuming that PAR makes 42% from total irradiance, how can I convert this to solar ...
-7
votes
1answer
31 views

Simple harmonic oscillators [on hold]

A block whose mass m is 680g is fastened to a spring whose constant k is 65N/m . The block is pulled a distnce x=11cm from its equilibrium postion at x=0cm on a frictionless surface and released ...
-3
votes
0answers
27 views

Maximum speed in Simple harmonic motion [on hold]

If a vertical spring block executes SHM at a frequency of 10 Hz . At the upper extreme position of the block the spring is unstretched . Then how can we find the maximum speed . I tried The ...
-1
votes
0answers
25 views

Please help me to solve this work and energy problem [on hold]

A child of mass 35 Kg is sitting on trolley of mass 5 Kg.The trolley is given push by apply a force so that it began moving at a velocity of 4 m/s.The trolley comes to rest after covering a distance ...
1
vote
0answers
21 views

Electric Field From a point P

A rod of length $L$ and total charge $Q$ creates an electric field at a point $P$, $x$ units away. We need the electric field at this point. I am working on a physics problem for homework and the set ...
0
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0answers
7 views

What is temperature dependence (Schottky defects)?

The problem requires us to use the following property to determine the temperature dependence: For a system in thermal equilibrium in a heat bath at temperature $T$ the Helmholtz free energy is a ...
1
vote
0answers
22 views

Carnot cycle in steam processes

Our professor told us that in steam processes, such as Clausis-Rankine process, Carnot's cycle is impossible to achieve. Can someone explain why?
0
votes
1answer
16 views

Phase Portraits Given Hamiltonian

Given a Hamiltonian say $$ H = 5p^2 $$ What is the correct procedure for producing a phase portrait. My initial thoughts were to solve the system of equations $\frac{dq}{dt} = 0$ and $\frac{dp}{dt} ...
0
votes
1answer
38 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
28 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
0
votes
1answer
19 views

Conduction problem [on hold]

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 ...
-1
votes
0answers
35 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
26 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 $H$ ...
0
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0answers
24 views

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

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
50 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
1answer
21 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
66 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
27 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
36 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
40 views

Spinor on an eigenstate of $J_z$ [on hold]

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
38 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
31 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
19 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 ...
3
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
34 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
489 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
41 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
20 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
23 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
139 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
37 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
45 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
35 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!
2
votes
0answers
29 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
33 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
28 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
46 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 ...