# 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.

learn more… | top users | synonyms

1answer
35 views

### Diagonal matrix in k-space

I'm having some trouble with an integration I hope you guys can help me with. I have that: ${{\mathbf{v}}_{i}}\left( \mathbf{k} \right)=\frac{\hbar {{\mathbf{k}}_{i}}}{m}$ and ...
2answers
30 views

### Voltmeter forming a closed circuit

A battery is connected to a 10Ω resistor as shown in Figure 2. The emf (electromotive force) of the battery is 6.0 V. When the switch is open the voltmeter reads 6.0 V and when it is closed it ...
0answers
44 views

### Air pressure in balloon

I have to calculate the air pressure inside of an hot air balloon. After some searching I found out that I can use the ideal gas law: PV = nRT (from Wikipedia) So to get the pressure in the balloon I ...
1answer
65 views

### Proof $\left[ {\hat H,{{\hat p}_i}} \right] = - \frac{\hbar }{i}\frac{{\partial \hat H}}{{\partial {{\hat q}_i}}}$ [closed]

I have a problem with the Hamiltonian, I don't think anything to solve it!! So could you give me some hints! Knowing that: \left[ {{{\hat p}_i},{{\hat q}_k}} \right] = \frac{\hbar }{i}{\delta ...
1answer
41 views

### Center of mass of three particles of masses 1kg, 2kg, 3kg lies at the point (1,2,3) [closed]

Center of mass of three particles of masses 1kg, 2kg, 3kg lies at the point (1,2,3) and center of mass of another system of particles 3kg and 2kg lies at the point (-1,3,-2). Where should we put a ...
0answers
56 views

### Relativistic canonical transformation

What is relativistic canonical transformation? I need every piece of information about it. Does anyone know a reference or an article about relativistic canonical transformation? For example, in ...
1answer
41 views

### How large of a solar sail would be needed to travel to mars in under a year?

I'm attempting to approach this using the identity $$F/A = I/c$$ I can solve for Area easily enough $$A = F(c/I)$$ and I know the distance $d$ is $$d=1/2(at^2)$$ But I'm having difficulty trying to ...
1answer
202 views

### Potential due to a spherical surface charge

The potential at the surface of an insulating sphere (radius R) is given by $$V(R,\theta) = k \cos(3\theta)$$ where $k$ is a constant. Use separation of variables to find the potential inside the ...
1answer
74 views

### Simple harmonic oscillator system and changes in its total energy

Suppose I have a body of mass $M$ connected to a spring (which is connected to a vertical wall) with a stiffness coefficient of $k$ on some frictionless surface. The body oscillates from point $C$ to ...
1answer
77 views

### Killing vector argument gone awry?

What has gone wrong with this argument?! The original question A space-time such that $$ds^2=-dt^2+t^2dx^2$$ has Killing vectors \$(0,1),(-\exp(x),\frac{\exp(x)}{t}), ...
1answer
198 views

### Electric field due to nonconducting sphere

For calculating electric field outside a nonconducting sphere with a hollow spherical cavity. When I use the rule (Charge density= $dQ/dV$), I don't know exactly what is $dV$, is the volume here ...
1answer
209 views

### What is the general relativistic calculation of travel time to Proxima Centauri?

It has already been asked here how fast a probe would have to travel to reach Alpha Centauri within 60 years. NASA has done some research into a probe that would take 100 years to make the trip. But ...
2answers
108 views

### Geodesic equations

I am having trouble understanding how the following statement (taken from some old notes) is true: For a 2 dimensional space such that $$ds^2=\frac{1}{u^2}(-du^2+dv^2)$$ the timelike geodesics ...
0answers
49 views

### A slender rod with a ball at the end

A slender rod is attached to a block accelerating horizontally. The rod is free to rotate without friction. At the end of the rod is a ball. As the block accelerates, the slender rod will be deviated ...
1answer
96 views

### Chain of balls on an inclined plane

Suppose we have some inclined plane, and there is some chain of balls of length $l$ and mass $m$ lying on it. No friction at all in the system. 1) What is $x_0$ (the vertical hanging part of the ...
2answers
572 views

### Electric field and electric potential of a point charge in 2D and 1D

in 3D, electric field of a piont charge is inversely proportional to the square of distance while the potential is inversely proportional to distance. We can derive it from Coulomb's law. however, I ...
0answers
35 views

### Magnetic Field Lines predict? [closed]

Question: Magnetic field lines determine: (A) only the direction of the field (B) the relative strength of the field (C) both the relative strength and the detection of the field (D) only the ...
3answers
2k views

### Finding Angular Acceleration of rod given radius and angle

A uniform rod is 2.0 m long. The rod is pivoted about a horizontal, frictionless pin through one end. The rod is released from rest at an angle of 30° above the horizontal. What is the angular ...
1answer
53 views

### The second resonance of string?

What is the relationship between "the second resonance " and string and the wavelength. Like in this question: if the length of the string is 2cm with second resonance, then what is wavelength?
2answers
102 views

### Potential Inside Conducting Cube

A cubical box with sides of length L consists of six metal plates. Five sides of the box { the plates at $x=0, x=L, y=0, y=L, z=0$ - are grounded. The top of the box (at z = L) is made of a separate ...
1answer
65 views

### How is torque equal to moment of inertia times angular acceleration divided by g?

How is the following relation true $$\tau = \large\frac{I}{g} \times \alpha$$ where $\tau$ is torque, $I$ is moment of inertia, $g= 9.8ms^{-2}$, and $\alpha=$ angular acceleration.
0answers
46 views

### What force does the seat exert on the rider at the top and bottom of the ride? [closed]

A 75 kg person rides a Ferris wheel which is rotating uniformly. The centripetal force acting on the person is 45 N. What force does the seat exert on the rider at the top and the bottom of the ride?
3answers
71 views

### How to determine the direction of medium's displacement vectors of a standing wave?

Consider the following problem taken from a problem booklet. My questions are: What is displacement vector? And how to determine the direction of displacement vector at a certain point? Where is the ...
1answer
82 views

### Liquid benzene magnetic susceptibility

In a solid state physics problem, I'm asked to make a rough estimate of the contribution to the diamagnetic susceptibility of the outer electron of each carbon atom. The wavefunction of these ...
2answers
144 views

### Why does the quantum Heisenberg model become the classical one when $S\to\infty$?

The Hamiltonian of the spin $S$ quantum Heisenberg model is $$H = J\sum_{<i,j>}\mathbf{S}_{i}\cdot\mathbf{S}_{j}$$ I have read that when the spin quantum number $S\to\infty$, quantum fluctuation ...
1answer
86 views

### How can the derivative of this trace be constrained?

I am studying for my exam on relativity and I am going through some problems sets including ones where I was not very successful in so I want to know how to do this problem. (Convergence of ...
3answers
948 views

### “Find the net force the southern hemisphere of a uniformly charged sphere exerts on the northern hemisphere”

This is Griffiths, Introduction to Electrodynamics, 2.43, if you have the book. The problem states Find the net force that the southern hemisphere of a uniformly charged sphere exerts on the ...
1answer
26 views

### How to find time taken for a spinning top to stop? [closed]

The angular position of a spinning top is given by $\theta = t^3 - 72t$, where $t$ is in seconds and $\theta$ in "radian".
1answer
44 views

### A theoretical problem on Mechanics [closed]

Two particles with masses $m_1$ and $m_2$ are moving in 3D space with some Cartesian coordinate system. There are known the laws of motion of these particles, i.e. the position vectors $\vec{r_1}(t)$ ...
3answers
111 views

### Projectiles and escape velocity

Q: The escape velocity for a body projected vertically upwards from the surface of earth is 11 km/s. If the body is projected at an angle of $45^\circ$ with vertical, the escape velocity will be? ...
0answers
26 views

### Assume the following vectors [closed]

Assume the following vectors: $a = \frac{1}{\sqrt{2}} [0,1,1]$ and $b = [1,2,3]$. (a) Calculate the scalar product c = a c b (b) Calculate the value of vector d where d = b - ca (c) What is the ...
1answer
46 views

### Forces on a particle moving in a vertical circle

In the diagrma, a particle (A, mass 0.6kg) is moving in a vertical circle. The question is: When it gets to the lowest point, what is the tension in the light rod that is between the center of the ...
0answers
35 views

### Physical Optics [closed]

Monochromatic light is used to illuminate a pair of narrow slits 0.3 mm apart and the interference pattern is observed on a source 0.91 mm away. The second dark band appears 3.0 mm from the center. ...
0answers
23 views

### Absolute Viscosity of Water at certain temperatures

I just started my class on fluid mechanics. There's a problem that requires absolute viscosity $u$ of water at 25C. I looked up my table in the book and I only have it for water at 20C and 30C. I ...
4answers
2k views

### Calculating impact force for a falling object?

Good evening, I'm trying to calculate what kind of impact force a falling object would have once it hit something. This is my attempt so far: Because $x= \frac{1}{2} at^2$, $t=\sqrt{2x/a}$ $v=at$, ...
2answers
105 views

### Why is $r'/r^2 = -1/r$? [closed]

If $r=r(t)$, why is $\frac{r'(t)}{(r(t))^2}$ = $\frac{1}{r(t)}$ where $'$ denotes the derivative? I saw it in a lecture. Can you please explain?
1answer
287 views

### Simple work and energy problem

I have the following problem: A man that weights 50kg goes up running the stairs of a tower in Chicago that is 443m tall. What is the power measured in watts if he arrives at the top of the tower ...
0answers
45 views

### Work And Energy Question [closed]

$H = 3\text{ m}$,$m=2\text{ kg}$ The right side is rough. I want to figure: what is the coefficient of friction $\mu$? How high and exceed the maximum return on the plane right body? I know ...
1answer
81 views

### When should angles be expressed in degrees vs. radians?

I am trying to calculate the albedo of a given latitude by following the methods of Brutsaert (1982), I have copied the formula below: 3.6 Shortwave and long-wave radiative fluxes Albedo ...
2answers
48 views

### Constant of gravity in earth fixed coordinate system

I have this problem: If the constant of gravity is measured to be $g_0$ in an earth fixed coordinate system, what is the difference $g-g_0$ where $g$ is the real constant of gravity as ...
0answers
49 views

### Calculating pressure in accelerated fluids in closed and open vessels?

The question asked was "what should be the acceleration such that the pressure at both the points marked by thick dots be equal? the vessel is open and cubic with side 5m?" Initially i considered ...
1answer
90 views

### Joule-Thomson effect of Van der Waals gas

I'm supposed to calculate the inversion pressure $p_i$ of a Van der Waals gas. The state equation of the Van der Waals gas is: $$(p + \frac{a}{V^2})(V-b) = RT.$$ To get a hold of the inversion ...
2answers
51 views

### Electron in an infinite potential well

Does this problem have any sense? Suppose an electron in an infinite well of length $0.5nm$. The state of the system is the superposition of the ground state and the first excited state. Find the ...
0answers
30 views

### Lagrangian of electromagnetic tensor in light cone coordinates? [closed]

I have Lagrangian Density of Electromagnetic field Tensor in light cone coordinates using D'Alembertian operator and Lagrangian density in Cartesian coordinates. I couldn't figure out the way to ...
0answers
28 views

### Mercury's Orbital Precession in Special Relativity

I am researching Mercury's orbital precession. I have considered most perturbations and general relativity. I am still not satisfied. I need your help. I need a solution to Exercise 13, Chapter 6, in ...
4answers
133 views

### Resistors in Parallel

From my book: "A length of wire is cut into five equal pieces. The five pieces are then connected in parallel, with the resulting resistance being 2.00 Ω. What was the resistance of the ...
2answers
54 views

### Angle between acceleration and velocity

Problem: A particle is constrained to move in a circle with a 10-meter radius. At one instant, the particle's speed is 10 meters per second and is increasing at a rate of 10 meters per second ...
2answers
66 views

### Is it possible to “add cold” or to “add heat” to systems?

Amanda just poured herself a cup of hot coffee to get her day started. She took her first sip and nearly burned her tongue. Since she didn't have much time to sit and wait for it to cool down, ...
4answers
291 views

### Trace and adjoint representation of $SU(N)$

In the adjoint representation of $SU(N)$, the generators $t^a_G$ are chosen as $$(t^a_G)_{bc}=-if^{abc}$$ The following identity can be found in Taizo Muta's book "Foundations of Quantum ...
2answers
43 views

### Simple conservation of momentum and frame of reference problem

I'm making a very simple physics engine based on momentum, and I'm solving what response to use for a collision from each involved object's frame of reference. However, something about how I'm ...
{"extraction_info": {"found_math": true, "script_math_tex": 0, "script_math_asciimath": 0, "math_annotations": 0, "math_alttext": 0, "mathml": 0, "mathjax_tag": 0, "mathjax_inline_tex": 50, "mathjax_display_tex": 10, "mathjax_asciimath": 0, "img_math": 0, "codecogs_latex": 0, "wp_latex": 0, "mimetex.cgi": 0, "/images/math/codecogs": 0, "mathtex.cgi": 0}, "config": {"markdown_headings": true, "markdown_code": true, "boilerplate_config": {"ratio_threshold": 0.3, "absolute_threshold": 10, "end_threshold": 15, "enable": true}, "remove_buttons": true, "remove_image_figures": true, "remove_link_clusters": true, "table_config": {"min_rows": 2, "min_cols": 3, "format": "github"}, "remove_chinese": true, "remove_edit_buttons": true, "extract_latex": true}, "english_confidence": 0.9243711829185486, "perplexity_flag": "middle"}