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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-2
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0answers
15 views

A problem from the Rudolf Ortvay Competition in Physics [on hold]

PROBLEM STATEMENT: A uniform circular disc is kept face down on a table which has a uniform friction coefficient.It is pushed in such a way that there is translational as well as rotational motion of ...
0
votes
0answers
7 views

Correlator $bc$ system

I have da doubt with bc system. Polchinski says (2.5.10) $$ b(z)b(0)~=~O(z). \tag{2.5.10} $$ I tried to compute the correlation function With eom, using eq (2.5.6b) by Polchinki, removing the ...
1
vote
1answer
15 views

Complex scalar theory: annihilation and creation operators give wrong commutators with Hamiltonian

The theory of a real (hermitian) scalar field can be found in many books and everywhere online. On the other hand, if we take the field non-hermitian, then I can only find notes on path integrals. I ...
0
votes
1answer
47 views

Can forces in N be thought of as a length?

A solid uniform 45.0 kg ball of diameter 32.0 cm is supported against a vertical friction-less wall using a thin 30 cm wire of negligible mass. (a)make a free body diagram for the ball and use it to ...
2
votes
1answer
30 views

Polchinski Equation (10.4.7)

I'm having a problem to interpret eq. (10.4.7) in Polchinski: $$ \gamma(z)\delta(\gamma(0)) = O(z), \qquad \beta(z)\delta(\gamma(0)) = O(z^{-1}). $$ What does he mean by $\delta$? He tries to ...
-2
votes
0answers
20 views

Finding root mean square speed of an ideal gas [on hold]

Again a question ive never seen in my life... The molecules of an ideal gas have a root mean square speed of 520 meters per second at a temperature of 27 degrees celcius. Calculate the r.m.s speed ...
0
votes
0answers
11 views

To change the path of an electron beam travelling horizontally using a uniform electric field [on hold]

Problem: A beam of 20kV electrons is travelling horizontally. If one was to bend the path to make them travel vertically at the same speed, using a region with a uniform electric field, what are the ...
-1
votes
2answers
45 views

Find an angle in 3 dimensions [on hold]

I've had a good attempt at this question trying everything I could think of and the answer I got was 60 degrees yet still incorrect. It would be great if anyone could help me out. The boom ...
-2
votes
2answers
24 views

Finding velocity using given angle

Question: A body is projected with $50 \text{m/sec}$ at an angle of $53$ degree with the horizontal. After what time velocity makes an angle $33.7$ degree with horizontal? ($\tan 33.7 = 2/3$) My ...
0
votes
1answer
20 views

2-Dimensional motion query [on hold]

Question: A body is projected with speed u at an angle theta with the horizontal. (Following are the three parts in the question). Find the change in velocity in time t. Find the speed of body at a ...
0
votes
1answer
19 views

Electric field of a non-conducting, non uniform sheet

I have to find an expression for the electric field everywhere (mag and direction). The sheet is lying on the yz plane with the center at the origin. The width of the sheet is equal to 2b. The charge ...
1
vote
1answer
49 views

Can mass-less spring system be solved?

Suppose we have typical chain of strings with masses, attached to the walls (W) at each side W-----m-----m--------W x=0 x=6 x=12 x=21 So if we let ...
3
votes
2answers
60 views

Twin paradox modified for a periodic universe

I had received a homework problem by my professor. Please consider part (b) of the problem. Can part (b) be solved only using special relativity? And how can you solve this? I think that Alice will ...
0
votes
2answers
52 views

If $[A_1, H] =[A_2, H] = 0$ but $[A_1, A_2] \neq 0$, $H$ must be degenerate?

I am having a difficult time understanding this problem. Suppose $[A_1, A_2] \ne 0,$ $[A_1, H] = 0,$ $[A_2, H] = 0.$ Show that the energy eigenstates of $H$ are in general ...
-3
votes
0answers
14 views

On Fig 4.1, draw the normal to the mirror at end P

I got given some homework to do over summer. It's a completely different exam board with a different specification. I understand it's hard to draw lines on this site but any help you could give me, ...
-1
votes
1answer
21 views

Problem related to Motion in 2-Dimension [on hold]

Question: The height y and the distance x along the horizontal place of a projectile on a certain planer (with no surrounding ...
-4
votes
0answers
25 views

Relative motion problem [on hold]

A fighter plane flies at a velocity of 300 m/sec. On the fighter plane there is a gun which shoots at a rate of 40 rounds per second with a muzzle velocity of 1200m/sec. The shots are aimed at another ...
-6
votes
0answers
52 views

Speed of the train is faster than a car, why? [on hold]

What are the physical quantities required to prove this?
-9
votes
1answer
48 views

Can you help me with physics homework please? [on hold]

An object with a mass M is suspended from an elastic spring with a spring constant k. The object oscillates with period T on the surface of Earth. If the oscillating system is moved to the ...
1
vote
0answers
22 views

Simple mean-field “lattice gas” model

I'm having some conceptual problems with a simple mean-field "lattice gas" model and I'd be glad if someone could help me go further. Basically the model consists of the following hamiltonian: $$ H ...
0
votes
2answers
30 views

Reaction Force on String Wrapped Around Circular Peg

A massless inextensible string is wrapped around a frictionless circular peg. The string is taut, with tension $T_2$ and $T_1$ at the points where it leaves of the pg as shown. The segment wrapped ...
-2
votes
0answers
23 views

Calculating deceleration time - help! [on hold]

I'm struggling with a question due to not having the correct formula, so I hope someone can help! A 10000kg object is travelling at 15m/s, and a force of 600N is applied in the opposite direction to ...
1
vote
1answer
38 views

Electric field between capacitors [on hold]

A parallel-plate capacitor consists of two parallel, conducting plates of area $A$, separated by a distance $d$. Each carries a charge of magnitude $Q$; positive on one, negative on the other. ...
-5
votes
0answers
19 views

Find tf and end distance [on hold]

Mass = 30kg, Velocity = 30, Acceleration = unknown, end distance = unknown, force = -20N. Please breakdown formula in elementary physics terms thanks! a)Find time the object came to a stop b)the end ...
-5
votes
0answers
23 views

Blackbody thermal radiation [on hold]

By definition, the absorptivity of every blackbody radiator equals 1.0 (100% of the incident radiation is absorbed). So from this fact, you can conclude that the reflectivity and transmissivity of a ...
2
votes
0answers
37 views

Determining potential in coaxial cylinders

If I have two conducting, coaxial cylinders as shown: The potential at the outer cylinder is = 0. And I apply a potential difference across both cylinders in the form $V_a - V_b = V_{ab}$, how can ...
0
votes
1answer
59 views

Finding the action of a discretized Lagrangian

I am trying to find the action associated with the Lagrangian density $$ \mathcal{L} = \frac{1}{2}\left( \frac{\partial\phi}{\partial x} \right)^2 + \frac{1}{2}m^2\phi^2. \tag{1} $$ I am supposed to ...
0
votes
1answer
29 views

What is force of static and kinetic friction? [on hold]

While I am asking more than one question in this thread they are all small concept tests that you can answer with a yes or no, and is all related to me understanding kinetic and static friction. ...
0
votes
1answer
18 views

Can the electric potential corresponding to the negative plate ever be equal to anything bigger than zero?

I am doing my homework. I read my textbook. It says that the negatively charged plate is the low potential plate. It also shows a example of lower plate equal to 0V, I guess for simplicity. But what ...
7
votes
2answers
208 views

Deriving Newton's first law from the principle of least action

Newton's first law states that if the net force on an object is zero, then this object moves with constant velocity. I'm interested in the derivation of this law from the principle of least action. ...
0
votes
0answers
39 views

How can I obtain a unit vector of a shifted spherical system? [migrated]

I hope that I can explain myself clear enough, Assuming i have a sphere that has been moved down in the Z-axis. I know that the radial unit vector when the sphere is not shifted can be expressed as: ...
0
votes
1answer
63 views

Commutation relations in second quantization

I know that for operators $a(\chi_1), a(\chi_2)$ of the same type (fermionic or bosonic) $$ [a(\chi_1), a(\chi_2)]_{-\xi} = [a^\dagger (\chi_1), a^\dagger (\chi_2)]_{-\xi} = 0 \tag{1}$$ where $$\xi ...
3
votes
2answers
103 views

Where does the equation $p=\frac{1}{c}\sqrt{T^2 +2mTc^2}$ come from?

Where does the relativistic formula $$p~=~\frac{1}{c}\sqrt{T^2 +2mTc^2}$$ come from? What is the derivation from Einstein's formula? $T$ is the kinetic energy $m$ is the mass $p$ is the momentum.
0
votes
1answer
21 views

how covalent bonding lower the energy of potential energy

Considering the potential energy of interacting particles, how does covalent bonding lower the energy of the system?
1
vote
2answers
74 views

Klein-Gordon Green's function: derivative of delta distribution?

In Peskin/Schroeder there is an explicit calculation showing that the retarded Green's function of the real Klein-Gordon field $$D_R(x-y) ~\equiv~ \theta(x^0 - y^0) \langle 0 | [\phi(x), \phi(y)] ...
0
votes
2answers
62 views

Why is this circuit component on without a potential difference?

From the following graph, determine whether (h) will be on or off: I think (h) should be off because there is no potential difference across it. Is this correct? Edit: Using the method provided ...
2
votes
1answer
38 views

What do field lines between two like-charged parallel plates look like?

I couldn't find any image to answer my question on Google. But I guess it looks somewhat like this, correct?
0
votes
1answer
23 views

How does the size of conducting sphere affect the distribution of charge?

I am doing my homework. One question gives individual charges on two spheres. And asks what the resulting charge is on each after touching. But one thing interest me is that in the answer key, it's ...
1
vote
1answer
40 views

Transfer function of an RC circuit

I'm trying to reach the transfer function of an RC circuit which I am told is $$\frac {V_R} {V_\text{in}} = \frac{R}{\sqrt{(R^2 + \frac {1}{\omega^2 C^2})}} \cos(\omega t - \theta) \, .$$ Is this ...
3
votes
3answers
122 views

What is the binding energy of a neutron star?

Neutrons which constitute a neutron star have a rest mass that is greater when separated from the star because they are bound with a certain potential energy. This potential energy causes the system ...
0
votes
1answer
39 views

When will a ball running between two downwards slopes stop going back and forth?

Two angles $a$ and $b$ meet at a point, making an angle $c$ between them. $a$ and $b$ are all $0 < \theta < π/2$. $c$ is $π/2 < \theta < π$ $a$ is to the left and opens to the left, $b$ ...
0
votes
0answers
66 views

Finding the tension/acceleration of a pulley

I redid the entire thread with pictures. Please click the link to see a picture of pulley with it weights. You see the equation at the bottom of the picture. ...
0
votes
1answer
52 views

Magnetic field tangential surface integral

When deriving: $\vec{E}=-\frac{\partial\vec{A}}{\partial t}-\nabla\phi$ from Helmholtz decomposition of $\vec{E}$, It seems necessary to show: $$\iiint \nabla' \times \frac{\vec{B}(\vec{r')}}{|\vec{r} ...
2
votes
1answer
68 views

Maxwell's equations in integral form using differential geometry

So I've been trying to convert from Maxwell's equations in terms of differential forms to the integral versions of Maxwell's equations that we know from vector calculus. We have, in vector calculus ...
1
vote
0answers
27 views

A functional average calculation confusion within Gaussian planar model's RG

I am trying to follow some detailed calculation in a famous paper [John, B. Kogut, Rev. Mod. Phys. 51, 659 (1979), An introduction to lattice gauge theory and spin systems]. More precisely, please ...
1
vote
1answer
81 views

How many days it takes to travel to Cygnus? [duplicate]

BACKSTORY I read today newspaper and made discovery of planet Cygnus (in news paper of course :v) that human can live there! The distance is 1400 Lightyear. Assuming I travel with the world fastest ...
0
votes
0answers
18 views

Breaking $SO(10) \rightarrow$ Pati-Salam: Which Higgs field gets a vev? (= What's the VEV direction?)

Which Higgs field or linear combination of Higgs fields gets a vev if we want to break $$ SO(10) \rightarrow SU(4) \times SU(2) \times SU(2) ? $$ The rank of the Pati-Salam group $SU(4) \times SU(2) ...
0
votes
0answers
15 views

Differential pressure reading at a slope [on hold]

What is the differential pressure reading when a mercury manometer is at a slope of 15 degrees to the horizontal and its original vertical differential pressure is 0.2 metres. The answer involves ...
-2
votes
0answers
25 views

Electromagnaetizma [on hold]

The figure shows a schematic diagram of a simple mass spectrometer, consisting of a velocity selector and a particle detector and being used to separate singly ionized atoms (q = +e = 1.602e-19 C) of ...
-1
votes
0answers
54 views

Solving for the resonant frequencies of a mass-spring system [on hold]

How do I solve for the resonant frequencies of a mass-spring systems. Given 3 masses ($M_1, M_2, M_3$) connected linearly with 2 springs ($K_1,K_2$), let $X_1,X_2,X_3$ be the displacement of the ...