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
27 views

Proof of Poisson equation in a gravitational field

Can anybody show me how to prove Poisson's equation in Newtonian Gravitational field?
0
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3answers
30 views

What is extension produced in spring?

Consider a spring -block system given in the figure : Note: K is spring Constant. Initially, the spring was unloaded. After that, a block of mass M is loaded to spring and as a result, the spring ...
-1
votes
0answers
15 views

Are there any benefits from doing Physics MCQs without a given answer key? [on hold]

If I have a answer key, I can know where I'm wrong and solve again. But many books give MCQs without answer key, how should I use them?
1
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0answers
26 views

Angle sum of triangle in Schwarzchild solution

Curvature of space is often intuitively explained as angles of a triangle not adding up to 180 degrees. My questions concerns that. Suppose you have a perfectly spherical star of uniform density - so ...
11
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5answers
766 views

Conical train wheels

I've been reading about how the conical shape of train wheels helps trains round turns without a differential. For those who are unfamiliar with the idea, the conical shape allows the wheels to shift ...
-1
votes
0answers
30 views

Using Lagrangian to get the equations of motion [on hold]

The thin hollow cylinder rolls on the plane without slipping. I want to write the Lagrangian, but am confused about the degrees of freedom of the motion and the coordinate axes to consider for ...
0
votes
1answer
25 views

Calculate constant acceleration for a length of track with start at rest and final velocity known [on hold]

My maths and physics are rusty... I'm a bit ashamed, but I have this problem: Let's have a 1000m linear accelerator (mass driver) propel a payload at constant acceleration, with a final velocity of ...
-1
votes
0answers
41 views

Commutation relation between linear momentum and vector Potential [on hold]

Does the linear momentum and vector potential commute? How can we show their commutation relation ? I am actually trying to find the commutation relation between both linear momentum of an electron ...
-4
votes
2answers
67 views

Would a piece of paper look as big as a small bedroom to an atom?, or bigger? [on hold]

This evening my six year old asked me "Would a piece of paper look as big as this room to an atom?, or bigger?" ('this room' being a small sized bedroom) A friend suggested it would probably ...
1
vote
2answers
36 views

Magnetic field of three infinite co-planar wires

I'm reviewing test questions from an old exam, but getting a little confused. a) I thought one of the locations of 0 magnetic field would be right where the middle wire is, since the fields from ...
1
vote
0answers
40 views

Showing that a mass moves a half cycle

Consider a mass $m$ at position $x(t)$ on a rough horizontal table attached to the origin by a spring with constant $k$ (restoring force $-kx$) and with a dry friction force $f$ $$\begin{cases} ...
-1
votes
1answer
15 views

Help with this cyclic thermodynamic process question? [on hold]

So we had this question on a midterm: A heat engine runs by taking in 2000 J of heat energy, doing 1450 J of work, and expelling the remaining energy to the cold reservoir. If the cold reservoir is ...
1
vote
0answers
60 views

How to integrate this physics model? [on hold]

OK, this might seem easy... but I'm having trouble solving it. Let's say I have a force acting on a body whose mass is known, by this law: $$F(t) = bt^2$$ where $b$ is a known constant, and $t$ is ...
0
votes
1answer
12 views

magnetic intensity of cylinder sheet of charge having surface charge density K

Here is the problem I have by using $$a_z\times R = (0,0,1) \times (\rho-1.2,0,-z) = (\rho -1.2)a_\phi $$ $$H=\int_{-\infty}^{\infty}\int_{0}^{2\pi}\frac{2.4(\rho-1.2)d\phi dz}{4\pi ...
1
vote
2answers
45 views

Finding out the direction of $E$ field

In the Figure we can see a conducting loop of radius $r$ situated in a uniform magnetic field that is perpendicular to the plane of the loop. If we change the magnetic field, then according to the ...
2
votes
1answer
36 views

Lorentz-invariant phase space of a three-body decay process

I am not following the use of delta function in the 3-body decay process. In $\gamma^* \to gq\bar{q}$ process, with $\gamma^*$ being a virtual photon, we have a phase space factor $$d^9R_3 = ...
-2
votes
0answers
21 views

Vertical throwing exercise [on hold]

I'm struggling with this question Assume an object is resting on the ground, in $t=0$ there is a force that starting to work on it, which grants him vertical acceleration towards up, $a=3m/s^2$, ...
-2
votes
0answers
20 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 ...
1
vote
0answers
20 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
28 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
53 views

Can forces in N be thought of as a length? [on hold]

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 ...
1
vote
1answer
55 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
25 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
12 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
83 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 ...
-3
votes
2answers
30 views

Finding velocity using given angle [on hold]

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 ...
-1
votes
0answers
23 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
22 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
51 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 ...
6
votes
2answers
83 views

Twin paradox modified for a periodic universe [on hold]

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
84 views

$[A_1, H] =[A_2, H] = 0$ but $[A_1, A_2] \neq 0$?

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
19 views

On Fig 4.1, draw the normal to the mirror at end P [on hold]

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
29 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
53 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
50 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
26 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
3answers
50 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
51 views

Problem with OPE (from Polchinski) [on hold]

I was reading Polchinski, Vol. 2 pag 12, while I found (10.3.12a): $$ e^{iH(z)}e^{-iH(z)}=\frac{1}{2z} + i\partial H(0) + 2zT^H_B(0) + O(z^2).\tag{10.3.12a} $$ Now I tried to do the OPE, what I ...
-2
votes
0answers
24 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
40 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
20 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
24 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
38 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
30 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
212 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
64 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 ...