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 questions.

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-1
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0answers
22 views

Formula to find angle of projectile motion [on hold]

I want to be able to modify the angle at which the bullet follows projectile motion in order to hit the object which is moving horizontally (from right to left) and the projectile motion is starting ...
4
votes
3answers
765 views

Which ball touches the ground first?

This is a very well known problem, but I can't find an answer in the specific case I'm looking for. Let's consider two balls : Ball 1 weighs 10 kg Ball 2 weighs 1 kg Balls have identical volumes ...
1
vote
1answer
28 views

CPT invariance of Dirac equation

We know that Dirac equation is \begin{equation} ( i \partial _\mu \gamma ^\mu - m ) \psi ~=~0. \end{equation} How can we show that Dirac equation is invariant under CPT transformation?
-1
votes
0answers
26 views

Exercise on the Oscillatory Motion [on hold]

Exercise A resistance force $F_r(t) = -bv(t)$ and a sinusoidal force $F(t) = F_0 \sin{\phi}$ act on an oscilating body. Find the energy that needs to be provided in order for the system to perform ...
2
votes
0answers
38 views

S-Matrix Generating Functional (Problem 4.1 in Weinberg)

I'm currently working through Weinberg's QFT book, but I'm somewhat stuck at problem 4.1, which states: Define generating functionals for the S-matrix and its connected part: \begin{equation} ...
0
votes
1answer
65 views

Minimum separation problem

I am trying to solve this problem where two point charges are infinite distance apart(horizontally) but separated by some finite distance vertically. Both are positively charged and projected ...
1
vote
2answers
76 views

Given eigenvalues of $\vec l^2$ and $\vec s^2$, calculate the eigenvalue for $\vec j^2$

There was an exam question that read approximatly: Let $\vec j = \vec l + \vec s$. Given eigenvalues of $\vec l^2$ and $\vec s^2$, calculate the eigenvalue for $\vec j^2$. We came up with $$\vec ...
-6
votes
0answers
25 views

Potential Energy and Kinetic Energy [on hold]

how does potential energy related to height? how does kinetic energy related to velocity? how does potential energy related to those kinetic energy? how does potential energy and kinetic energy ...
-1
votes
0answers
24 views

How do I know which force to use in moments calculation?

I'm working on some Statics problem, with the method of joints, etc and for all of them, we use sum of moments = 0. However, I'm very confused about which forces I should use to find the sum of ...
2
votes
1answer
38 views

Gas pressure and centrifugal force

I think about a rotating torus (simplified tire) filled with ideal gas. Mass of gas is $m$ and molar mass is $M$. Pressure in non rotating torus is $p_0$. Temperature is constant $T$. Inner radius of ...
2
votes
2answers
53 views

Mass of air inside air balloon

I have an exercise here with a solution. The basic outline of the problem is: Calculate the mass of air inside a hot air balloon, given the mass of the air balloon at rest is $m_b = ...
0
votes
0answers
28 views

Poisson equation solver with specific boundary condition

I want to solve 2d Poisson equation with this Boundary conditions below $$u(-5,y) = 0 , \\ \frac{\partial u(x,y)}{ \partial x} = 0 \,\,{\rm at}\,\, x = -5\\ u(x,-5) = u(x,5)$$ Now my question is ...
-2
votes
0answers
24 views

problem: polarized sphere [on hold]

Need some help with a practice exam question: Given a solid sphere with radius R, with polarization density given by $P(\vec{r})) = a(R-r) \hat{z}$ . I need to find the potential inside the sphere, ...
-1
votes
0answers
24 views

Trying to calculate the average speed from this question [on hold]

I already checked the average speed question that was posted before and tried applying the methods to my question but nothing seemed to work. I would like to know if anyone can explain to me how to ...
2
votes
1answer
57 views

Invariance of canonical Hamiltonian equation when adding the total time derivative of a function of $q_i$ and $t$ to the Lagrangian

The following is exercise 8.2 in 3rd edition (and exercise 8.19 in 2nd edition) of Goldstein's Classical Mechanics. Adding the total time derivative of a function of $q_i$ and t to the Lagrangian ...
0
votes
0answers
20 views

calculate ph if C=0.02 [on hold]

we have a solution CH3COOH (acetic acid) (0.02 mol/l) and its KA is 1.8*10^(-5) calculate the PH of the solution above I tried everything but I failed and couldn't do it wasted about hours on this ...
1
vote
1answer
43 views

Torque due to Gravity

I have learned that a torque on an object due to the gravity can be calculated as if the gravity acts on the center of mass of the object when the object is near the surface of the earth. Now I want ...
3
votes
1answer
49 views

Show that getting parallel transported does not change angle between them- Tensors [on hold]

I must tell you that I have never seen this kind of question in Tensor Analysis. Our professor had set up this question in our exam, but I don't know whether it belongs to tensors or not. The question ...
3
votes
3answers
136 views

How to solve the Laplace Equation in the hollow square region?

Suppose the values of $a$, $b$, $V_1$ and $V_2$ is given. I want to find the solution of the Laplace equation, $$\frac{\partial^2 \phi}{\partial x^2}+\frac{\partial^2 \phi}{\partial y^2}=0$$ in the ...
1
vote
0answers
45 views

How to find the force to cause certain trajectory? [closed]

So I got a little board today and decided to do some physics. What I came up with is confusing me alot. Say someone throws a rock and when this happens you get the known's of. From here is there a way ...
1
vote
2answers
40 views

How do I find the perpendicular velocity of a particle to a varying magnetic field?

I am trying to find the component of velocity perpendicular to a magnetic field. This was easy when the magnetic field was static and pointing in only one direction (the $z$ axis), but now I need to ...
-1
votes
0answers
40 views

Moving object under water [closed]

If a cube of side 20ft is at 50 ft below in fresh water and it extends 20 feet horizontally giving it a measurement of 20 ft high, 40 ft long by 20 ft wide and the moving section weighs 700lbs. How ...
0
votes
0answers
28 views

Kinetic and potential energy of riser [closed]

Could some one help me , how can i get the kinetic and potential energy of the riser system : Kinetic energy EK : where x and t represent the independent spatial and time variables, respectively, ...
0
votes
0answers
36 views

Gauss' Law, charge distribution on conductor [closed]

_A point charge $q$ is placed in a cavity in a metal block. If a charge $Q$ is brought outside the metal, will the charge q feel an electric force? Explain how the external charge Q affects charge ...
0
votes
0answers
34 views

Writing Schrodinger equation with central potential in Atomic unit

I'm struggling to write Schrodinger equation with a central potential in Atomic unit. $$ ...
1
vote
1answer
110 views

GPS Satellite - Special Relativity

I'm going through an old relativity assignment, and I've been asked to calculate the time dilation for a satellite which orbits the earth in 12 hours at 26000km from the surface, and travels at a ...
0
votes
0answers
30 views

Finite rod aligned with a time varying magnetic field

A finite length rod is aligned parallel with a magnetic field that has a changing magnitude (let's say a sinusoidal change), given the conductivity and permeability of the rod and assume a large ...
3
votes
2answers
65 views

Gravity on flat object [duplicate]

I was wondering how gravity would behave on object of different shapes. If the Earth was squeezed into a thin disk what would the gravitional acceleration be at the center of the flat surface? Would ...
0
votes
0answers
54 views

Proving the contracted Bianchi identity [closed]

I know how to derive the contracted Bianchi identity $$\nabla_\mu(R_{\mu\nu}-\frac{1}{2}g_{\mu\nu}R) = 0.$$ But I am having trouble proving that the LHS of this is 0. Can someone help me please? ...
0
votes
3answers
28 views

Converting moment of inertia (kg per square m) to torque or moment of force (in/lb or n/m) [closed]

My problem is that I have an dc starter generater and they say that to test it I must have a flywheel that can provide the starting inertia of .522 kg per square meter. What I need to know what will ...
1
vote
2answers
147 views

When is this integral zero?

I have a particle with total energy $E$ confined in a potential $$U(x) = -\frac{\cos^4x}{2} - m \cos x - f \sin x. $$ The constants $f$ and $m$ are both in the range (-2,2). The energy is such that ...
0
votes
0answers
53 views

Variation of a lagrangian, least action, metric, covariant derivate [closed]

I am doing the variation of a Lagrangian, but I am having problem with a particle terminus. My action is: $$S=\int d^4x \sqrt{-g}[ (\nabla_\mu A^\mu)^2] $$ I want to get the $T_{\mu\nu} $ ...
-4
votes
0answers
26 views

what is the reflective index of given liquid? [closed]

In this question the refractive index of a liquid is to be calculated considering the given two cases.
0
votes
3answers
55 views

Superposition principle

If $S=(v_{1},v_{2}......v_{n})$ is a basis for vector Space V, then every vector v in V can be expressed in the form of $v=c_{1}v_{1}+.......c_{n}v_{n}$ in an unique way. Explain the significance of ...
1
vote
0answers
31 views

Future of a universe with scalar field

Assume that $\phi=\phi_0\gg M_{Pl}$, what can you say about the future of a universe in a model with $V(\phi)=\frac{1}{2}m^2\phi^2+\epsilon \phi$, where $|\epsilon|\ll m^2M_{Pl}^2$, is a small ...
4
votes
3answers
452 views

Does the weight of fluid in a conical container act entirely on the base?

Scenario description Lets assume both containers have a capacity of 300 litres. One is a vertical tube as shown in pic 1 Other one is more or less a V shaped containers as shown in pic 2 Both are ...
1
vote
0answers
44 views

About Stellar Evolution [closed]

During its formation, the Sun contracted under the force of its own gravity, until hydrostatic equilibrium was established. In this contraction, its temperature increased from $3\times10^4\text{ K}$ ...
0
votes
1answer
46 views

Modern physics photoelectric effect [closed]

In a photo electric experiment, energy of photon is 5eV incident on a metal surface. They liberate electrons which are just stopped by an electrode at a potential of -3.5V w.r.t the metal. The work ...
0
votes
2answers
70 views

Electrical force between two objects

I tried to solve the following problem: There are 2 objects . The object m1 with charge q and the object m2 with charge q.(same charge).The object m2 is connected with a rope to the ceiling. at the ...
3
votes
2answers
88 views

Difference between weight of water and pressure of water

(I didn't even have a basic formal education in physics. I'm learning through the internet out of my own interest, so if there are any silly mistakes, kindly bear with and guide me through.) ...
1
vote
1answer
32 views

Calculate electricity generation

A friend of mine stays near a water stream and wanted help to use its water. The height up to which the water could be raised using the water flow is 12ft. How much electricity can it produce? Is it ...
0
votes
0answers
18 views

Does a pivot exert an additional force, normal to the centripetal force, on an object rotating about it?

A thin plank of mass $M$ and length $l$ is pivoted at one end. The plank is released at 60$^{\circ}$ from the vertical. What is the magnitude and direction of the force on the pivot when the plank ...
0
votes
1answer
46 views

error propogation MCQ [closed]

This MCQ below recently appeared in an entrance exam. I'm not asking for the solution but just wanted to know if there is any flaw in the steps mentioned below and also if anyone is sure of the answer ...
-4
votes
0answers
26 views

Electric field-potential [closed]

help to solve this question. 1-magintude of image charge? 2-the distance to the image charge from the center o of the earted conducting shape along oA?
1
vote
1answer
42 views

How is capacitance of a cylindrical capacitor calculated?

I've just begun learning capacitance, and my lecture notes have a section on calculating capacitance for capacitors in vacuum of various shapes, e.g. two parallel plates and concentric spherical ...
-1
votes
0answers
24 views

Kinematics of point moving on a circle [closed]

Let $A$ be a fixed point on a circle. Let a point $P$ move along the circle in such a manner that the acceleration vector always points to $A$. Show that the acceleration is proportional to ...
0
votes
0answers
38 views

Atomic physics - electron density [closed]

Question: Which of the following has zero electron density in the xy plane? (A) $d_{z^2}$ (B) $d_{x^2 - y^2}$ (C) $p_{z}$ (D) $d_{xy}$ Can't the answer either (A) or (C)? The ...
-1
votes
0answers
10 views

Volume change of rod under loading given Poisson's ratio [closed]

I would usually use a combination of the following equations to solve these type of problems: But not sure how to calculate volume change as I am not given any lengths such as diamter, ...
1
vote
1answer
57 views

Operator Product Expansion in Massless 2D QED

In Peskin & Schroeder chapter 19 page 656, where the axial current anomaly of massless 2D QED is discussed, the authors go from: $$ ...
3
votes
1answer
58 views

$c^4$ in Einstein field equations

I have read many derivations of Einstein field equations (done one myself), but none of them explain why the constant term should have a $c^4$ in the denominator. the $8{\pi}G$ term can be obtained ...