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.

learn more… | top users | synonyms (1)

0
votes
0answers
11 views

Show: Lorentz-invariance of solution of Klein-Gordon equation [on hold]

Assume $\psi$ is a solution of the Klein-Gordon equation (KGE). Let $\Lambda$ be a Lorentz transformation. Show: $\phi = \psi(\Lambda^{-1} \cdot )$ is also a solution of the KGE. I try to ...
1
vote
2answers
12 views

Diagram of forces exerted on objects

Assuming that there is an interaction between 1 and 3 (they attract each other), what are the forces between 1 and 2? I know it is as if the force acts on a different body (1+2), but I want to know ...
-1
votes
0answers
15 views

How to calculate period compared to that of a hypothetical asteroid? [on hold]

A new comet is discovered orbiting the Sun. If it has a perihelion of 3 AU and an aphelion of 12 AU, what is (a) the orbital eccentricity and (b) the period of the orbit? (c) How does this ...
-1
votes
0answers
32 views

Definition of force in a scalar field theory

How do we define the force for a general scalar field theory? In particular what is the scalar force of the below equation of motion: where $\tilde{T}$ is the energy stress tensor of matter and $A$ ...
0
votes
2answers
27 views

How to Calculate Resistance? [on hold]

A Copper Cylinder is Reshaped, its length becomes Half the original Length How to calculate the new Resistance how can i follow the law of the resistance and i dont have the Area?
0
votes
0answers
13 views

Elementary thermodynamics problem? [on hold]

The maximum height reached by an aluminum rocket is $115\,\text{km}$. Upon reaching Earth, it is traveling with a speed of $1500\, \text{m}/\text{s}$. if 50% of the heat generated by friction with the ...
0
votes
0answers
18 views

Where is the maximum center of mass so it doesn't fall over? [on hold]

I have some sort of barrow with brakes, which its deceleration due to the braking never makes the crate slide down: it results in $\mathbf{a} \leq g(\mu\cos\theta - \sin\theta)$ (note that it can ...
1
vote
1answer
39 views

How to prove this equation?

$W_b = \int$Pdv $PV^{n}$=C $P=CV^{-n}$ W=$\int cv^{-n} $dv =c$\int_{v1}^{v2}V^{-n}$ dv =c[${v^{-n+1} \over 1-n}]_{v1}^{v2}$ How to get rid of the (-n+1)? to prove $W_b= {P_2V_2 - P_1V_1 \over ...
2
votes
1answer
81 views

Is there an alternative to dark matter?

I've been reading about dark matter. I understood that its existence is inferred by the discrepancy between the gravity exerted by a galaxy (total mass) and the movement of the stars that compose it. ...
0
votes
0answers
18 views

Buoyancy / Drag Problem

Buoyancy / Drag Problem Just a little bit of help would be nice. I have a spherical particle of radius $R$ and density $\rho$, surrounded in a fluid of density $\phi$ and viscosity $\eta$. I'm ...
0
votes
1answer
25 views

Getting maximum power/brightness from 5 lightbulbs with different resistances [on hold]

Hi I am trying to figure out the best way to arrange 5 lightbulbs to get all of them to have the maximum brightness with a 30V battery. The 5 five lightbulbs have different resistances: 3ohms, 6ohms, ...
1
vote
1answer
19 views

Calculate coefficient of friction from deceleration with and without conservation of energy

How can I solve the following problem? A block that moves at the speed of 120 cm / s keep going for 70 cm before stopping. What's the coefficient of friction between block and table calculated ...
0
votes
2answers
57 views

How to deal with a $0\Omega$ resistor connected in parallel? [on hold]

There is a side without resistance, what is the equivalent resistance? Can anyone help me solve this?
-1
votes
1answer
33 views

How fast must Nadia travel so that she is the same biological age as her twin upon returning to Earth? [on hold]

Two twins, Nadia and Aidan, decide to have an adventure when they turn 21. Aidan chooses to travel to a distant star 10 light years away at a speed of 0.8c. Nadia decides to travel to a closer ...
3
votes
2answers
67 views

Why are some forces not included in the sum of forces?

I was given the following notes in class: 4.32 A skier of mass $65.0\text{ kg}$ is pulled up a snow-covered slope at constant speed by a tow rope that is parallel to the ground. The ground ...
-2
votes
0answers
14 views

Calculate the maximum displacement of mass 'm' body, when the system is released from rest at euqilibrium? [on hold]

Image Describes about a system at equilibrium, which consists of springs, pulley, and a body of mass 'm'.
0
votes
1answer
18 views

Race Car Tire Pressure Change Due To Temperature Change [on hold]

I need a simple calculation for a race car tire pressure change. I have a 33" diameter x 16" width tire mounted on a 15" diameter x 16" width wheel. If I have 8 psi at 75° what psi will I have at ...
1
vote
1answer
40 views

Riemann Curvature Tensor Symmetries Proof

I am trying to expand $$\varepsilon^{{abcd}} R_{{abcd}}$$ by using four identities of the Riemann curvature tensor: Symmetry $$R_{{abcd}} = R_{{cdab}}$$ Antisymmetry first pair of indicies ...
0
votes
1answer
67 views

A question about finding average velocity and average acceleration

I have trouble understanding the following problem's answer: Which of the following relations are true for any arbitrary motion in space? (a) $v_{avg}=\frac{r(t_2)-r(t_1)}{t_2-t_1}$ ...
0
votes
1answer
18 views

Find distance with zero net electric force [on hold]

I am new to the site (and physics!), sorry if this is too elementary or something. I have this problem that I can not solve - perhaps you could help?! Thank you!
0
votes
0answers
39 views

Group velocity and phase velocity of a wave packet [on hold]

A wave packet in a dispersive medium is represented by the following equation, $$y(x, t) = \cos(1.0x - 5.0t)\cos(0.2x - 0.4t)\cos(0.1x - 0.2t).$$ Find the group velocity and the phase velocity ...
0
votes
0answers
14 views

I need help using Faraday's Electromotive Equation? [on hold]

I don't understand why the answer in my book is -17.93 mV for Practice Exercise 9.2. I tried solving it on my own but I'm certain that it should be 0 since the dot product is 0. Could someone show me ...
0
votes
0answers
12 views

Finding the contained charge in a box, given the $E$-field [on hold]

I'm not sure whether my approach is correct for this problem, it's been a while since I've touched E&M. I thought it best to check, as I'm not sure whether $0$ is the proper answer. I've got a ...
0
votes
0answers
18 views

How to compute Pressure, if acceleration is not given on the Force? [on hold]

I just need clarification on this if my theory is right. This to acquire the pressure applied on a foot of a person. Supposed that the measured mass or weight in the right foot is 50kgs when the ...
0
votes
0answers
41 views

Stern-Gerlach experiment theory question [on hold]

I have an assignment problem which I've been having some difficulty with. i know the answer but I want to know how to arrive at it correctly, and want to know where I've gone wrong. Determine the ...
0
votes
1answer
25 views

Horizontal distance covered by an object from an initial height to the ground

We are given: An angle $\theta$ with respect to the ground, its initial height $h_0$ and the final distance $d$. How can we find the initial velocity from this information? I tried a bunch of things, ...
-1
votes
0answers
25 views

addition of a component of a vector to the same vector [on hold]

I am having a problem in the last part of this question-(D).My book says that it is not meaningful. I think that supposing a vector for instance 2i^ has a component 2i^. so when they are added it is ...
5
votes
1answer
70 views

Stable muon density inside a white dwarf star?

It occurs to me (though I'm hardly the first) that the decay $$ \mu^- \to e^- + \bar \nu_e + \nu_\mu $$ should be forbidden in electron-degenerate matter, since there must be an empty state available ...
3
votes
4answers
798 views

Can we add any two vectors?

Can we add any two vectors? If not, why is that so? I think this is not true, but I am not sure. My book says it is true, but I guess it is a misprint. For example, adding acceleration to velocity.
1
vote
0answers
40 views

Bending of a pipe filled with sand

There is this industrial pipe made out of steel that is 1 meter in diameter and 6 meters long (and volume 4,71 cubic meters). The thickness of the pipe "wall" is 4 centimeter. The pipe is filled ...
1
vote
1answer
36 views

Eigenfunctions for $1s$ hydrogen Schrodinger equation

I am a computer scientist and started my Phd in material science. The second course os my Phd is material simulation by computer. One the task is show the verification of the eigenfunction $1s$ from ...
0
votes
1answer
40 views

The meaning of: “a vector quantity has magnitude and direction” [on hold]

At a tutorial the teacherasked a student if he could explain vector and scalar quantity in concepts of everyday life, not by using any formula. they answered ''it is a vector because it has magnitude ...
0
votes
3answers
68 views

I wonder why I cannot charge a capacitor with alternating current?

Why can't I charge the capacitor with AC? How do the plates block the flow of electrons with DC but not with AC. Somebody told me that the DC is blocked by the capacitor, so the capacitor gets ...
3
votes
1answer
85 views

Could $\int (\ddot\phi + \mu\dot\phi^2)d\phi$ ever be negative? How to show it?

I am working on the following problem: Given a mass $m$ at rest at the base of an incline surface shaped like 1/12th of a circle with radius $R$. The mass is pulled up the incline by a rope that ...
0
votes
1answer
24 views

Estimating the rotational speed of DC motor

What is the best strategy to roughly estimate the RPM of a simple DC motor as seen in this video?. I want to estimate the max possible rotation of the coil that has $n$ number of turns and has moment ...
0
votes
0answers
38 views

Projectile under quadratic air resistance force

So here's the problem background: A projectile of mass m is shot straight up with initial velocity $v_0$ and feels a quadratic air resistance force of the form $F = −kv^2$, where $k ≡ mg/u^2$. ($k$ ...
3
votes
1answer
131 views

Directional derivatives in the multivariable Taylor expansion of the translation operator

Let $T_\epsilon=e^{i \mathbf{\epsilon} P/ \hbar}$ an operator. Show that $T_\epsilon\Psi(\mathbf r)=\Psi(\mathbf r + \mathbf \epsilon)$. Where $P=-i\hbar \nabla$. Here's what I've gotten: ...
-1
votes
2answers
62 views

Is $V= V_1 + V_2$? horrible question but im confused [duplicate]

So yeah , the answer seems true but i'm very skeptical about it since D3 is in the middle
-1
votes
1answer
40 views

Direction of radial acceleration same in circular motion?

I have a big confusion. There is a question in my book which basically says that a ball is tied to a string and rotated. and it asks me to tell whether the following statement is true of false. ...
0
votes
2answers
97 views

$V = V_1 + V_2$ confusion? Why is my “proof ”incorrect? [on hold]

Why isn't $V= V_1 + V_2$? $V=V_a - V_c = V_a - V_b + V_b - V_c$, $V_a - V_b= V_1$ and $V_b - V_c = V_2$ Doesn't that prove that $V = V_1 + V_2$? Regardless of $V_3$, If i'm wrong , is there a way ...
2
votes
2answers
62 views

Charge outside Gaussian Surface doesn't contribute to Flux?

I roughly understand the explanation for this: any electric field line that enters the surface, must leave it, since field lines can't terminate abruptly in space. My question is, what if you have a ...
0
votes
3answers
35 views

Net Electrostatic Field inside a Conductor is $\vec{0}$

I've heard an explanation for this, and my professor wasn't really able to clarify my questions, so I was hoping someone could help: Suppose there is an electrostatic field $\vec{E_{net}}\ne\vec{0}$, ...
-3
votes
0answers
66 views

How many seconds should you accelerate to complete the run on time? [on hold]

Your goal is to complete your 10,000-m run in less than 30.0 min. Apparently you've started too slowly because at 27.0 min, you still have 1100 m to go! Part A You can accelerate at 0.20 m/s2. For ...
1
vote
1answer
54 views

First order dependency

In a question I am doing it says: Show explicitly that the function $$y(t)=\frac{-gt^2}{2}+\epsilon t(t-1)$$ yields an action that has no first order dependency on $\epsilon$. Also my textbook ...
0
votes
2answers
35 views

Parallel and series circuit problem

When two resistors, each of resistance 4.0 ohms, are connected in parallel with a battery, the current leaving the battery is 3.0 A. When the same two resistors are connected in series with the ...
1
vote
0answers
35 views

How to derive the scale factor for special conformal transformation? [on hold]

By definition a conformal transformation of the coordinates is an invertible mapping $x\rightarrow x'$ which leaves the metric invariant upto a scale factor: \begin{equation} g_{\mu\nu}'(x') = ...
0
votes
1answer
39 views

2 simple Generators = Im lost (intensity)

What happens? we have 2 Generators and 2 intensities, when I2 and I3 come together (they have opposite sides ) what happens? I tried writing down the voltage relations but I'm stuck because I don't ...
2
votes
2answers
93 views

The variation of the Lagrangian density under an infinitesimal Lorentz transformation

I'm trying to introduce myself to QFT following these lectures by David Tong. I've started with lecture 1 (Classical Field Theory) and I'm trying to prove that under an infinitesimal Lorentz ...
0
votes
1answer
82 views

What's the relation between output voltage and time to boil water given the same kettle?

An electric kettle rated 220V, 2000W needed 10 minutes to boil water when it is half filled with water in Singapore where the output voltage is 220V. Estimate the amount of time needed to do the ...
-4
votes
0answers
33 views

Position Vector confusion [on hold]

I am having a little trouble understanding why the position vector r in calculating the $v_a$ is given by $225/\sqrt2$ (-i -j) If it is the position of C with respect to A shouldn't it just be $225$ ...