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. Please READ THE GUIDANCE IN META before asking homework-like questions.

learn more… | top users | synonyms (3)

-1
votes
1answer
11 views

Find the impulse momentum

A boy of weight 40 kg stands on a board of weight 60 kg. The board is on ice (friction force =0): Find the velocity of the movement of the boy and the board when he moves from one side to another.
0
votes
0answers
9 views

Convert this to a Hamiltonian system and find Hamiltonian [on hold]

I tried solving this by using the conjugate momentum and position formulae: but could not get the answer. EDIT: I have used the same procedure as mentioned in this Phys.SE answer but the ...
0
votes
0answers
28 views

Help with my schoolwork

I have to do project about Newton's Movement Laws, and I decided to write about the situations, where do NML not work. For example, why does photon have force and momentum, when it has 0 mass. It's ...
1
vote
1answer
42 views

By what force does a man attract Earth? [on hold]

By what force does a man attract Earth? a) 800N b) 80N c) he doesn't attract it By putting some values into the formula for gravitation force I get about 785N. Is it (a) then?
-2
votes
0answers
8 views

Solving for initial velocity required to launch a projectile to a given destination at a same height formula [on hold]

with the premise that gravity (gravity is 10m/s) is the only force acting on it after initial launch (no air friction) the angle is 30 deg and the destination is 40 meters
0
votes
0answers
20 views

Ames Room, how to construct properly?

I have been assigned to create an Ame's Room for a physics project. However, the teacher told us very little about it, and wanted us to figure it out through trial and error. Today I spend 6 ...
0
votes
0answers
9 views

Transistor Series Regulator [migrated]

I am in the 6th week in Navy school and we are on voltage regulators or circuits that have components that maintain a constant voltage output. I need an explanation on how to work out this problem ...
1
vote
2answers
111 views

Triple integral $\iiint_{\mathbb{R}^3} d^{3}q ~\delta^{3}(\vec{q})\frac{(\vec{p}\cdot\vec{q})^2}{q^{2}} $ involving Dirac Delta function [on hold]

I am trying find $$\iiint_{\mathbb{R}^3} d^{3}q ~\delta^{3}(\vec{q})\frac{(\vec{p}\cdot\vec{q})^2}{q^{2}},$$ where $\vec{p}$ is some fixed vector. The answer should be $\frac{p^2}{3}$. Below is ...
0
votes
0answers
35 views

Allowed Values of Angular Momentum for a Rotating Mass

I am attempting to calculate all possible values of angular momentum, $L_z$, which can be found by making a measurement on the following system: A small mass, $M$, is attached to the end of a rigid, ...
1
vote
1answer
18 views

Calculating energy density in the early Universe

I've been told to calculate the energy density in the early Universe. It states that it is completely dominated by neutrinos (3 species), photons, electrons, and positrons. Now, I've found an ...
0
votes
1answer
25 views

Finding acceleration only with known acceleration distance, final velocity, and mass for Newton's Third Law [on hold]

So I have this problem that I am trying to figure out. I need to find the acceleration of a 4 kg object, that is accelerated through a distance of 1 m to the left to a final velocity of 6 m/s. This ...
0
votes
2answers
19 views

How to determine the difference between object distance and image distance? [on hold]

How do we derrive the last equation? Whatever I tried it didn't work... Oh man I just saw, that all the did is write $b=3m-g$, how was is it possible for me to miss it...
-1
votes
0answers
41 views

how to prove the following equations? [on hold]

Equations in this image include some confusing steps for me, I tried but no results came out. please if some one can solve it I'll be very thankful.
0
votes
0answers
46 views

Writing this in a neater more compact way [on hold]

If one has $$ (\frac{d\omega^2}{dx^1} - \frac{d\omega^1}{dx^2}) (\theta^1 \wedge \theta ^2) +(\frac{d\omega^3}{dx^2} - \frac{d\omega^2}{dx^3}) (\theta^2 \wedge \theta ^3) +(\frac{d\omega^1}{dx^3} - ...
1
vote
2answers
122 views

Why is $\nabla\cdot(\hat{\bf r}/r^2)$ giving 0 as answer? [on hold]

While I was reading I encountered the statement $\nabla\cdot(\hat{\bf r}/r^2)$ (r cap divided by $r$ square) is 0. Can anyone explain proof of the statement why is it giving 0?
-1
votes
0answers
21 views

Find angular speed? [on hold]

The propeller blades of an airplane are 4.0 m long. The plane is getting ready for takeoff, and the propeller starts turning from rest at a constant angular acceleration. The propeller blades go ...
0
votes
0answers
37 views

Geometric optics question

This question appeared on this site Q17: here A concave mirror is broken into two parts and these parts are separated by a distance if 1 cm. The focal length of the mirror is 10 cm. Find the ...
0
votes
2answers
12 views

What happens to the electric charge of a rod and a cloth if the rod is rubbed with a cloth? [on hold]

We have a rod and we rub it with a cloth. Will the rod have a positive charge and the cloth have a negative charge? Will both the rod & cloth have a positive charge? Both rod & cloth have a ...
0
votes
0answers
15 views

Antiunitary operator, momentum operator [on hold]

Assuming the time-reversal operator $T$ $T|x>=|x>$ Now I want to calculate $TpT^{-1}$ So, $TpT{-1}|x>=Tp|x>=\int\int T|x'><x'|p|p'><p'|x>=\int\int ...
0
votes
0answers
21 views

Metallic coin over wooden block in glass of water

So there is some water inside a container; the height of water inside the container is $l$. I placed a wooden block on the water and it's floating to some height $x$, on top of the block is a metallic ...
1
vote
1answer
47 views

Eigenvalues of hamiltonian [on hold]

Q: THe hamiltonian which describes the motion of a particle in an one dimensional potential V(x) is $H_0=\frac{p^2}{2m}+V(x)$ , where $p=-i\hbar \frac{d}{dx}$ is the momentum operator. $E_n^0$ , ...
0
votes
0answers
27 views

Uniqueness of the Einstein tensor

This is related with an exercise 17.4-a in MTW Here what i want to show is the Einstein tensor $G_{\alpha\beta} = R_{\alpha\beta} - \frac{1}{2} R g_{\alpha \beta}$ is the only second-rank, symmetric ...
0
votes
0answers
22 views

Constructable components on metric and its derivatives [on hold]

This is exercise 17.2 in MTW Show that there exists no tensor with components constructable from the ten metric coefficient $g_{\alpha\beta}$ and their 40 first derivatives, ...
0
votes
1answer
26 views

Polarisation vectors in the quantization of electromagnetic field [on hold]

How can we prove $ \sum^{3}_{\lambda =0} \sum^{3}_{\lambda^{'}=0} \epsilon^{\lambda}_{\mu}(k)\epsilon^{*\lambda^{'}}_{\nu}(k) g^{\lambda \lambda^{'}} = g_{\mu\nu}$ where the $\epsilon$ are the ...
0
votes
0answers
9 views

Introducing a conductor in a capacitor [on hold]

1) A capacitor is taken and a Conductor (Not a dielectric) is inserted in between the plates (made of a different material other than the conductor) like: What will be the new capacitance, How much ...
1
vote
1answer
15 views

Identification of massless, chargeless $x$ in a nuclear reaction

On Friday, we had our Physics test. We (the tenth grade students) have the basic introduction to Radioactivity and a few nuclear reactions in our syllabus. In the test, the following question was ...
-2
votes
0answers
12 views

Diffusion Equation [on hold]

Please give me some guidance on my following assignment of Simulations in fluid flow: Thank you. Solve the following differential equation for transport of f(x,y,z,t) by MS Excel ∂f/∂t+Ux ∂f/∂x+Uy ...
0
votes
0answers
15 views

How to simplify logic circuit? [on hold]

Only the AND, OR, NOT gates are there in my course and I know only how to write results including only a single of these, but my course has no mention of solving these: I just wrote ...
1
vote
0answers
105 views

Horrifying electron gas model

I am given the Hamiltonian, in an exercise called plasmons, and where $\langle, \rangle $ denotes the expectation value. $$ H = \sum_{k} \varepsilon_k a_k^{\dagger} a_k + \sum_{k_1,k_2,q} V_q ...
1
vote
1answer
30 views

How to determine velocity of two objects after they collide? [on hold]

Let's say an object of mass 1 kg is moving 10 m/s. A second object of mass $m_2$ is at rest. When the two objects collide, they stick together and move 8 m/s. what is $m_2$? I understand how to solve ...
-1
votes
0answers
14 views

How to relate the height from which a object can fall with the young modulus of that object? [on hold]

How to relate the height from which a object can fall without breaking with the Young modulus of that object?
-3
votes
0answers
21 views
1
vote
0answers
16 views

Suspended mass in motion free-body diagram [on hold]

A 20kg mass is held by two ropes which span 90 from each other. The mass is being accelerated at 1m/s^2 to the right. Draw a free body diagram for the mass and find the numerical values for each ...
0
votes
0answers
19 views

Determine coefficients so that $f(x,y)$ is a conserved quantity [migrated]

Given the function $f(x,y) = ax^2y + by + ct^2 $ with $x(t)=2t$ and $y(t)=3t^2$ I shall determine the coefficients a,b,c so that $f(x,y)$ is a conserved quantity. My approach is $$\frac{df}{dt} = ...
0
votes
0answers
57 views

Computation of spin connection 1-form [on hold]

If stationary metric is given by $$ds^2 = e^{2U}(dt+\omega_idx^i)^2 - e^{-2U}dx^2$$ and I want to try to solve for this. After doing the effort to find the metric and its inverse, I used the tetrad ...
0
votes
1answer
33 views

Deduce the 3 dimensional Anti de Sitter space from the 4 dimensional case [on hold]

From $$AdS^4=\big\{(u,w,x,y,z) \in \mathbb{R} | -u²-w² +x² + y² + z²=-1 \big\}$$ parametrized by $$f(t,\rho, \theta, \phi) = (\sin t \cosh \rho, \cos t \cosh \rho, \sinh \rho \cos \theta, \sinh \rho ...
0
votes
1answer
48 views

Uncertainty Principle and single-slit diffraction [on hold]

A beam of 50eV electrons travel in the x direction towards a slit of width 6 micro metres which is parallel to the y direction. The diffraction pattern is observed on a screen 2 metres away. Use ...
0
votes
1answer
67 views

Does varying centripetal force produce circular motion?

Centripetal force $\vec{F}$ is responsible for circular motion and is given by $$ \left|\vec{F}\right| = \dfrac{m\left|\vec{v}\right|^2}{r}$$ If the linear velocity $\vec{v}$ varies, the centripetal ...
0
votes
0answers
16 views

capacitors - Square circuit of two capacitors charging in a changing magnetic field

I've been studying capacitors and DC circuits, and I received this question as part of my homework: The above circuit is a square circuit with side a. E and F are the midpoints of AB and CD ...
0
votes
0answers
44 views

Please help with quantum mechanics [on hold]

Let the Hamiltonian of two nonidentical spin 1/2 particles be where and are constants having the dimensions of energy. Find the energy levels and their degeneracies.
0
votes
0answers
22 views

Translational acceleration of cylinders [on hold]

Two cylinders with total mass $M$ and radius $R$ are connected by a massless rod along their axis of rotation and rest on a horizontal surface. A frictionsless ring at the center of the rod is ...
1
vote
1answer
45 views

Calculate the mass of a Schwarzchild black hole with Komar integral

In Wald's GR, Komar integral is Eq. (11.2.9): $$M=-\frac{1}{8\pi}\int_S\epsilon_{abcd}\nabla^c\xi^d$$ $S$ can be chosen as a 2-sphere, the boundary of a spacelike hypersurface $\Sigma$ such that the ...
0
votes
1answer
62 views

Work-Energy theorem vs conservation of mechanical energy? [on hold]

Bodies A and B are moving in the same direction in a straight line with constant velocities on a frictionless surface. the mass and the velocity of A are: 2kg and 10m/s. The mass and the velocity ...
0
votes
1answer
50 views

Spring on a cart problem [on hold]

The problem is two dimensional. At t = 0, a stationary cart at initial position (0,0) starts to accelerate with acceleration a in the negative "x-axis" direction. On the cart, there is an isosceles ...
-2
votes
0answers
84 views

Why the sum of energy don't increase in this device? [on hold]

The black arm is turning clockwise at $w1$. The blue axis is fixed to the ground. The grey cruz (grey arms) is turning at $w2$, with $w2 < w1$, so the grey cruz is turning around itself ...
30
votes
7answers
4k views

“Falling upward” - how far you have to be from Earth to start falling to the Moon?

Talking about gravity with my 9 y/o she asked when do we start "falling upward" to the Moon. What is the distance at which the Moon's gravitational attraction is higher than that of the Earth and thus ...
2
votes
1answer
45 views

Integration of $e^{-it\sqrt{\mathbf{p}^2 + m^2}}$ for QM amplitude

My question might be more about maths than physics, but it originated in a Physics context. Take $\hbar$ = $c$ = 1. I was looking at the amplitude for a free particle to propagate from an initial ...
1
vote
1answer
31 views

Calculating speed of spheres in an asymmetric Newton's cradle?

If we assume there is an asymmetric Newton's cradle with 3 spheres of different masses and the first one is initialized with some certain speed can we calculate the outcome speed of the all the three ...
0
votes
0answers
23 views

How are Newton's Laws of motion related to the amount of work exhibited by the balloon vehicle? [on hold]

A long string is running through a straw with a balloon taped onto the straw, and a penny taped onto the balloon.
1
vote
1answer
56 views

What is the point of this type of graph? (Projectile) [on hold]

What is the point of graphing vs range, during kinematic motion?