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1answer
960 views

Calculating (Glass) thermal conductivity

I read that glass has a thermal coefficient of 0.8-1 W/mK. Given that my window window thickness is around $5\ mm$, then I would calculate my heat loss per area being $160-200\ Wm^{-2}K^{-1}$. Yet ...
0
votes
1answer
327 views

Electric Fields

The problem I am working on is, "In the figure below, determine the point (other than infinity) at which the electric field is zero. (Let $q_1 = -2.45\ \mu C$ and $q_2 = 6.5\ \mu C$) Here is a little ...
3
votes
1answer
483 views

Electric Field due to a charged sphere

Suppose we have a spherical surface with a surface charge density varying as $cos(\theta)$. Apparently one can find the electric field both outside and inside such a spherical surface by superposing ...
1
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1answer
98 views

Thermodynamics of a bar, specific heat

I have a problem and I don't understand how the book solves it. It says I have a bar such that $$F=aT^2(L-L_0)$$ where $F$ is the force or tension of the bar, $T$ is the temperature, and $L$ the ...
1
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1answer
399 views

Monkey and tree - projectile motion

The famous scenario: A hunter is trying to shoot a Monkey hanging from a tree. However, this question doesn't mention the monkey jumping down from the tree or trying to escape. (The hunter uses a ...
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1answer
798 views

How do I find the magnitude and angle of the velocity?

I'm confused about the following problem. A ball is shot from the ground into the air. At a height of $9.1\text{ m}$, its velocity is $v = (7.6\hat{\imath}+ 6.1\hat{\jmath})\text{m/s}$, with ...
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1answer
636 views

Infinite square well in momentum space

As we know the eigenfunctions for a particle of mass m in an infinite square well defined by $V(x) = 0$ if $0 \leq x \leq a$ and $V(x) = \infty$ otherwise are: $\psi_n (x) = \sqrt{(2/a)} sin(n \pi ...
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2answers
2k views

Particle in infinite potential well which is doubled in size at $t_0$

I am currently studying for an exam in Quantum Mechanics and came across a solution to a problem I have trouble with understanding. The Problem: A Particle sits in an infinite potential well ...
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1answer
350 views

Electric field of a flat metal plate and a point particle

I'm currently studying electric potential, and I'm having trouble with one of the problems on my homework: A) A point particle with charge $+q$ is on the x-axis at a distance $d$ from the origin, ...
3
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3answers
741 views

Irreversible process

I have this problem. I have an ideal gas that goes through an irreversible adiabatic decompression. I have the initial state (P,T,V), and the final pressure, and I have to calculate the entropy ...
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2answers
1k views

Constant pressure and temperature mixing of 2 different ideal gases - possible work and heat?

A simple question I hope... Initially, have two separate containers of 2 different ideal gases, 1.) N1, P, T, V1 and 2.) N2, P, T, V2. After mixing, the pressure and temperature are still P and T, ...
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1answer
88 views

Relationship between angular momentum of Earth and recession rate of the Moon

So the problem goes like this: Two masses $m_1$ and $m_2$ orbit each other with semimajor axis $a$. The orbit is circular, and $m_1 \gg m_2$. The body $m_1$ has a rotational moment of intertia $I_1$ ...
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0answers
25 views

Movement of a ball on a curved ramp [duplicate]

Possible Duplicate: What is a natural movement of a ball on a upward curve (the two arrow lines pointing upward) given no external force? http://i47.tinypic.com/mt55s7.jpg -See for image ...
3
votes
1answer
405 views

Expectation values-Wavefunction

I'm a bit puzzled about an excercise in which I have to find the expectation values for position and momentum. Normally this should be pretty easy but in this case I just don't get the point. ...
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3answers
1k views

Partition function for quantum harmonic oscillator

Hi guys I'm currently trying to solve a mock exam for an exam in a few days and am a bit confused by the solutions they gave us for this exercise: Exercise: A solid is composed of N atoms which ...
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2answers
712 views

Work done in projectile motion

A projectile is shot at some inclination to the ground. It falls at another point having R distance from the initial point on the ground. Is there any work done? If initial velocity vector is $(u\cos ...
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0answers
45 views

Heat contained in compressed gas [closed]

I am trying to compute the amount of energy released in the form of heat if I compress 1 cubic meter of Hydrogen to 0.1 cubic meter while submerged in a large body of water of a given temperature - ...
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0answers
40 views

stopping distance [closed]

if it takes a 4 door sedan x amount of feet to go from 65 mph to 45 mph (and i don't know how many feet that is) on a dry straight road, how many feet would it take for a large Mack truck ...
3
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4answers
922 views

Paradox- Object thrown parallel to the ground will never fall down

Suppose an object is thrown parallel to the ground. The gravity acts downward (ie. perpendicular to the direction of motion of the object). The work done by gravity on that object will be given by : ...
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3answers
1k views

Preventing a block from sliding on a plane (with friction)

Assume a small square block $m$ is sitting on a larger wedge-shaped block of mass $M$ at an upper angle $\theta$ such that the little block will slide on the big block if both are started from rest ...
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0answers
602 views

Neglecting friction on a pulley?

So, this is how the problem looks: http://www.aplusphysics.com/courses/honors/dynamics/images/Atwood%20Problem.png Plus, the pulley is suspended on a cord at its center and hanging from the ceiling. ...
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1answer
251 views

Conservation of Energy as Applied to Point Charges

The following question appears to be extremely straight forward, but I can't seem to be able to obtain the correct answer. The question is: A charge Q1 = 1.3uC is at rest and is located 2.3 cm ...
3
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1answer
4k views

What does it mean to find acceleration in terms of g?

I'm having trouble understanding what a problem I have is seeking. To simplify the problem: A particle reaches a speed of 1.6 m/s in a 5.0 micrometer launch. The speed is reduced to zero in 1.0 ...
2
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1answer
377 views

Finding orbital eccentricity

I have this problem: They give me, from a satellite that is in orbit in earth, a value for the period, and the closest height to earth surface, the ask me what the eccentricty of the orbit is. I have ...
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1answer
2k views

Electric force in DNA molecule

Given that distance between O-H and N-H bonds are 0.11 nm and How do I compute the net force exerted from Thymine and Adenine? A hint given is: Hint: To keep calculations fairly simple, yet ...
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2answers
454 views

Kinetic energy of a cylinder

It is a long cylinder (you can approx $R=0$), and it has a fixed point in one os its ending points, it's rotating on a plane and I have to calculate the kinetic energy from reference systems situated ...
0
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1answer
114 views

Spring extensions

A certain spring has attached to it a mass of 25 units: on increasing the load by 6 units it extends 2.5 cm. a) What is the time of oscillation under the original load? b) What will be ...
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2answers
758 views

Problem of Landau, Lifshitz - Mechanics - Integration of the Equations of motion [closed]

I'm studying Landau, Lifshitz - Mechanics. Could someone help me with this problem ? =) Problem 2 (Page 27 3rd Edition) Determine the period of oscillation, as a function of the energy, when a ...
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1answer
3k views

What does y with a line over it represent?

I've been asked to complete this chart and have never come across this symbol before, nor can I find anything about it on google: http://postimage.org/image/oe7hb9cy3/ What does the y with the line ...
2
votes
1answer
788 views

Why can we say there is no tangential acceleration in this specific problem?

I'm looking at a problem involving a simplified rollercoaster with only 3 cars (connected with weightless rods) for which the friction and distance from the carts to the rails can be ignored. The part ...
2
votes
0answers
306 views

Prove that the first order perturbation theory overestimates fundamental state [closed]

This was a question on my exam and I don't know how to solve it. Use the variational principle to prove that the first order perturbation theory always overestimates the energy of the fundamental ...
2
votes
1answer
201 views

Transformators (primary and secondary windings)

Consider the following problem: Robby wants to put a light in the shed so he puts a cable between his house and the shed. In the cable there are 2 wires with a combined resistance of $0,92 \space ...
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2answers
193 views

Rotating a reference system on a vector

Assume you have a vector $\vec{x}=(\sin(\vartheta)\cos(\varphi),\sin(\vartheta)\sin(\varphi),\cos(\vartheta))$ given in spherical coordinates in a reference System "R". I want to rotate the reference ...
3
votes
3answers
358 views

Is this interpretation of $\psi=\frac{1}{\sqrt{\pi a^{3}}}e^{-r/a}$ correct?

Apologies if this is stating the obvious, but I'm a non-physicist trying to understand Griffiths' discussion of the hydrogen atom in chapter 4 of Introduction to Quantum Mechanics. The wave equation ...
1
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1answer
188 views

Show that $\partial_\nu T^{\mu\nu} = - j_\nu F^{\mu\nu}$

In a theoretical physics homework problem, I have to show the following: $$\partial_\nu T^{\mu\nu} = - j_\nu F^{\mu\nu}$$ Where $T$ is the Energie-Momentum-Tensor, $j$ the generalized current and $F$ ...
0
votes
3answers
5k views

How to derive the period of spring pendulum?

So I wanted to find out how to (simply, if that's possible) derive the formula for a period of spring pendulum: $T=2\pi \sqrt{\frac{m}{k}}$. However, Google doesn't help me here as all I see is the ...
1
vote
1answer
96 views

Magnetic force on capacitor

I have this problem: I have a capacitor formed by two parallel square-shaped plates, of side $a$, and with distance $d$ between them. They're asking me out of other things, the magnetic force as a ...
2
votes
1answer
154 views

Write $\epsilon_{\mu\nu\alpha\beta} F^{\mu\nu} F^{\alpha\beta}$ as a total divergence $\partial_\mu G^\mu$

I have the following homework problem in theoretical electrodynamics: Show that the gauge invariant Lagrange density $\epsilon_{\mu\nu\alpha\beta} F^{\mu\nu} F^{\alpha\beta}$ can be written as a ...
1
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0answers
652 views

Lagrangian of pendulum with pivot point rotating about a circle [closed]

I just would like to know If I am on the right track with this stuff. I have scanned the work I have done, sorry if it is a little messy. Disregard the top most $\ddot x$ A disk of radius ...
4
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3answers
312 views

How can a vertical force cause motion at an angle?

I just started learning physics 3 days ago and am having trouble understanding what I am doing wrong. Can someone please explain my error(s)? Thanks! We have a 1kg object on a plane at a 30 degree ...
1
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0answers
106 views

Massless Dirac equation is Weyl covariant

Does somebody know how to show that the following equation is Weyl invariant? $$\gamma^ae_a^\mu D_\mu \Psi=0$$ where: $D_\mu \Psi=\partial_\mu\Psi+A_\mu^{ab}\Sigma_{ab}\Psi$ is the spin-covariant ...
3
votes
3answers
439 views

What is the current through the lamp?

We have the following circuit: A neon lamp and a inductor are connected in parallel to a battery of 1.5 $V$. The inductor has a 1000 loops, a length of $5.0 cm$, an area of $12cm^2$ and a ...
3
votes
2answers
338 views

The direction of the Lorentz Force

We have a square loop (I believe it's called) in a uniform magnetic field between the 2 poles of a permanent magnetic field (green is N, red is S). P is connected to the positive pole of a voltage ...
3
votes
2answers
2k views

block slides on smooth triangular wedge kept on smooth floor.Find velocity of wedge when block reaches bottom

Find the velocity of the triangular block when the small block reaches the bottom: Here is what I did: The final velocity(at the bottom)of the small block of mass m is $\sqrt{2gh}$ along the plane ...
2
votes
1answer
713 views

Calculating projectile range from known maximum height and time traveled

I've been stuck on this problem for many hours and I think I'm onto the right solution but I'm uncertain about my math. I've got a projectile that I know its maximum height and it's hang time and I ...
3
votes
2answers
461 views

Dynamics of a Vertical Mass-Spring Simple Harmonic Oscillator with Gravity

I am having some trouble obtaining the elastic potential energy and gravitational potential energy of a simple mass spring system. In this experiment, masses attached to a spring were dropped from a ...
2
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0answers
2k views

Rotate vector in spherical coordinates

I have two arbitrary vectors $\vec{x}$ and $\vec{x}'$ given in spherical coordinates $(|\vec{x}|=x,\theta,\phi)$ (as convention I take the "physics notation" given on Wikipedia ...
3
votes
0answers
358 views

Shape of a string/chain/cable/rope?

The height of a string in a gravitational field in 2-dimensions is bounded by $h(x_0)=h(x_l)=0$ (nails in the wall) and also $\int_0^l ds= l$. ($h(0)=h(l)=0$, if you take $h$ as a function of arc ...
3
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4answers
1k views

why does a larger thermal conductivity provide a smaller temperature gradient?

I was thinking about Fourier's Law in heat transfer today and for some reason I am just not understanding the relationships it gives us. Fourier's tells us that if the heat transfer rate is kept ...
5
votes
1answer
250 views

What is the change in flux through a loop that has been rotated?

We have a number of field lines perpendicular to one loop of wire with an area $A = 10\textrm{ cm}^2$. The magnetic field is$B= 7.2\times10^{-5}\textrm{ T}$. You turn the loop and the flux decreases ...