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Resultant vector problem

$\newcommand{\v}[1]{\vec #1}\newcommand{\i}{\hat i}\newcommand{\j}{\hat j}$ Problem statement (1,2) A shopper at the supermarket follows the path indicated by vectors $\v A, \v B, \v C, \v D$ in ...
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2answers
103 views

Quantum yield and spontaneous decay

I'm trying to figure out how many atoms are decaying spontaneously in a span of 2 seconds. Let's say that the quantum yield is 0.45, and that the lifetime "τ" (tau) is 10 microseconds. Then I found ...
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1answer
81 views

Help for density of charges in electric field [closed]

This is for a homework assignment so it would be great to have a hint or an explanation of how to do it. In a certain region in space, the electric field is given by $\vec{E}(r) = \dfrac{A}{r} ...
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2answers
193 views

Electron in a uniform magnetic field and center of the circumference

I have an electron in a 3D space and there is a uniform constant magnetic field $B$, oriented according to z-axes. At $t=0$, we have $z=0$ and $\dot z =0$. I have proved that the trajectory is a ...
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1answer
504 views

Minimal Kinetic energy for particle in a box

This is driving me crazy! The question goes as follows: A proton is enclosed in a zone of length 2pm along the x-axis. The minimal kinetic energy of the proton lies closest to: 5000eV 0.5eV 50eV ...
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2answers
459 views

Convert state Vectors to Bloch Sphere angles

I think this question is a bit low brow for the forum. I want to take a state vector $ \alpha |0\rangle + \beta |1\rangle $ to the two bloch angles. What's the best way? I tried to just factor out ...
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1answer
301 views

Projectile motion problem with upward acceleration and horizontal velocity [closed]

An electron in a cathode-ray tube is traveling horizontally at 2.10×10^9 cm/s when deflection plates give it an upward acceleration of 5.30×10^17 cm/s^2 . B.) What is its vertical displacement during ...
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2answers
251 views

A simple logical problem regarding pressure and force

Suppose a 2 meter long pipe, with area of cross section is $1 m^2$. The pipe is vertically placed, with the bottom end closed. Inside the pipe, at the closed bottom end, there is $1 m^3$ of air, ...
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0answers
278 views

How to solve fixed-fixed beam with finite difference method?

What equations to use on this system to form a matrix $A$ with dimensions $[n,n]$ and load vector $q$ with dimension $[n]$ ? I am trying to get vertical displacement $w$. $$w = A^{-1}\times q$$ ...
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3answers
128 views

How do I determine where I can no longer hear sound?

A teacher stands well back from an outside doorway that is 95 cm wide and blows a frequency of 600 Hz. The door faces north. Assume the speed of sound is 340 m/s and that there are no reflections. At ...
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0answers
109 views

A question on charge renormalization in QED

Let us work with charge renormalization in QED. Consider 2-point photon correlation function $\Pi_2(q^2)$ at one loop level. We normalize the coupling constant at $q^2=0$ (point of normalization). ...
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0answers
138 views

How can I calculate all of the tensions in the cords? [closed]

I have been stuck on this problem for the last half, and to be honest I don't even no where to begin. I'm being asked to calculate all of tensions for the cords. I know I'm supposed to use $\cos$ to ...
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3answers
736 views

where did I lose the negative sign when spring is in compression in a mass-spring problem?

Most books just tells you that spring force is $-kx$ since it opposes motion and to just write $mx''=-kx$ regardless if the spring is in tension of compression. But when I try to derive this using ...
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1answer
481 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 ...
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1answer
955 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 ...
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2answers
450 views

Calculate force of electric charges “suspended” by strings [closed]

In a question: Two small plastic balls hang from threads of negligible mass. Each ball has a mass of 0.110g and a charge of magnitude q. The balls are attracted to each other, and the ...
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1answer
135 views

Vertical velocity to reach a certain point given a flat horizontal velocity? [closed]

Suppose I have two points such as $a(4,6,9)$ and $b(32,5,12)$. If I have a flat velocity pointing $a$ to $b (b - a)$ which has an arbitrarily defined magnitude. Given gravity $g$,how can I calculate ...
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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 ...
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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 ...
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1answer
795 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
631 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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4answers
996 views

Spring-mass system with two springs and three masses

I'm trying to solve a system of springs and masses that is confusing me. First, the balls are all lined up linearly. Secondly, the ball in the middle has a smaller mass $m$ while the first and last ...
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3answers
730 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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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 ...
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3answers
126 views

How can I modify the bullet trajectory based on the ballistic coefficient? [duplicate]

Possible Duplicate: How to modify the bullet trajectory based on the ballistic coefficient? I am new to the physics surrounding bullet trajectory and how it is calculated. I am a software ...
3
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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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1answer
508 views

Diffraction through a single slit?

I'm having some trouble finding the answer to this homework question: A wide beam of light from a laser with a wavelength of 450.0 nm is incident on a long narrow opening that is 0.300 mm wide. ...
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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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0answers
44 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 ...
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3answers
2k views

Charge density in concentric spheres

Question: If there are two conducting spherical shells and the inner shell is grounded, what will be >the charge density in the inner shell if there is a charge Q placed on the outer shell? ...
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0answers
598 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 ...
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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 ...
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 ...
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2answers
444 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 ...
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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
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 ...
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2answers
751 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
2k views

Force on Earth due to Sun's radiation pressure

I have been asked by my Classical Electrodynamics professor to calculate the force that the Sun exerts in the Earth's surface due to its radiation pressure supposing that all radiation is absorbed and ...
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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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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 ...
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1answer
783 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
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0answers
304 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 ...
3
votes
3answers
438 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 ...
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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 ...
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3answers
357 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 ...
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2answers
3k views

Preventing a block from sliding on a frictionless inclined plane

I want to demonstrate what force $F$ you would have to exert on an inclined plane of angle $t$, mass $M$ to prevent a block on top of it with mass $m$ from sliding up or down the ramp. I worked out ...
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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 $F$ ...
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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 ...