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.
-1
votes
1answer
46 views
Organic Proliferation In Terms of Speed [closed]
OK for 5 stars...Dealing with Einsteins theory of special relativity:
Here is the question:
As observed on earth, a certain type of bacterium is known to double in number every 24.0 hours. Two ...
1
vote
1answer
76 views
Given a potential energy function, find expression of the force of a particle?
This comes from an AP review packet. I'm given a potential energy functon, $$U(r)=br^{-3/2} + c,$$ where $b$ and $c$ are constants, and need to find the expression for the force on the particle.
...
1
vote
1answer
34 views
Sign issue on electrostatic potential energy
I have been working on a problem about finding the electrostatic potential energy stored on a capacitor of concentric spheres with inner radius $a$ and outer radius $b$ and with charge $Q$. I've got ...
3
votes
2answers
170 views
Firing machine question
Suppose we have a firing machine on a frictionless surface at point $x=0$. It fires a bullet of mass $m$ every $T$ seconds. Each bullet has the same constant velocity $v_0$. There's a body of mass ...
-2
votes
0answers
49 views
Rolling a perfect square [closed]
If a 20-sided fair die with sides distinctly numbered 1 through 20 is rolled, the probability that the answer is a perfect square can be expressed as a b where a and b are coprime positive ...
2
votes
0answers
34 views
Difficulty in obtaining the Lorentzian lineshape for natural broadening [migrated]
Not sure if this maybe belongs more in the maths section, but since it comes from a physics problem i'll post here.
when calculating the natural broadening lineshape for a laser we have to take the ...
1
vote
0answers
57 views
Double Slit Problem Involving Superposition of Wave Equation [closed]
Here's my question:
To be clear it's part (iv) that's unclear to me.
I can see that the important bit is that the exposure is over a LONG time. Hence, this must have some implication on the manner ...
0
votes
1answer
38 views
Period of oscilation [duplicate]
Two masses $m_1$ and $m_2$ are connected by a spring of spring constant $k$ and slide freely without friction along horizontal track. What is period of oscillation? No force influence.
0
votes
0answers
53 views
Equilibrium of a sphere in a water tank
A rigid sphere of radius $R_S$ made from a material with specific gravity $SG_s$ is completely
submerged in a tank of water with radius $R_t$ and initial depth $L$ as shown in the figure
The ...
0
votes
1answer
38 views
Rotational Kinetic Energy of a rigid root - Full workings Provided?
Part v is where I'm stuck.
Here is the question with my workings
If I can find Torque I can find L and using the equation $L=Iw$ I can find the w value and hence using $w=2pi*f$ I can prove it ...
0
votes
1answer
62 views
A sphere rolling down a rough wedge which lying on a smooth surface
A sphere of mass $m$ and radius $r$ rolls down from rest on an inclined (making an angle $\phi$ with the horizontal ) and rough surface of a wedge of mass $M$ which stays on a smooth horizontal floor. ...
0
votes
1answer
58 views
Problem about moments and equilibrium. [closed]
A plank is on top of 2 branches. It is in contact at 2 points, A and B. The plank is 90 cm long. A is on the left of B. A is 15 cm from the left end of the plank. B is 20 cm from the right end of the ...
1
vote
1answer
55 views
Rømer's determination of the speed of light
I am trying to understand Rømer's determination of the speed of light ($c$). The geometry of the situation is shown in the image below. The determination involves measuring apparent fluctuations in ...
0
votes
1answer
52 views
Neutron conversion to energy? [closed]
Numerical:
If a neutron be entirely converted into energy how much energy would be produced. ($M_n = 1.6743\times 10^{-27}$kg.)
Explain
1
vote
1answer
84 views
Finding the wavelength of an electron in its ground state?
To find the wavelength of an electron in its ground state in a hydrogen atom, would I or could I do the following?
Use the ground state energy (-13.6eV) in $E^2 = m^2c^4 + p^2c^2$
Solve for $p$
Use ...
0
votes
0answers
47 views
Finding the coefficients of a spinor
From the Schrödinger equation of a system I'm investigating, where the wave function is a 4-component spinor of coefficients $C_1, C_2, C_3, C_4$, I am able to obtain the expression
$\begin{pmatrix} ...
-1
votes
1answer
81 views
Differentiating a sinusoid [closed]
http://i.stack.imgur.com/ndLgm.png
I don't see the how the author derived that $\frac { dv }{ dt } \leftrightarrow\overrightarrow { V } j\omega$.
1
vote
1answer
64 views
Ratio of Size of Atom to Size of Nucleus
I have the following problem:
In nuclei, nucleons exists in nuclear energy levels and in atoms, electrons exist in atomic energy levels. The order of magnitude of nuclear energy is 1MeV whereas ...
0
votes
1answer
64 views
Question on hadamard gate and cnot gate circuit tables
I'm trying to solve this problem for homework:
Now show that if the CNOT gate is applied in the Hadamard basis - i.e. apply the Hadamard gate to
the inputs and outputs of the CNOT gate - then ...
2
votes
1answer
111 views
Wedge and Sphere problem
A sphere of radius $R$ is in contact with a wedge. The point of contact is $\frac{R}{5}$ from the ground as shown in figure. Wedge is moving with velocity $20~\text{m/s}$ towards left then the ...
0
votes
0answers
47 views
The electrical field of a conductive sphere containing a charge - gounded vs not grounded
Let's suppose we have a sphere but unlike theoretical ones it'll has have some thickness say $\Delta r$ and inner radius $R$. What I was wondering about is how will it behave if we place some charge ...
0
votes
1answer
92 views
How to design a AC/DC generator and motor? [closed]
Through electrodynamics mechanics learning, it is possible to design a simple and elegant design for the following four machines:
1. AC generator
2. AC motor
3. DC generator
4. DC motor
I have some ...
0
votes
1answer
19 views
Gas transformation for different molar number
When a gas at normal conditions (1atm, 273K, 22.4L) we say like the $$\frac{PV}{T} = 0.0820...$$
And by the know equation: $$PV = nRT$$
Where R is equals to $0.08205...$ and $n$ is the number of mols ...
-4
votes
1answer
86 views
How fast will I get if I burn a comet? [closed]
Assuming I have a spacecraft which is $30,000\,\mathrm{kg}$ (roughly the size of the Apollo spacecraft).
If I take a comet and (theoretically) electrolize it perfectly to hydrogen and oxygen. I know ...
0
votes
1answer
56 views
Confusing mirror problem
A piece of thin spherical shell that has a radius of curvature of 106 cm is silvered on both sides. The concave side of the piece forms a real image 79.5 cm from the mirror. The piece is then turned ...
1
vote
2answers
140 views
Relativistic Doppler effect derivation
This is about a step in a derivation of the expression for the relativistic Doppler effect.
Consider a source receding from an observer at a velocity $v$ along the line joining the two. Light is ...
3
votes
2answers
88 views
How can a circuit function with two negative battery terminals facing each other?
Here is a drawing of the circuit that is confusing me:
I don't quite understand how batteries work in this diagram. If a battery has a negative and positive terminal, there must be a barrier ...
0
votes
1answer
38 views
Finding the work required to move an object under pressure without knowing volume
"What is the least amount work you must issue onto a silver ingot with a mass of 38 kg at 26 meters depth to bring it to the surface?"
I'm not sure how to do this one. Surely it depends on the volume ...
0
votes
0answers
27 views
Defining collision point in 3D [closed]
I have a problem with finding collision point of two objects with given speed vectors in 3d. Eg:
...
2
votes
1answer
93 views
Proton-proton collisions
I have a question about proton-proton collisions at the LHC. Firstly, the 4-momentum $p^\mu=(E/c,\vec{p})$ can be represented as $p^\mu =(m_T \cosh \Psi, p_T \cos \phi , p_T \sin \phi, m_T c \sinh ...
0
votes
0answers
38 views
Solution of Maxwell's equation for simple, time-harmonic wire
I would like to compute the electric field $\boldsymbol{E}$ in the time-harmonic case for a (thick) wire parallel to the $z$-axis, but I can't quite get to it.
What I've got so far:
The current ...
4
votes
1answer
110 views
Quantum Mechanics - Hidden Variables
In Steven Weinberg's Lecture on Quantum Mechanics (p. 342), he writes:
The correlation between the spins of the two particles can be
expressed as the average value of the product of the ...
3
votes
1answer
73 views
Tunneling and transmission
Lets say we have a tunelling problem in the picture, where $W_p$ is a finite potential step:
If particle is comming from the left a general solutions to the Schrödinger equations for sepparate ...
-1
votes
1answer
40 views
Mass of a jumper, given height jumped and force exerted [closed]
This is the question:
An exceptional vertical jump from rest would raise a person 0.83m off
the ground. To do this, a constant force of 2593N would need to be
exerted against the ground. ...
0
votes
0answers
36 views
Changing Momentum [closed]
A high school football player hits and pushes a tackling dummy during practice. The following graph shows the amount of force that acts on the player.
a. What is the impulse that acts on the ...
0
votes
0answers
28 views
Changing Momentum [closed]
Changing Momentum
The illustrations show two ways of providing impulse to change the momentum of a vehicle. In the top illustration, the vehicle collides with a massive concrete barrier. In the ...
1
vote
1answer
66 views
Movement of the particle in a magnetic field
This is the numerical question.
A particle having a mass of 0.05 mg carries a charge of $5\times10^{-6}C$. The particle is given an initial horizontal velocity of 5000 m/s, what is the magnitude ...
1
vote
1answer
64 views
$\frac{dt}{d\tau}=\gamma$ in special relativity
I hope this is not too silly a question: We often see
$$\frac{dt}{d\tau}=\gamma=\frac{1}{\sqrt{1-v^2}},$$ taking $c=1$.
Problem:
I don't understand why...
In the Minkowski metric, using the ...
2
votes
1answer
128 views
Two masses attached to a spring
I'm trying to understand the solution of the following problem.
Two masses $m_{1}$ and $m_{2}$ slide freely in a horizontal frictionless track and are connected by a spring whose force constant is ...
1
vote
1answer
53 views
What is the coefficient $\mu_\text{air}$?
I am answering some exercises in thermodynamics, and I am still in trouble with some values and constants.
In the exercise, it is given: $\mu_\text{air}=1.8\cdot10^{-5}~\text{kg/m s}$. What for is ...
2
votes
1answer
58 views
$\require{mhchem}$ $\ce{\beta^{+}}$ decay for $\ce{_9^18F}$, computing $\Delta m$
For the decay:
$$\require{mhchem}\ce{_9^18F\to_8^18O +e+ +{v}}$$
To compute $E$, I need $\Delta m$, the provided answer looks like:
$$m_i = 18.000938~u$$
$$m_f = 17.999159~u + 2~(5.49 \times ...
0
votes
1answer
53 views
Computing $\Delta m$ in $\beta^-$ decay
$\require{mhchem}$What is the energy $Q$ released when $\ce{^131_53I}$ decays and $\ce{^131_54 Xe}$ is formed? The atomic mass of $\ce{I}$ is $130.906118~u$ and the atomic mass of $\ce{Xe}$ is ...
2
votes
1answer
138 views
Mechanics question
The following is a question from a past exam paper that I'm working on, as I have an exam coming soon. I would appreciate any help.
A fairground ride takes the form of a hollow, cylinder of radius ...
0
votes
0answers
25 views
Calculate the amount of water needed to extinguish a vehicle fire? [closed]
Essential Data:
Petrol has a calorific value 45000 KJ/Kg
Temperature at which petrol burns ~ 300 Degrees Celcius
Specific heat Capacity of Water = 4.2 KJ/Kg
Specific heat Capacity of Steam = 2.0 ...
2
votes
1answer
100 views
Energy density of a quantum mechanical ensemble
How do we determine the energy density of a given system? I have seen that the density operator
$$\rho~=~\frac{\exp(-\beta \hat{H})}{\text{tr}(\exp(-\beta \hat{H}))}.$$
What does this mean exactly ...
1
vote
1answer
67 views
Basic energy calculation for N identical spin system
We have a system that has N identical spins $n_i$, and each spin can be in state 1 or 0. The overall energy for the system is $\epsilon\sum_{i=1}^{N}n_i$.
My understanding: There is only one ...
0
votes
3answers
131 views
Why does a rod rotate?
I'm a physics tutor tutoring High School students. A question confused me a lot.
Question is:
Suppose a mass less rod length $l$ has a particle of mass $m$ attached at its end and the rod is ...
0
votes
0answers
26 views
Problem in geometrical optics [duplicate]
I was trying to solve a problem taken from an Physics Olympiad when I came across a curious and complex mathematical expression. I can not prove with what I know so far about mathematics, does could ...
0
votes
0answers
47 views
Trigonometry in the plane mirror [closed]
I was trying to solve a problem taken from an Physics Olympiad when I came across a curious and complex mathematical expression. I can not prove with what I know so far about mathematics, does could ...
1
vote
1answer
33 views
How can I find the temperature of this system?
A system was given a small amount of thermal energy dE, and its number of states G grew by 25%. How can I find the system temperature?
The system contains gas particles, I know that $dE << ...
