7
votes
2answers
318 views

Sailing against the wind. Is this a fair model?

I was ruminating the explanations about how boats can sail against the wind (or "into the wind"), and wondered if one could devise a simple mechanical model without hydrodynamics involved. Imagine a ...
6
votes
2answers
6k views

Ice bath is always 3C, why?

I've always learned that a mixture of ice and water should reach equilibrium at approximately 0C. I've actually tried to create that a number of times in different contexts and always fail. First, ...
1
vote
0answers
82 views

Help wanted achieving cryogenic temperatures [closed]

My son's science project involves liquifying air. I found the Linde process which is the way it was done first. We are trying and not getting very far so I am looking for advice. I have an air ...
5
votes
2answers
452 views

Time-ordered operator in Srednicki

On page 51 Srednicki states, "Note that the operators are in time order...we can insert $T$ without changing anything". This I agree with. But then on the next paragraph he states "The time order ...
0
votes
1answer
120 views

Will a copper plate spin near a motor?

I'm revising old physics papers and I'm unable to answer the following question. If a horizontal copper plate is suspended at the centre by a thread, over a motor whose axis is aligned vertically, ...
-1
votes
3answers
346 views

According to relativity, can a statement be true and false in same time?

Say I write following statement on a paper: It's year 2014 at earth. Then I give it to my friend. My friend then starts to leave me and earth to outside with velocity of light. Einstein says the ...
2
votes
2answers
359 views

A change of units in Bethe formula

I'm reading through the following paper: Monte Carlo simulation of non relativistic electron scattering by W. Williamson and G. C. Duncan. In the following paragraph, I want to know how to arrive at ...
0
votes
2answers
893 views

Is it possible to project an image onto water? [closed]

Is there a way to create an image/projection in the water? Or form the water to project an image, little dot/pixel? Use some type of frequency to modulate the little dot in the fluid?
3
votes
2answers
1k views

Second order energies of a quartic pertubation of a harmonic oscillator

A homework exercise was to calculate the second order perturbation of a a quantum anharmonic oscillator with the potential $$ V(x) = \frac{1}{2}x^2 + \lambda x^4 $$ We set $\hbar = 1$, $m=1$, etc. ...
0
votes
3answers
463 views

Non-Conservative Behaviour of Static Electric Field

Static electric fields are supposed to be conservative in nature and therefore give $0J$ work if traversed over a loop. However in the following problem I got non zero work by a static electric field. ...
1
vote
0answers
145 views

Conduction– conservation of energy

So I know that conduction is transfer of thermal energy by direct contact via molecular collisions. For conduction, $Q/t$ is the rate of heat flow and is heat current ($I$). My textbook says that ...
0
votes
1answer
292 views

Quantum state with zero standard deviation of position operator

Is any quantum state $|\psi\rangle$ possible such that the standard deviation $\sqrt{\langle\psi|(\Delta\hat{x})^2|\psi\rangle}$ of the position operator $\hat{x}$ is zero? If not, why?
3
votes
0answers
126 views

picture and isomorphisms

I found a paper by A. Belopolsky : "Picture changing operators in supergeometry and superstring theory" where he says there exists a possible physical state in one picture that doesn't exist in ...
7
votes
2answers
221 views

Why does a small object stay in one place after being shot from a big gun?

This video shows what I mean. Basically they shoot a bullet from a huge gun at an iPhone. What I don't get is why does the phone move only so little after being shot? As far as I can tell it isn't ...
0
votes
1answer
354 views

Charge density in depletion layer for pn-junction

I just came across this Wikipedia article on pn-junctions: pn junction What I don't get is why the negative charge density is narrower/smaller than the positive one in Figure A. I'm guessing the ...
1
vote
2answers
303 views

Relativity and speed of light again - two opposite light sources [duplicate]

Two light sources emit light at the same moment but in opposite directions. At what speed the distance between two light fronts is increasing? c or c * 2? Note, that there is only one coordinate ...
1
vote
1answer
100 views

What do they mean with: photon scattering with $q^2=-Q^2\leq 0$

In a scattering problem, let q denote the four-momentum of the photon. Is $q^2=-Q^2\leq 0$ simply a statement of what metric one uses and simultaneously a definition of $Q^2$?
1
vote
0answers
72 views

Synchronizing time between two points

Suppose we have two points on earth, A and B, both with clock. Suppose we can send some information between points, with a random delay: $$ min < delay < max $$ ($min$ theoretically exists ...
7
votes
3answers
424 views

Does Super Mario physics work in reality? [duplicate]

This illustration summarizes my question: Please assume no air resistance (unless that makes a large difference). Here's what we concluded, but I could use both confirmation and also help with the ...
0
votes
1answer
135 views

What happens to matter when in a thermodynamic equilibrium?

I am trying to gain a better understanding of thermodynamic equilibrium. Here's what (I think) I know: If a system is in Thermal, Radiative, Chemical, and Mechanical Equilibrium, then it is in ...
134
votes
15answers
48k views

How do I explain to a six year old why people on the other side of the Earth don't fall off? [closed]

Today a friend's six year old sister asked me the question "why don't people on the other side of the earth fall off?". I tried to explain that the Earth is a huge sphere and there's a special force ...
1
vote
2answers
240 views

Can a bullet leave a gun and tumble to the ground?

This question seems to have been asked a few times in different configurations, but none of them answer my variation. I've struggled to understand this for nearly 15 years and had conflicting answers ...
0
votes
1answer
52 views

What is the nature of Young's Double Slit Experiment with detectors?

If a detector is kept at the two slits the fringes disappear. But, when the detectors are removes do the fringes come up immediately without any significant time lag? Can there be a way to switch on ...
0
votes
1answer
184 views

Conclusions From a Junior School Science Experiment

If you stick a tissue to the bottom of a glass and immerse this glass fully (upside down) into a tumbler of water, then this tissue remains dry. Why is this so? / What conclusion would you draw from ...
1
vote
3answers
4k views

How the apparent weight varies due to the rotational motion of Earth?

I learned that as the earth rotates about its axis, the bodies on the earth also follow a circular path. In most books I read, they give the example of a person standing on a weight balance at the ...
0
votes
0answers
84 views

About divergence of a vector field and geodesic sphere

I have a question. I want to know the difference between the sphere and the geodesic sphere. Another question: given a vector field, $Y$, on a manifold $M$ defined by: $Y(p)=p$ for every point $p \in ...
1
vote
0answers
371 views

Free Energy of N Spin 3/2 Particles

This question is from the book "Introductory Statistical Mechanics" by Bowley and Sanchez. The question is as follows: Calculate the free energy of a system with N particles, each with spin 3/2 with ...
5
votes
1answer
722 views

Velocity distribution in Plummer's models and others mass distributions

The Plummer's sphere is an model for the mass density in a globular cluster of stars. For an $N$-body simulation I have initialized the position of $N$ masses with a Monte-Carlo technique but cannot ...
1
vote
0answers
65 views

How does a liquid crystal on Silicon affect the phase of incident light?

When a polarized light beam is directed to a phase-only LCoS system how is the phase modulated ? What is the physical effect behind ? Does voltage modulation has an impact on birefringence ?
1
vote
1answer
326 views

SIngle slit diffraction and location of dark spots

In single slit diffraction, why (according to the equation for $\theta$) do successive dark spots exist at whole number intervals ($n=1,2,3, \ldots$) and not half? The correct path difference for ...
0
votes
2answers
114 views

Is it possible for a human being to travel at the speed of light? [closed]

Can our human body travel at the speed of light in a space shuttle? Indeed, can any material object with nonzero mass travel at the speed of light with respect to a valid inertial frame? If not, why ...
3
votes
2answers
380 views

Projectile motion maximum distance to origin

Is there an elegant or easy way to derive the maximum distance from the origin to a point of the parabola created by projectile motion (assume initial position is the origin)? Other than ...
0
votes
1answer
825 views

Minimum separation distance between two masses cushioned by a spring [closed]

I think this problem is much more difficult than what I've learned so far. B) is the problem I'm having a hard time with. I think it is much more difficult to consider because as the red object ...
3
votes
0answers
118 views

Sound - for purposes of vibration

What is the best way to distribute noise from more than one source (I'm envisioning a system with many), within a dome, with the ground as its primary target, at optimal frequencies and volumes to ...
4
votes
1answer
353 views

In a positively biased PN junction, where do the injection carriers come from?

I am not quite understand i-v character of PN-junction diode. Here is the model in textbook. The PN junction diode can be divided into three regions. They are One depletion region near the PN ...
1
vote
1answer
168 views

General Thermodynamic equation of state

I heard my professor saying that the equation $$ PV = \frac{2}{3}U $$ is valid for any non-relativistic gas, be it Ideal or Real gas(includes quantum ideal gases). Is this true, If it is how can we ...
3
votes
1answer
102 views

Can a room resonate at a frequency lower than the principal frequency?

I'm the bass in our high school a cappella group. Recently we had a performance in our auditorium, and I am almost certain I heard the pitch an octave lower than I was singing. I was singing E3 (164 ...
31
votes
4answers
681 views

Why doesn't a typical beam splitter cause a photon to decohere?

In many experiments in quantum mechanics, a single photon is sent to a mirror which it passes through or bounces off with 50% probability, then the same for some more similar mirrors, and at the end ...
0
votes
1answer
68 views

Multiverse explnation of fine tuning of cosmic constants

Physicists proposed the idea of multiverses to explain extraordinary fine tuning of the cosmic constant and physical laws which is essential for rise of life on earth. Is there any experiment that ...
3
votes
1answer
168 views

Non-determinitric flow of water stream on vertical porcelain surface

I noticed several times while washing my hands that accumulated water when starts streaming vertically does not flow vertically in straight line on vertical porcelain surface under the force of ...
1
vote
2answers
299 views

Total Resistance in a Tetrahedal loop [closed]

Imagine if I placed resistors of 1 ohm each in shape of tertahedal. What would the total resistance be between A and D? I can't find any website that gives me a good explaination.
1
vote
0answers
37 views

Observations used to derive orbital and timing information required for GPS satellites

Although I have studied GPS receiver position finding procedure in some detail, I am struggling to find enough information to allow me to deduce how the essential highly accurate orbital and time ...
5
votes
3answers
2k views

Can a megatsunami have a height of 30,000 feet or more?

Is it possible on Earth? Roughly what size would an impactor have to be to cause a tsunami of this height? And it's speed, energy equivalent in tons, megatons, etc.?
3
votes
2answers
5k views

Relationship between measures of light (candela and lux)

I am trying to understand the relationship between two measures of light: lux and candela. It is probably easy for those with longer experience, but here it goes: I have learnt (here for instance) ...
-3
votes
1answer
428 views

Collapse in Quantum Field Theory? [duplicate]

I do not want answers telling me that wave-function collapse is not real and decoherence is the answer (I know the situation with that). I am asking a question purely on the basis if wave-function ...
-1
votes
1answer
84 views

Does alternative reality theoretically prove? [closed]

Does Hugh Evert theory means Alternative-Reality (or in general, Parallel-Worlds) and Many-Worlds-Interpretation theoretically prove? Or are scientists have any progress to prove it?
10
votes
5answers
2k views

Noether Theorem and Energy conservation in classical mechanics

I have a problem deriving the conservation of energy from time translation invariance. The invariance of the Lagrangian under infinitesimal time displacements $t \rightarrow t' = t + \epsilon$ can be ...
4
votes
5answers
4k views

Conductors and their charge?

Why does excess positive charge stay on the surface of a conductor? This is what I understood from: How does positive charge spread out in conductors? and other resources on the web: If there is a ...
7
votes
4answers
1k views

What {R,G,B} values would represent a 445nm monochrome lightsource color on a computer monitor?

Is it possible to answer my question definitely (assuming the monitor is perfect)? What would be the formula for calculating RGB values for a visible monochrome light with given wavelength?
0
votes
0answers
124 views

Light cones and reference frames

I'd like to know what does it mean exactly to find a reference frame in which two events occur at the same time or in the same space coordinates. As I picture it if we have two events A and B in a (x, ...

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