# All Questions

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### Riddle question in quiz

The closer you look, the less you'll actually see. Get closer to your question. How close can close be? Hi guys i had this PHYSICS quiz i my school and this is one of the questions that was ...
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### Why the Doppler Ultrasound beam needs to be looking directly down at a pipe

So I was doing a question about "Doppler Ultrasound" It states that a Doppler ultrasound uses soundwaves at a frequency much higher than what humans can hear. They can me used to measure the speed of ...
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### Alkali metal transparent to UV - is cesium (somewhat) transparent to visible blue light?

Most optics texts will mention that alkali metals can become transparent in the near ultraviolet in the sections on reflections from metals, plasma frequency, and electron density. I remembered this ...
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### How much water did I actually drink?

Water expands at temperatures over $5^\circ\mathrm C$ strictly monotonic. If you now drink $1~\mathrm{L}$ water ($5^\circ\mathrm C$) which will expand in your body on $37^\circ\mathrm C$, did you ...
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### Fierz identity in Spin(5,5)

I am trying to understand(derive) equation 3.8 in 1208.5884 which is First, here $\gamma$ is $16\times 16$ chirally projected gamma matrices. which $\alpha=1,2,3,4,...16$ and $a=1,2,3,4,5,6,7,8,9,10$...
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### What is the importance of torque direction?

I understand the right-hand rule and direction of the force on an arm, but what exactly is this 'direction' that results from the cross product of the vectors $T = r \times F$? On paper, what is this ...
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### Kinetic Hysteresis

I have been wondering about this question for quite some time, but, until now, I was not sure how to phrase it without sounding naïve. Let's take Newton's Cradle (just three congruent balls will do). ...
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### hamiltonian in a rotating frame [on hold]

ˆ H0 = h(ωopt − Gx)a†a + hΩb†b··· this is the hamiltonian of optomechanical system how this can be solve after switching to a frame rotating at the incomming frequency ωL.
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### Mass density of a cylinder increasing with the square of the radius

I’ve had to deal with a question like this on a test on friday and I just can’t get it out of my mind. I was supposed to calculate the moment of inertia of a cylinder with an inhomogenous yet ...
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### High School Physics-Electromagnetism

An insulated smooth square frame ABCD of side $a$ is able to rotate about one of its side taken as positive axis. The plane is initially located at x-z plane. A magnetic field present in the region ...
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### magnetization curve for motors

I have query that if you observe magnetization graphs they are plotted between flux and armature induce voltage keeping speed constant but how does it happen ? Did every time to keep speed constant ...
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### Origin of time or origin of space-time? [on hold]

This might be a mere theoretical concept and I might even get down voted for asking this. I read in several sources that winding back general relativity equations leads to $t=0$, origin of time. ...
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### Why move from Special Relativity to General Relativity? [on hold]

The concept of relativity is simple. Why take Maxwell's equations to a new Special Relativity level with the unproven assumption that photons have no mass? All other physical data (momentum, affected ...
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### Does “Schrodinger's cat” need entity knowledge to work?

I've never studied physics, but am interested in whether this paradox, from an interpretation of quantum mechanics, can still be stated if we don't know anything about unobservable entities. My ...
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### Pressure loss in an infinite pipe

I feel lost in this problem. I know pressure loss occurs in a pipe flow due to friction. Say given a constant diameter pipe, if we keep lengthening the pipe horizontally, and static pressure will ...
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### Speed of sound at phase transition — does percolation theory describe it?

I was discussing with a friend whether we expected liquid water or ice to have a higher speed of sound, near freezing; we figured it would be water since it was more dense. It turned out to be ice (by ...
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### basic question about work exercise

I am learning independently from Tobochnik's statistical physics text. He asks a warm up question about work: A person pushes a block up a frictionless ramp. Is the work positive, negative, ...
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### How can IR cameras see things colder than the camera?

I’ve read that only cold-blooded animals develop IR vision to spot warm prey because a mammal would blind himself with his own heat and not be able to see anything. But I’ve seen a FLIR camera, ...
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### Spectral covers and a specific exact short sequence

I have a question about the spectral cover construction of Friedman, Morgan, and Witten (typically used to map a description in heterotic string theory into F-theory). I realise this is a highly ...
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### Initial Velocity Question

What is the initial velocity vertically when a person jumps 0.5 metres into the air, neglecting horizontal movement. Is it just 0 m/s? And the final velocity would be -9.8m/s^2 * the time it took took ...
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### How much energy is released by decompressing water?

How much energy would be released if 2L of (relatively pure) water compressed to 1L at room temperature was suddenly decompressed to atmospheric pressure? Even a first-order approximation would be ...
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### Frequency range possible from a mechanical music box

Here is a small mechanical music box. The tines of a metal comb are tuned to emit a particular frequency when plucked. The material and structural characteristics of the comb, plucking cylinder, ...
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### If we harvest energy generated by Coriolis Effect of the earth, will the earth slow down?

Coriolis Effect causes strong winds in the upper atmosphere, and it's purely due to differences in the linear velocity of Earth's rotation across latitudes. Now the Earth is spinning with the energy ...
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### When we say particle in a box has quantized energy, is that kinetic or total energy?

In quantum mechanics, it is usually said that energy of the bound (constrained) systems such as particle in a box (infinite potential well) is quantized. It confuses me exactly what type of energy is ...
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### Motion in a Plane Velocity

If I were to throw a rock of a cliff at 12 m/s as a horizontal velocity, what will the final velocity of the rock become when it hits the ground? I am tempted to say 12 m/s because the horizontal ...
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### Action at a Distance

Since photons move at $c$, do they experience time or distance? If they don't, doesn't this explain action at a distance? From the point of view of the photons, there is no time, so the action at a ...
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### Could a strongly enough charged black hole allow for a strongly charged particle to escape the event horizon?

If there is a strongly charged black hole, and a particle with a strong charge of the same sign goes inside of the black hole's event horizon while moving quickly, is there a reason why it would not ...
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I'm trying to calculate the symmetric traceless tensor for the octupole moment in cartesian coordinates... I have to deal with the electrostatic potential of the form: $\Phi^{(4)}(\textbf{r})=\int d^{... 1answer 27 views ### What happens with QM particle at constant velocity? [on hold] In QM particles have wavefunctions that disperse quite fast. The FWHM for gaussian wavepacket increases in time as$\Delta x(t)= \Delta x_0\sqrt{1+(t/\tau)^2} $,$\tau=2m(\Delta x_0)^2/\hbar$What ... 0answers 30 views ### Why do these experiments contradict the accepted theory about puddles? A few months ago, I tried to tackle the problem of finding the shape, the height and other properties of a puddle (whenever a few mL of some liquid are dropped in a "uniform way" so that the puddle ... 1answer 37 views ### How feasible is it to power your home with a bicycle? This article claims "60 Minutes On This Bicycle Can Power Your Home For 24 Hours!" How feasible is this claim? Sounds great - but the skeptic in me says "yeah, right." Hope this is an acceptable ... 0answers 26 views ### How does this dumbbell rotate? I am reading the book Spacecraft Attitude Determination and Control by J.R. Wertz, and on page 488 it says: My question is: how does the axis of rotation,$\hat{\omega}$, rotate? How does ... 0answers 7 views ### Motion in a Plane Question [on hold] I don't understand how to do this problem: You are the pilot of a small plane and want to reach an airport, 600km due south, in 4.0h. A wind is blowing at 50km/h[S35oE]. With what heading and ... 0answers 10 views ### Interpretation of Evanescent Elastic Waves for Material Damage In elastodynamic theory, when the slowness vector is imaginary, the resulting elastic waves are called evanescent. I have read that this corresponds to exponential decay. I have also read that complex ... 1answer 20 views ### Heating water in a pot from a constant heat source What are the equations used to calculate the amount of energy transferred from a constant heat source "like a stove" to a pot which would then transferred energy to water contained in that pot. I don'... 2answers 46 views ### Why do magnetic coils consist of many thin wires? Magnetic coils are usually made of thin wire turned into spring shape: Magnetic field which is generated by this solenoid is proportional to the number of turns N... 1answer 30 views ### Is it possible to quantify the work required to introduce order? Is there a thermodynamic analysis that allows us to say "in order to introduce this much order into this subsystem, this much work must have been done on it by the rest of the system"? I assume that ... 1answer 35 views ### X-Ray diffraction - Braggs law "Based on the Bragg equation, if crystal A has larger spacing in its diffraction pattern than crystal B, what conclusion can you draw about the spacing between layers of atoms in A compared with B?" ... 3answers 2k views ### Are black holes perfect spheroids? What I know about black holes (correct me if I'm wrong) is that they are the most compact objects in the universe that have been discovered. Due to all that gravity, wouldn't black holes be a perfect ... 2answers 49 views ### Does the charge of a black hole affect its space-time geometry? Does a black-hole of a given mass and angular momentum have the same space-time geometry regardless of its charge? I'm pretty sure that an electric field can accelerate a charged particle but doesn't ... 2answers 308 views ### What's smaller: a neutrino, or a string from string theory [on hold] I've recently read an article that stated "If an atom were as big as the solar system, a neutrino would be the size of a golf ball". I watch the science channel, and on (I believe) the show How the ... 2answers 38 views ### Question regarding elastic collisions and the conservation of momentum / energy [on hold] I've been working on this question for a few hours and I cannot quite convince myself that my answer is correct. I've read the relevant section in my textbook, but I guess the lightbulb just isn't ... 0answers 22 views ### How far would the atmosphere have to extend before we couldn't see the sun? Ignoring dust, haze, clouds or ice crystals. Just 100% clean air. 0answers 14 views ### Chemical Potential of Interacting Particles I'm interested in studying the diffusion of (classical) particles that have some interaction with each other. More specifically, let the potential energy of any two of particles separated by a ... 2answers 67 views ### Two particles system Source: this video For a system with two particles (09:30), why is its wave function a product of each particle's wave function? E.g. $$\psi(x_1,x_2)=\psi_a(x_1)\psi_b(x_2)$$ For indistinguishable ... 1answer 41 views ### When should parameters be replaced while changing basis? I have the following equality: $$a\lvert 00\rangle + b\lvert 11\rangle = a\lvert ++\rangle + b\lvert --\rangle$$ for any$a,b \in C$that satisfy the normalization condition. For$a = \frac{1}{\...
This is very basic but I just suddenly got confused. Any state can be expressed as complete set of eigenkets with discrete eigenvalues: $$|P\rangle = \sum^n c_n |p_n\rangle$$ I understand the above. ...