# All Questions

2k views

### How to calculate air velocity in a fan?

How to calculate air velocity in a fan and what data should I know ? So if I have a fan motor, fan blade and a battery, what data should I get from these items ? I think I will need to know the ...
764 views

### Do asteroids create sonic boom while entering the atmosphere?

Because of the lack of any materials to propagate sound, it can't spread in space. There's the air in the atmosphere, but its volume mass increases constantly (either linear or exponential, I don't ...
151 views

### Weird definition of Bohr radius on Wiki

On the Wikipedia I found that the Bohr radius is equal to: \begin{align} \boxed{r_b=\dfrac{4\pi\varepsilon_0\hbar^2}{m_e{e}^2}} \end{align} but while we have been learning Bohr's model we derived ...
340 views

### Isentropic vs Total pressure

I am confused between these two quantities: Isentropic pressure and Total pressure. Are they the same?
461 views

### Particle slightly disturbed radially in stable circular motion under central force

A particle of mass $m$, moves around a central force whose potential is $$V(r) = kmr^3 \; (k>0),$$ then $(c)$ if the particle is slightly disturbed from the circular motion, what is the ...
813 views

### Measuring speed of a vehicle

I'm very primitive with my thought. So please help if you can in layman terms with an answer to this question. Here we go- by using a street power pole as the source responder, is it possible to read ...
1k views

### Is the standard explanation for the ring launcher incomplete?

Related: Faraday's law in a ring The ring launcher is a standard introductory physics demonstration that I assume almost everyone has seen (if not, YouTube it). The explanation of why the ring is ...
227 views

### Saturn V specific impulse issue in velocity modelling with Tsiolkovsky's equation against actual values?

I've been trying to model the Saturn V's velocity using Tsiolkovsky's ideal rocket equation, and in the process, I think I may have made a mistake with regards to the specific impulse? I've come up ...
362 views

### Derivation for 7.14 in Atomic Physics by Foot

I was going thru Ch7 of Foot and trying to fill in the gaps. However I got stuck on (7.14). So Foot was working with a two level system with a small perturbation in the Hamiltonian resulted from an ...
251 views

### Can spheres leaking charge be assumed to be in equilibrium?

I am struggling with the following problem (Irodov 3.3): Two small equally charged spheres, each of mass $m$, are suspended from the same point by silk threads of length $l$. The distance between ...
112 views

### Surface integral of a line

Suppose I am calculating the (homogenous) electric flux through a straight line. How do I calculate the surface integral of a straight line? Is it simply a line integral, or is it more complex (I ...
65 views

### Could it be a limit in our measuring the speed of photon, instead of photon having a speed?

Are we (not) limited by our own speed through time (1 sec / sec) to measure highest possible speeds(c)? I fire a laser which hits a mirror and comes back and I precisely measure the time, and knowing ...
22k views

### Relationship between torque and angular momentum

I want to know how to deduce the equation $\vec{\tau}=\vec{\omega} \times \vec{L}$, where $\vec{\tau}$ is the moment of force (also known as torque), $\vec{L}$ is the angular momentum, ...
484 views

### How small can you be and still hear?

According to my understanding, at the smallest level sound is just a collection of particles colliding on a wave. So, assuming we can arbitrarily reduce the size of any human being, how small can we ...
53 views

### I've been reading that it's possible to create a “mostly magnetic” wave, and I have a few questions

Would it be possible with the "mostly magnetic wave" to have it behave in such a way that it would be undetectable by radio triangulation? I read about the monument at the CIA headquarters that was ...
82 views

### Adiabatic quantum Hamiltonian of variable dimension

Is adiabatic quantum Hamiltonian of variable dimension possible? This is very hypothetical and I am afraid may not have enough merit to belong to this forum. I would still like to elaborate. Here is ...
9k views

### Why does a car go down the hill backwards even though the engine keeps running? [closed]

I dont get why a forward gear allows sliding a car backwards. Any ideas? So this is a car with an automatic transmission. The transmission is in the drive mode (move forward). Car is at the slope ...
116 views

### Difference between Higgs contribution to electroweak boson mass, and fermion mass?

Thanks in advance to anyone who takes the time to answer this, and apologies in advance for what is probably (yet another) question here due to unfamiliarity with The Math. I had a chance recently to ...
172 views

### How many seconds is a temporal meter?

Is there a proof that time is a 4th dimension? If it is, then why not measure it in units of the previous three? Logical right? How many seconds is a temporal meter?
87 views

### Transmutation with cosmic radiation possible?

Can cosmic radiation (alpha radiation) transmute the material of a space craft, particular carbon, titanium and aluminum? Where can i find transmutation tables or formulas to calculate the ...
83 views

### Possible abuse of notation in statistical mechanics

I know that it often occurs that we need to take a derivitive with respect to $\beta$ in statistical mechanics. However, I think it is poor style to use equations with both T and $\beta$ in them ...
276 views

### Potential of a dipole with actual physical extension?

I think everybody here knows the equation that gives the potential of a point like dipole, but how does the field look like if you have e.g. a metal sphere with radius $R$ and a certain dipol moment, ...
833 views

### Help on applying a Hadamard gate and CNOT to two single q-bits

I am stuck on a few issues in this video. (Note: It is at the frame concerning this question.) In it, from what I understand (which could be wrong) we first apply the Hadamard gate to a qbit in the ...
6k views

### Why does the Moon or the Earth revolve anti-clockwise?

The question is very simple: Why does the Moon or the Earth or any another planet revolve anti-clockwise? And can any planet (or satellite) revolve clockwise? What is the physical law for this ...
257 views

### Photons moving relative to photons

I know we can't move at the speed of light, but if we were to travel on a photon how fast would we see other photons going? The speed of light is constant... So do photons see other photons moving at ...
108 views

### How to measure the arbitrariness of a quantum state?

An arbitrary qubit is represented as $\alpha|0\rangle+\beta|1\rangle$ with $|\alpha|^2+|\beta|^2=1$. If we know either $\alpha$ or $\beta$, the state can be completely identified. The 'arbitrariness' ...
600 views

### Why terminal velocity exists?

Why the acceleration of free-falling bodies become zero after some time? My only idea would be that the closer to the core of Earth we are, the value of "g" the lower becomes (since the mass below the ...
441 views

### Center of mass, center of gravity of a spoon

I went to a restaurant yesterday and while I was eating dessert, I saw a standard stainless steel spoon (note the alliteration!) balancing on a plate. It was balancing on its handle, and I started ...
3k views

### Why is there a size limitation on animals?

Why is there a size limitation on human/animal growth? Assuming the technology exists for man to grow to 200 feet high, it's pretty much a given that the stress on the skeletal structure and joints ...
1k views

### Does gravity slow down a horizontally thrown baseball?

I have a simple question: Does gravity slow down a horizontally thrown baseball? Assuming when a baseball is thrown it has a vertical velocity as well, does it slow does the ball? Any help is much ...
239 views

### Ising model Mean field theory and translational invariance

In the Ising model the mean value of any particular spin is: $$m = \left<s_i\right> = \frac{ \sum_{s_i}e^{-\frac{H}{T} }s_i} { \sum_{s_i} e^{-\frac{H}{T} } } .$$ I read in several ...
1k views

### Conditions for Conservation of energy law

In two-dimensional motion, which conditions are needed to be satisfied so the conservation of energy law holds? (for example, simple pendulum motion)
269 views

### Steering forces on a bicycle

Ok, I always notice this weird thing and try to overcome it but cant. Ok as shown in the image when I ride the bike by just one hand and pull the handle back say from the right side so as commonly the ...
74 views

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### Mass, momentum and energy…How many equations for more then one elements? [closed]

Basically, for solving 1D flow equations, one uses the mass, momentum and energy equations (Also known as Euler equations). Say now I have $N$ elements in series, will I be using $3N$ equations?
448 views

### Is there any physics behind covariance and contravariance of indices of tensors?

Is there any physics behind covariance and contravariance (up and down) of indices of tensors?
313 views

### Steel rod to Mars vs the speed of sound, how is supersonic travel possible?

I remember reading this passage in the "Feynman Lectures", where Dr. Feynman describes an experiment in which a theoretical metal rod of length equal to the distance between Mars and Earth is arranged ...
1k views

### Can a wave be two dimensional?

I am having a hard time picturing waves, the image that comes to mind is a bobbing device submerged in still water which generates pulses in all directions (similarly in air). Then how can a wave be ...
366 views

### What does string theory say about the metric expansion?

Specifically, what happens to those small intertwined hidden dimensions? Do those expand too?
714 views

### Why electron clouds in atoms don't radiate? [duplicate]

I was reading that Bohr assumed electrons in orbit simply did not radiate, and my professor told me that the actual case is that electrons are clouds of probability. Even so, aren't they still moving ...
190 views

### How to obtain the free energy of the canonical ensemble in Euclidean general relativity?

If the gravitational field couples with matter fields, such as a charged scalar field, I know the partition function of the grand canonical ensemble naturally relates to the path-integral expression ...
762 views

### Electromagnetic Momentum

My book says : The fact that electromagnetic radiation of energy carried momentum was known from classical theory and from the experiments of Nichols and Hull in 1903. This relation is also consistent ...
8k views

### In an ideal gas, if temperature increases, will pressure or volume increase?

This question came up when I was talking about the atmosphere. Someone had mistakenly claimed that as temperature increases, the density of the atmosphere should increase as well. I reasoned from the ...