# All Questions

328 views

### Is an “infinitely sharp blade” possible?

A staple of science fiction and fantasy is a blade (knife, sword, ...) that cuts through literally any solid object (wood, steel, concrete, skulls, ...) without effort, often even without the need to ...
269 views

### How do we know that photons are exactly massless and travel exactly with speed $c$ in vacuum?

I know that this question may sound stupid, but what I mean is that photons have some energy and no mass, yet the mass and energy are said to be equivalent (or maybe I got that part wrong). In an ...
137 views

### How rigorous can conservation of energy be made?

The principle conservation of energy is often taken as an obvious fact, or law of nature. But it seems to me the definition of energy is far from obvious, or natural: ...
2k views

### Prove EM Waves Are Transverse In Nature

Why we say that EM waves are transverse in nature? I have seen some proofs regarding my question but they all calculate flux through imaginary cube. Here is My REAL problem that I can't here imagine ...
248 views

### What is the third law action pair of buoyancy?

Consider a block floating in a fluid. The force due to gravity balances the buoyant force exerted on the block. But there must be a third-law action pair to the buoyant force. If the block exerts a ...
113 views

### Entropy and probability

I read "The NEW world of Mr. Tompkins" and I'm not sure with one of the Gamow's equation. When he calculated the probability of entropy, he used this reasoning: "How likely is a situation that all the ...
164 views

### Is it possible to have a negative attraction to gravity just like how magnets with same pole does?

Layman question here, is it possible for an object to be negatively attracted to a gravitational pull of another object? Like how same pole magnets repels each other, can it happen on gravity too?
243 views

### Why consider only direction cosines?

Why are these called direction angles? Why do we consider only direction cosines and not direction sines or tans. What is its actual significance? And How to use them? Why are they called ...
4k views

### Why does a human body only emit infra red radiation and not other types of electromagnetic radiation?

What causes humans to emit infra red radiation and why don't we also emit other types of electromagnetic radiation such as ultra violet or microwave? Sunlight contains UV and our body takes it in, but ...
810 views

### Why doesn't the potential energy of any object equal 0

Consider a particle on the ground. This particle is raised by a force of magnitude $mg$ to a height $h$ above the ground. At this point, the work done on the particle by the force is $mgh$, which is ...
169 views

### Is there a physical quantity which is the reciprocal/multiplicative inverse of time?

Is there a physical quantity which is the reciprocal/multiplicative inverse of time? If time =distance/speed what is speed/distance. Please forgive my ignorance if there is a really simple ...
436 views

### Does Newton's third law of motion fail anywhere?

Does Newton's third law of motion fail anywhere, including particle physics and other advanced topics?
350 views

### Is the triangle stronger?

I have a friend who cedes that a framework that is a triangle would be stronger than a square framework. But he maintains that a square solid would not be stronger than a triangle solid. Ie a set of ...
639 views

### Gravitational collapse and free fall time (spherical, pressure-free)

A very large number of small particles forms a spherical cloud. Initially they are at rest, have uniform mass density per unit volume $\rho_0$, and occupy a region of radius $r_0$. The cloud collapses ...
5k views

### Why does cold water weigh more than hot water in a fixed volume?

Take a bucket of hot water and the other bucket of cold water. Why does the bucket full of cold water weigh more than bucket full of hot water?
169 views

### Why in some cases $0\alpha$ component of stress-energy tensor don't form 4-vector?

In electrodynamics there is Poynting vector and energy density, which refer to $0\alpha$ components of stress-energy tensor, don't create 4-vector. Analogous situation with mass density and mass ...
542 views

### Deriving the law of moments

Recall the Law of Moments for a one dimensional rod: "When an object is in equilibrium the sum of the clockwise moments is equal to the sum of the anticlockwise moments." I understand that we ...
121 views

### Can we make usable energy from subnuclear particles?

I understand mass and energy are the same, but in this question I will be talking about mass being turned into usable energy (electricity/heat/etc). We can make our energy through chemical reactions ...
165 views

### Are communications between computers faster by electrical signals via copper cables or electromagnetic signals?

Assume that there are two computers which are connected with a copper cable, e.g. Ethernet. Also, there is a radio connection between them, e.g. AM radio, in order to exchange data. When we try to ...
406 views

### Resistors in series vs parallel

I understand that for resistors in series we need to add R for each resistor to get the equivalent resistance, where R is resistance. I also understand the mathematical manipulations that show that ...
2k views

For some reason, I feel like the concept of voltage is escaping my grasp. I've done much research on these forums and through texts, and come across answers that seem quite well thought out, but still ...
507 views

### Proof that the ground pushes you up [duplicate]

It has been said that when you try to jump you are exerting your force on the ground and the ground pushes you up. I was wondering if this was an over-simplification or not. If so, what's the full ...
84 views

### How can a string be unidimensional if they can be open ended or close ended?

I just don't understand how a object with 2 ends can be unidimensional.
171 views

### A complicated question about $E=mc^2$

I know this is a little outside the normal question and there may not be a direct answer, but it is an interesting thought experiment. Starting with a supermassive black hole, if you were able to ...
169 views

### Microscopy types and techniques? [closed]

I didn't get the basic difference in between the different types of microscopy. for example there are several different microscopy techniques are available such as Bright field, Dark field, Confocal, ...
2k views

### Does a Faraday cage block all magnetic field frequencies lower than x?

Where x is a function of the dimension of its holes. I mean, if I build a faraday cage able to shield magnetic field at 100kHz, will it we able to shield also all frequencies lower than 100kHz, such ...
312 views

### How does a dehumidifier work?

I have read sites like this, but I am just not getting it yet. I'm looking to understand the variables involved, which I think are: air moisture content air flow (cfm?) condenser surface area? ...
188 views

### Are there moments in particle collision experiments where the particle beam is in open air?

I keep encountering the story of Anatoli Bugorski, who apparently got hit in the head by a proton beam at the U-70 PS in Protvino, Russia. I find it difficult to believe that this is actually ...
560 views

### Intuitive understanding of centripetal vs. centrifugal force

I am having trouble understanding how centripetal force works intuitively. This is my claim. When I have a mass strapped on a string and spin it around, I feel the mass pulling my hand. So, I want ...
223 views

### Will a moving neutron produce a magnetic field around it?

Will a moving neutron produce a magnetic field around it? I know that neutrons have no charge, so magnetic field should not be produced along it...but neutrons have a magnetic moment. So why can't ...
274 views

### How to imagine the first few moments of an LR circuit?

For example, there's a very simple circuit which only contains on resistor. So according to Ohm's law, we have: $\mathrm{emf} = IR$ As we know when time $t = 0$, the current must be $I = 0$. However, ...
13k views

### Why work is a scalar and not a vector?

Work (in physics) is a scalar but why? and why not a vector?
822 views

### Block and inclined plane (INPhO Problem)

The figure shows two blocks on an inclined plane of mass 1kg each.The coefficient of static as well as kinetic friction is $0.6$ and angle of inclination is $30^\circ$ . Find the acceleration of the ...
289 views

### Vanishing of the Ricci tensor in higher spacetime dimensions

I understand how, if the Riemann tensor is 0 in all its components, since we construct the Ricci tensor by contracting the Riemann, Ricci tensor would be 0 in all components as well. I've read that ...
1k views

### Where does the energy of a light bulb come from?

Where does the energy of a light bulb come from? Is it from the coil of wire, magnet in the generator, mechanical input to the generator or plug where the generator is connected to the wall?
497 views

### Simple Quantum Mechanics Question about The Commutator of Translation Operators

Say there is $\hat{J} = \exp[-i \hat{p} l/ \hbar]$ and $\hat{U}= \exp[-i\hat{H}t/ \hbar]$, where $\hat{H}$ is time-independent. Can we say anything about $[\hat{J},\hat{U}]$? Is it zero? How do we ...
156 views

### Does the black hole only increase?

I don't understand this: The more mass a Black Hole has, the more gravity it create! The more gravity it has, the more mass it can get! With this lines, someday, all the universe will be a black ...
175 views

### A machine which copies any object with 100% accuracy?

Does physics allow for a machine that copies an object with 100% accuracy?
553 views

### Light-like Interval

In SR, the interval $I$ between two spacetime events is called light-like if $I=0$. Griffiths in his Introduction to Electrodynamics book says that [page 503], If $I=0$ we call the interval ...
3k views

### why does headlights dim when a starter motor is turned on

okay first of all, what exactly does the starter motor do? does it increase the current flow and decrease voltage? and does starter motor have its own internal resistance? i understand how there is a ...
148 views

### If the velocity of particle $A$ exceeds that of $B$, is the acceleration of $A$ greater than $B$?

Two particles $A, B$ are travelling along parallel straight paths. At some point, the velocity of $A$ exceeds that of $B$. Does this necessarily mean that the acceleration of $A$ is greater than the ...
668 views

since electromagnetic radiation possess the property of both wave and particle(photon). and both theory are applicable but how we have to find out that which theory is suitable or applicable in ...
157 views

### Dimension & non - locality problem in string theory

I have some questions with string theory: Why is it that there is exactly 4 large spacetime dimensions while the rest remain small? It is a nonlocal QFT. How could that fit in GR?
105 views

### Will a diamond helm be more protective against bullets?

Will a helm made entirely of diamond be more protective against bullets than standard steel or kevlar helmlets?
196 views

### How to superpose an wavefunction for an infinite potential well / interval - $-d/2 <x<d/2$

I know that two functions which describe the state of a particle in an infinite square (on interval $-d/2<x<d/2$) well are like: \begin{align} \psi_{even}&= ...
70 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 ...
3k views

### If an event happens 10 light years away and we observe it here, did it actually happen 10 years ago?

I'm sorry if this question has been answered a squillion times, I need someone with smarts to explain it to me. If an event happens 10 light years away, and we observe it here, did it actually happen ...
95 views

### Is the gravity between objects other than celestials observable?

I've always read that all matter has gravity. But, can we observe it? I mean, The Earth pulls us but what about small daily objects? For example, if we release 2 small objects in space, do they get ...