# All Questions

12k views

### Getting started self-studying general relativity

What are some good books, videos, websites for getting started with general relativity? I would prefer mathematically rigorous sources.
954 views

Take the following gedankenexperiment in which two astronauts meet each other again and again in a perfectly symmetrical setting - a hyperspherical (3-manifold) universe in which the 3 dimensions are ...
9k views

### Home experiments to derive the speed of light?

Are there any experiments I can do to derive the speed of light with only common household tools?
2k views

### How can I measure the mass of the Earth at home?

How can I measure the mass of the Earth at home? How was the mass of the Earth first measured?
594 views

### Why were the SI base quantities chosen as such?

The reasons for choosing length, mass, time, temperature, and amount as base quantities look (at least to me) obvious. What I'm puzzling about is why current (as opposed to resistance, electromotive ...
19k views

### Why don't electric fish shock themselves?

Fish like electric eels and torpedoes have specially designed nerve cells that allow them to discharge hundreds of volts of electricity. Now, while pure water is usually nonconductive, the dissolved ...
2k views

### Physics and Computer Science

Not sure if this is a 'real' question, but what is the relation between physics and computer science? A lot of physicists are also computer scientists and vice versa. My professor has a PhD in Physics ...
8k views

### The principle behind door peepholes

Hotels usually install peepholes in their doors so that a person inside a suite can see who is at the door without having to open it. I understand that there should be a convex lens within the ...
4k views

### How can I determine transmission/reflection coefficients for light?

When light rays reflect off a boundary between two materials with different indices of refraction, a lot of the sources I've seen (recently) don't discuss the relation between the amplitude (or ...
2k views

### Is electricity instantaneous?

My question is basically what exactly is electricity? I've simply been told before that it's a flow of electrons, but this seems too basic and doesn't show that electricity is instant. What I mean is ...
5k views

### Period of Precession [closed]

I'm trying to find the period of precession for a gyroscope. Now I was able to find the angular precession rate, which was 1.132 rad/s, but I have no idea how to convert this to a 'period', and google ...
322 views

### Angular Momentum and Force [closed]

I'm stuck on number 5. The answers to the first 4 are correct, but I dont know how to set up number 5. Any idea that I would have would require me having some kind of time information, but thats not ...
3k views

### Angular Momentum and Average Torque

Refer to number 6. This is the one I'm stuck on. So angular momentum is conserved right, so initial angular momentum is equal to final angular momentum. Initial is 7.87 so final must be 7.87, right? ...
6k views

### Planet orbits: what's the difference between gravity and centripetal force?

My physics teacher says that centripetal force is caused by gravity. I'm not entirely sure how this works? How can force cause another in space (ie where there's nothing). My astronomy teacher says ...
1k views

### Why can't the outcome of a QM measurement be calculated a-priori?

Quantum Mechanics is very successful in determining the overall statistical distribution of many measurements of the same process. On the other hand, it is completely clueless in determining the ...
1k views

### Particle physics getting started

I know classical physics, quantum mechanics, special relativity, basic nuclear physics. I would like to get into some particle physics. I want to get into that higgs boson, lepton, quark things :D ...
2k views

### Why did people expect the number of electrons emitted because of the photoelectric effect to rise with the radiation's intensity?

The number of electrons emitted because of the photoelectric effect rises with the frequency of the radiation is not influenced by the intensity of the radiation (As Chad points out, this is wrong, ...
2k views

### What determines the pitch of an explosion?

It's bonfire weekend here in the UK, and everybody is intent on shooting fireworks. I was wondering, what determines the pitch of the sound produced by an explosion? The sound has clearly a ...
4k views

### Why does water make a sound when it is disturbed?

When I disturb a body of water, what causes the familiar "water moving" sound?
202 views

### How do subgrid-scale parametrisations in climate models work?

Global Circulation Models typically have grids of 100-300km on a side. There are obviously lots of atmospheric processes that happen at smaller scales than this. Convection, cloud formation, the ...
4k views

### Is energy really conserved?

In high school I was taught energy was conserved. Then I learned that nuclear reactions allow energy to be converted into mass. Then I also heard that apparently energy can spontaneously appear in ...
20k views

### What really allows airplanes to fly?

What aerodynamic effects actually contribute to producing the lift on an airplane? I know there's a common belief that lift comes from the Bernoulli effect, where air moving over the wings is at ...
2k views

### How is it that angular velocities are vectors, while rotations aren't?

Does anyone have an intuitive explanation of why this is the case?
2k views

### What is the most counter-intuitive result in physics? [closed]

I think that relativity and quantum mechanics would provide some good examples.
7k views

### Why don't spinning tops fall over?

One topic which was covered in university, but which I never understood, is how a spinning top "magically" resists the force of gravity. The conservation of energy explanations make sense, but I don't ...
3k views

### Would wearing clothing that is black on the inside and white on the outside keep you cooler?

The Straight Dope ran an explanation of why nomads often wear black clothing - it absorbs heat better from the body. On the other hand, white clothing reflects sunlight better. Is it possible to get ...
830 views

### What physical forces pull/press water upwards in vegetation?

Each spring enormous amounts of water rise up in trees and other vegetation. What causes this stream upwards? Edit: I was under the impression that capillary action is a key factor: the original ...
2k views

### What determines the (minimum) angle at which a domino falls over?

Dominoes, when placed upright, remain that way. Sometimes, even if you tip them a little bit, they will go back to their upright position. However, if you tip them too far, they will fall over. ...
481 views

### Particle colliders: why do they need an accelerator chain

Particle colliders like the LHC or the Tevatron use a complex accelerator chain to have particles at a given energy before being accelerated. For example: The CERN accelerator complex to inject in ...
2k views

### The Many Body problem

(This is a simple question, with likely a rather involved answer.) What are the primary obstacles to solve the many-body problem in quantum mechanics? Specifically, if we have a Hamiltonian for a ...
9k views

### What property of objects allow them to float?

I used to think that the shape of an object determines its ability to float (boat-shaped objects are more likely to float, and spheres tend to sink). But my friend, who is fond of making me look ...
2k views

### Finding the volume of this irregular shape I have

I have an approximately basketball-sized non-hollow piece of aluminum sitting in my house that is of irregular shape. I need to find the volume of it for a very legitimate yet irrelevant reason. ...
1k views

### How does Positronium exist?

I've just recently heard of Positronium, an "element" with interesting properties formed by an electron and positron, and I was shocked to hear that physicists were actually working with this element, ...
6k views

### How should a physics student study mathematics? [closed]

Note: I will expand this question with more specific points when I have my own internet connection and more time (we're moving in, so I'm at a friend's house). This question is broad, involved, and ...
749 views

### acceleration of rings in aerotrim (human gyroscope)

I'm working on a graphic simulation (just for fun, for an open-source screensaver) of an Aerotrim - a "human gyroscope", one of those exercise/training machines with a human in the middle, perched on ...
2k views

### Software for physics calculations

What is some good free software for doing physics calculations? I'm mainly interested in symbolic computation (something like Mathematica, but free).
722 views

### What are insightful/impressive experiments for adults [closed]

Similar to this question: what experiements do you think are impressive for layman adults? I for example like how you can simulate an Einstein Ring with the bottom of a wine glass.
2k views

### Why can you “suck in” cooked spaghetti?

We all know that there is no "sucking", only pushing. So how are cooked spaghetti pushed into your mouth? The air pressure applies orthogonal on the spaghetti surface. Where does the component ...
6k views

### Would it help if you jump inside a free falling elevator?

Imagine you're trapped inside a free falling elevator. Would you decrease your impact impulse by jumping during the fall? When?
2k views

### Is the distance between the sun and the earth increasing?

M = mass of the sun m = mass of the earth r = distance between the earth and the sun The sun is converting mass into energy by nuclear fusion. \$F = \frac{GMm}{r^2} = \frac{mv^2}{r} \rightarrow r ...
4k views

### Fourier transformation in nature/natural physics?

I just came from a class on Fourier Transformations as applied to signal processing and sound. It all seems pretty abstract to me, so I was wondering if there were any physical systems that would ...
547 views

### Do current models of particle physics explain the chemical properties of elements/compounds?

I have a particle system of seven protons and seven (or sometimes eight) neutrons (each formed by their appropriate quarks, etc.) bound together in a state that can be macroscopically described as a ...
2k views

### How are forces “mediated”?

I hope this is the right word to use. To me, these forces seem kind of fanciful (except for General Relativity and Gravity, which have a geometric interpretation). For example, how do two charged ...
2k views

### What are some scenarios where FTL information transfer would violate causality?

I've always heard people saying, "Faster than light information transfer can't happen because it would violate causality! Effects can precede their causes!" However, I'm trying to think of a ...
14k views

### Best books for mathematical background?

What are the best textbooks to read for the mathematical background you need for modern physics, such as, string theory? Some subjects off the top of my head that probably need covering: ...
2k views

### What is the mechanism of magnetic core saturation?

Why does a magnetic core saturate? What is its actual mechanism?
2k views

### Finding angular acceleration from torque

We have to analyze this video Givens: An applied net torque due to the wind on the windmill is equal to 1500 N*m. Each (of the 3) propeller props weighs approximately 45 Kg and has a Moment of ...
2k views

### What is needed to claim the discovery of the Higgs boson?

As I understand the Higg's boson can be discovered by the LHC because the collisions are done at an energy that is high enough to produce it and because the luminosity will be high enough also. But ...