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1500 questions
84
votes
8 answers
Will a hole cut into a metal disk expand or shrink when the disc is heated?
Take a metal disc and cut a small, circular hole in the center.
When you heat the whole thing, will the hole's diameter increase or decrease? and why?
What will happen to the diameter of disc?
jojo
- 857
83
votes
4 answers
What causes the water in this fountain to reverse direction?
In a children's museum, I ran across this fountain. You can adjust the flow rate with a valve, visible at the bottom. At low flow rates, the sheet of water does more or less what you'd expect: it curves downward, eventually falling more or less…
Ben51
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83
votes
6 answers
Why does public mains power use 50-60 Hz and 100-240 V?
Is there a physical reason behind the frequency and voltage in the mains electricity? I do not want to know why exactly a certain value was chosen; I am rather interested to know why that range/order of magnitude was selected. I.e., why 50 Hz and…
SuperCiocia
- 24,596
82
votes
9 answers
How to treat differentials and infinitesimals?
In my Calculus class, my math teacher said that differentials such as $dx$ are not numbers, and should not be treated as such.
In my physics class, it seems like we treat differentials exactly like numbers, and my physics teacher even said that they…
Ovi
- 2,889
82
votes
4 answers
Why treat complex scalar field and its complex conjugate as two different fields?
I am new to QFT, so I may have some of the terminology incorrect.
Many QFT books provide an example of deriving equations of motion for various free theories. One example is for a complex scalar field:
$$\mathcal{L}_\text{compl…
BMS
- 11,493
82
votes
7 answers
Why is the Earth so fat?
I made a naive calculation of the height of Earth's equatorial bulge and found that it should be about 10km. The true height is about 20km. My question is: why is there this discrepancy?
The calculation I did was to imagine placing a ball down on…
Mark Eichenlaub
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82
votes
2 answers
Why doesn't a nucleus-like body made up of just neutrons exist?
We know that neutrons exert short ranged nuclear forces over other nucleons in a nucleus, and these forces are only attractive in nature. Also this force is universal and doesn't differentiate between charged or uncharged bodies. So why doesn't a…
Mehul
- 793
82
votes
10 answers
Can black holes form in a finite amount of time?
One thing I know about black holes is that an object gets closer to the event horizon, gravitation time dilation make it move more slower from an outside perspective, so that it looks like it take an infinite amount of time for the object to reach…
Itai Bar-Natan
- 1,383
82
votes
13 answers
Turbulent spacetime from Einstein equation?
It is well known that the fluid equations (Euler equation, Navier-Stokes, ...), being non-linear, may have highly turbulent solutions. Of course, these solutions are non-analytical. The laminar flow solutions (Couette flow for example) may be…
Cham
- 7,391
81
votes
8 answers
Why does fire make very little sound?
Sound is air particles vibrating (thus hitting each other to make longitudinal waves) and heat is the vibration of air molecules. Because we can only assume that heat made from fire is a higher intensity of vibration than sound (because we don't…
yolo
- 2,646
81
votes
10 answers
Why is light called an 'electromagnetic wave' if it's neither electric nor magnetic?
How can light be called electromagnetic if it doesn't appear to be electric nor magnetic?
If I go out to the sunlight, magnets aren't affected (or don't seem to be). And there is no transfer of electric charge/electrons (as there is in AC/DC…
makerofthings7
- 1,047
81
votes
6 answers
Why does water stop boiling immediately after turning off the heat?
When I am heating water on a gas stove, it begins to boil after some time and bubbles of air can be seen escaping out.
However, as soon as I increase the amount of heat in the stove, the rate of escape of air bubbles increases immediately, and as…
Mrigank Pawagi
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81
votes
6 answers
If a mass moves close to the speed of light, does it turn into a black hole?
I'm a big fan of the podcast Astronomy Cast and a while back I was listening to a Q&A episode they did. A listener sent in a question that I found fascinating and have been wondering about ever since.
From the show transcript:
Arunus Gidgowdusk…
shopsinc
- 929
81
votes
12 answers
What is time dilation really?
Please will someone explain what time dilation really is and how it occurs? There are lots of questions and answers going into how to calculate time dilation, but none that give an intuitive feel for how it happens.
John Rennie
- 355,118
81
votes
2 answers
Lev Landau's "Theoretical Minimum"
The great russian physicist Lev Landau developed a famous entry exam to test his students. This "Theoretical Minimum" contained everything he considered elementary for a young theoretical physicist. Despite its name, it was notoriously hard and…
jdm
- 4,207