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1500 questions
59
votes
6 answers
Why does cold metal seem colder than cold air?
(I apologize for this elementary question. I don't know much about physics.)
Let's say that I put a metal pot in the refrigerator for several hours.
At this point, I guess, the pot and the air (in the refrigerator) have the same temperature.
Now, I…
Niccolo M.
- 693
58
votes
6 answers
Why are rockets so big?
I'm curious why rockets are so big in their size. Since both the gravitational potential one need to overcome in order to put thing into orbit, and the chemical energy burned from the fuel, are proportional to the mass, so if we shrink the rocket…
xslittlegrass
- 873
58
votes
7 answers
Photons from stars--how do they fill in such large angular distances?
It would seem that far-away stars are at such a distance that I should be able to take a step to the side and not have the star's photons hit my eye. How do stars release so many photons to fill in such great angular distances?
Shookster
- 1,662
58
votes
3 answers
Why doesn't oil produce sound when poured?
I've noticed that unlike other liquids, when pouring olive oil for example, I don't hear any sound at all from it.
Usually you can hear an audible sound as a cup gets filled with water, as the sound increases in pitch.
What makes the oil behave this…
Moshe
- 731
58
votes
4 answers
How strong is the force of ice expanding when freezing?
Why does water contract on melting whereas gold, lead, etc. expand on melting? reminded me about something I've been wondering myself for some time.
We know that water expands as it freezes. The force is quite formidable - it can cause solid steel…
Vilx-
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58
votes
7 answers
If magnetic field lines don't exist, what are these iron filings doing around a magnet?
Obviously the iron filings can be seen aligning themselves along the virtual magnetic field lines produced by the permanent magnet, the virtual magnetic field line is made of electromagnetic field due to the alignment of electrons in the magnet but…
user6760
- 12,980
58
votes
15 answers
When a balloon pops and lets a brick fall, where does the energy come from?
Let's say a scientist attaches a 1 kg brick to a large helium inflated balloon, lets the balloon go, and then it reaches an altitude of 10 000 meters before it pops, dropping the brick.
The brick falls and hits the ground with with a kinetic energy…
CaptainCodeman
- 1,286
58
votes
14 answers
Why is cold fusion considered bogus?
Cold fusion is being mentioned a lot lately because of some new setup that apparently works. This is an unverified claim.
See for…
Sklivvz
- 13,499
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- 87
58
votes
5 answers
Why did the gamma ray burst from GW170817 lag two seconds behind the gravitational wave?
The ABC, reporting on the announcement of gravitational wave GW170817, explained that for the first time we could identify the precise source of a gravitational wave because we also observed the event in the electromagnetic spectrum. It notes…
curiousdannii
- 956
58
votes
12 answers
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?
Justin L.
- 5,980
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58
votes
2 answers
Does entropy depend on the observer?
Entropy as it is explained on this site is a Lorentz invariant. But, we can define it as a measure of information hidden from an observer in a physical system.
In that sense, is entropy a relative quantity depending on the computation, measurement…
veronika
- 2,706
58
votes
7 answers
Cyclist's electrical tingling under power lines
It's been happening to me for years. I finally decided to ask users who are better with "practical physics" when I was told that my experience – that I am going to describe momentarily – prove that I am a diviner, a psychic, a "sensibil" as we call…
Luboš Motl
- 179,018
58
votes
8 answers
Given that ice is less dense than water, why doesn't it sit completely atop water (rather than slightly submerged)?
E.g.
If we had a jar of marbles or something else of different densities and shook it, the most dense ones would go to the bottom and the less dense ones to the top.
(Image Source)
If I put a cube of lead in water it would sink all the way to the…
K-Feldspar
- 1,357
58
votes
13 answers
Why and how is the speed of light in vacuum constant, i.e., independent of reference frame?
I was told that the Galilean relative velocity rule does not apply to the speed of light. No matter how fast two objects are moving, the speed of light will remain same for both of them.
How and why is this possible?
Also, why can't anything…
SMUsamaShah
- 5,307
58
votes
11 answers
What is the logarithm of a kilometer? Is it a dimensionless number?
In log-plots a quantity is plotted on a logarithmic scale. This got me thinking about what the logarithm of a unit actually is.
Suppose I have something with length $L = 1 \:\mathrm{km}$.
$\log L = \log \mathrm{km}$
It seems that the unit of $\log…
Statec
- 715