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1500 questions
37
votes
3 answers
Why is rock or metal often cold to the touch but wood or plastic is not?
In a room at normal room temperature, certain materials, such as metal, glass, ceramic, or rock, will feel cold to the touch, but others, such as wood or plastic, do not so much.
Which physical properties do the former materials have in common that…
CPlus
- 928
37
votes
7 answers
Can spinning arms really help you balance on the edge of a cliff?
This is how many cartoons depict a character about to fall from the edge of a cliff. You can see movie characters do it. You can also see real people doing it. But does spinning arms like that actually help you to gain balance?
It would, ( through…
AlphaLife
- 11,871
37
votes
6 answers
What is canonical momentum?
What does the canonical momentum $\textbf{p}=m\textbf{v}+e\textbf{A}$ mean? Is it just momentum accounting for electromagnetic effects?
cpc333
- 1,137
37
votes
5 answers
Redshifting of Light and the expansion of the universe
So I have learned in class that light can get red-shifted as it travels through space. As I understand it, space itself expands and stretches out the wavelength of the light. This results in the light having a lower frequency which equates to…
QEntanglement
- 3,831
37
votes
4 answers
Why do we need insulation material between two walls?
Consider a slab made of two walls separated by air. Why do we need insulation material between the two walls. Air thermal conductivity is lower than most thermal conductivities of insulating material and convection cannot be an issue in the enclosed…
Shaktyai
- 1,849
37
votes
9 answers
How can a simple physical situation give rise to two different possible outcomes?
An object of mass $5\, \mathrm{kg}$ is projected with a velocity $20\, \mathrm{ms}^{-1}$ at an angle $60^{\circ}$, to the horizontal. At the highest point of its path, the projectile explodes and breaks up into two fragments of masses $1\,…
Tanisha Daharwal
- 487
37
votes
1 answer
Why can you see virtual images?
In optics it is widely mentioned real images are projectable onto screens whereas virtual ones can only be seen by a person. Isn't that contradictory? I mean in order to see the virtual image it has to be projected onto the retina (ultimately…
wnrph
- 1,287
37
votes
3 answers
Why does the metric system use "kilogram" as a base SI unit?
SI system uses all (that I know) measurement basic units as 1 (single) instance: meter, second, Ampère, etc, except for the KILOgram. It already defined with 1000 multiplier (kilo).
It prevents from using usual multiplier prefixes: mega, giga, tera,…
Sasha
- 497
37
votes
7 answers
When I put my hand on a hot solid why don't the particles transfering heat to my hand exert a force on it?
When I put my hand on a hot metal (say) solid, I can feel my hand heating up. I suspect this is caused mostly by particles (electrons, atoms, ...?) from the solid colliding with the particles that make up my hand thereby transferring kinetic energy…
Damian Birchler
- 545
37
votes
6 answers
Must observables be Hermitian only because we want real eigenvalues, or is more to that?
Because (after long university absence) I recently came across field operators again in my QFT lectures (which are not necessarily Hermitian):
What problem is there with observables represented by non-Hermitian operators (by observables, I obviously…
Quantumwhisp
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37
votes
6 answers
Is Nm the same unit of torque as mN?
A couple of days ago, I noticed that the torque unit used by my teachers is $mN$, and while reading on the internet it came to my notice that in all textbooks the official unit is $Nm$.
I asked one teacher about it and he insisted that I'm wrong,…
Abdullah Alhussni
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37
votes
3 answers
What is the current status of string theory (2013)?
I've seen a bunch of articles talking about how new findings from the LHC seem to disprove (super)string theory and/or supersymmetry, or at least force physicists to reformulate them and change essential predictions.
Some examples:
Did the Large…
aditsu
- 487
37
votes
8 answers
Why is it apparently not dangerous to fire a shotgun (such as when "skeet shooting") into the air?
If you fire a gun or rifle into the air, whether straight up or at an angle, as I understand physics, a metal projectile will gain surprising momentum on its way down again, more than capable of killing a grown human being, not to mention small…
user12989506
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37
votes
2 answers
What is (meant by) a non-compact $U(1)$ Lie group?
In John Preskill's review of monopoles he states on p. 471
Nowadays, we have another way of understanding why electric charge is quantized. Charge is quantized if the electromagnetic $U(l)_{\rm em}$ gauge group is compact. But $U(l)_{\rm em}$ is…
twistor59
- 16,746
37
votes
5 answers
Why is a leading digit not counted as a significant figure if it is a 1?
Reading the book Schaum's Outline of Engineering Mechanics: Statics I came across something that makes no sense to me considering the subject of significant figures:
I have searched and saw that practically the same thing is said in another book…
Vinicius ACP
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