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1500 questions
86
votes
14 answers
What is the difference between "kinematics" and "dynamics"?
I have noticed that authors in the literature sometimes divide characteristics of some phenomenon into "kinematics" and "dynamics".
I first encountered this in Jackson's E&M book, where, in section 7.3 of the third edition, he writes, on the…
nibot
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85
votes
15 answers
What are examples of Lagrangians that not of the form $T-U$?
My Physics teacher was reluctant to define Lagrangian as Kinetic Energy minus Potential Energy because he said that there were cases where a system's Lagrangian did not take this form. Are you are aware of any such examples?
Update: Here I'm of…
ZAC
- 1,297
85
votes
3 answers
What does general relativity say about the relative velocities of objects that are far away from one another?
What does general relativity say about the relative velocities of objects that are far away from one another? In particular:--
Can distant galaxies be moving away from us at speeds faster than $c$? Can cosmological redshifts be analyzed as Doppler…
user4552
85
votes
8 answers
Why is information indestructible?
I really can't understand what Leonard Susskind means when he says in the video Leonard Susskind on The World As Hologram that information is indestructible.
Is that information that is lost, through the increase of entropy really recoverable?
He…
HDE
- 2,899
85
votes
4 answers
If you are vacuuming your carpet and you wrap the cord around your body do you become a magnet?
If you wrap an active electric cord around your body, do you become an electromagnet?
Shawn Anderson
- 890
85
votes
2 answers
Why don't fluorescent lights produce shadows?
I have watched light sources such as incandescent lamps and other lamp sources; they have always made shadows. But a fluorescent lamp doesn't make any shadow. What is the reason behind the non-appearance of prominent shadow?
natarajan physicist
- 1,007
85
votes
9 answers
What is the physical significance of dot & cross product of vectors? Why is division not defined for vectors?
I get the physical significance of vector addition & subtraction. But I don't understand what do dot & cross products mean?
More specifically,
Why is it that dot product of vectors $\vec{A}$ and $\vec{B}$ is defined as $AB\cos\theta$?
Why is it…
claws
- 7,255
85
votes
11 answers
Does centrifugal force exist?
Currently in my last year of high school, and I have always been told that centrifugal force does not exist by my physics teachers. Today my girlfriend in the year below asked me what centrifugal force was, I told her it didn't exist, and then she…
Dylan
- 1,033
84
votes
1 answer
Why do chimneys have these spiral "wings"?
While walking around I noticed something very peculiar. Many chimneys had spiral "wings", while others didn't. I came up with two possibilities:
The wind circles around the chimney upwards which pushes whatever gases being released even higher into…
Adi
- 1,628
84
votes
24 answers
List of freely available physics books
I'm trying to amass a list of physics books with open-source licenses, like Creative Commons, GPL, etc. The books can be about a particular field in physics or about physics in general.
What are some freely available great physics books on the…
pcr
- 1,103
84
votes
6 answers
Why does a rubber band become a lighter color when stretched?
I was stretching a pink colored rubber band, and I noticed that the longer I stretch it, the lighter the pink becomes.
Why does this happen?
wavion
- 1,278
- 6
- 19
84
votes
4 answers
What is the mechanism behind the slowdown of light/photons in a transparent medium?
So light travels slower in glass (for example) than in a vacuum. What causes light to slow down? Or: How does it slow down? If light passes through the medium, is it not essentially traveling in the "vacuum" between the atoms?
Henry
- 1,043
84
votes
5 answers
Why doesn't water actually perfectly wet glass?
According to many high school textbook sources, water perfectly wets glass. That is, the adhesion between water and glass is so strong that it is energetically favorable for a drop of water on glass to spread out and coat the entire surface.…
knzhou
- 101,976
84
votes
3 answers
Can photons be detected without being absorbed?
I am thinking about a detector that would beep if light passes through it. Is it possible?
Arik
- 841
84
votes
4 answers
Where does the extra kinetic energy come from in a gravitational slingshot?
I read in this answer in this site that the KE a free-falling ball acquires is not originated by the attracting body but that energy was actually stored in the ball when it had been lifted to the height it dropped from.
In this way, it was said,…
Cory Baumer
- 851