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1500 questions
269
votes
11 answers
Don't heavier objects actually fall faster because they exert their own gravity?
The common understanding is that, setting air resistance aside, all objects dropped to Earth fall at the same rate. This is often demonstrated through the thought experiment of cutting a large object in half—the halves clearly don't fall more slowly…
ErikE
- 2,936
268
votes
5 answers
If I sliced the universe in half, would the slice go through a star?
This question is based on a discussion with a 10-year old. So if it is not clear how to interpret certain details, imagine how a 10-year old would interpret them.
This 10-year old does not know about relativistic issues, so assume that we are living…
user78574
249
votes
8 answers
How do towels stay on hooks?
Towels (and coats) are often stored on hooks, like this:
To the untrained eye, it looks like the towel will slide off from its own weight. The hook usually angles upwards slightly, but a towel does not have any "handle" to string around and hang on…
Caleb Stanford
- 1,979
247
votes
8 answers
Does Earth really have two high-tide bulges on opposite sides?
The bit that makes sense – tidal forces
My physics teacher explained that most tidal effect is caused by the Moon rotating around the Earth, and some also by the Sun.
They said that in the Earth - Moon system, the bodies are in free-fall about each…
Benjohn
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- 2
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243
votes
22 answers
What exactly is a photon?
Consider the question, "What is a photon?". The answers say, "an elementary particle" and not much else. They don't actually answer the question. Moreover, the question is flagged as a duplicate of, "What exactly is a quantum of light?" – the…
John Duffield
- 11,097
214
votes
1 answer
Strange ice found in my garden
This morning I found a really strange ice formation in my garden. I can't figure out how it appeared, because there was nothing above. The night was particularly cold (Belgium).
To give an idea, it has the size of a common mouse (5 cm of Height and…
snoob dogg
- 1,687
211
votes
15 answers
Why don't electrons crash into the nuclei they "orbit"?
I'm having trouble understanding the simple "planetary" model of the atom that I'm being taught in my basic chemistry course.
In particular,
I can't see how a negatively charged electron can stay in "orbit" around a positively charged nucleus.…
orome
- 5,087
208
votes
10 answers
If photons have no mass, how can they have momentum?
As an explanation of why a large gravitational field (such as a black hole) can bend light, I have heard that light has momentum. This is given as a solution to the problem of only massive objects being affected by gravity. However, momentum is the…
david4dev
- 2,764
206
votes
10 answers
Why is the detection of gravitational waves so significant?
LIGO has announced the detection of gravitational waves on 11 Feb, 2016. I was wondering why the detection of gravitational waves was so significant?
I know it is another confirmation of general relativity (GR), but I thought we had already…
Dargscisyhp
- 5,289
203
votes
8 answers
Why do we bend a book to keep it straight?
I noticed that I have been bending my book all along, when I was reading it with one hand.
This also works for plane flexible sheets of any material.
Illustration using an A4 sheet
Without bending the sheet:
With a bend along perpendicular…
AlphaLife
- 11,871
203
votes
4 answers
Why doesn't matter pass through other matter if atoms are 99.999% empty space?
The ghostly passage of one body through another is obviously out of the question if the continuum assumption were valid, but we know that at the micro, nano, pico levels (and beyond) this is not even remotely the case. My understanding is that the…
Bryson S.
- 3,876
201
votes
3 answers
Surviving under water in air bubble
An incredible news story today is about a man who survived for two days at the bottom of the sea (~30 m deep) in a capsized boat, in an air bubble that formed in a corner of the boat. He was eventually rescued by divers who came to retrieve dead…
Maxim Umansky
- 4,377
201
votes
7 answers
How do moving charges produce magnetic fields?
I'm tutoring high school students. I've always taught them that:
A charged particle moving without acceleration produces an electric as well as a magnetic field.
It produces an electric field because it's a charge particle. But when it is at rest,…
claws
- 7,255
199
votes
15 answers
What's the point of Hamiltonian mechanics?
I've just finished a Classical Mechanics course, and looking back on it some things are not quite clear. In the first half we covered the Lagrangian formalism, which I thought was pretty cool. I specially appreciated the freedom you have when…
Javier
- 28,106
196
votes
16 answers
Are units of angle really dimensionless?
I know mathematically the answer to this question is yes, and it's very obvious to see that the dimensions of a ratio cancel out, leaving behind a mathematically dimensionless quantity.
However, I've been writing a c++ dimensional analysis library…
Nicolas Holthaus
- 1,871