Most Popular
1500 questions
95
votes
7 answers
Why does a billiard ball stop when it hits another billiard ball head on?
(I'm repeating myself a lot here, but it's because I want to make my confusion clear.)
If 2 billiard balls are the same exact mass, and one hits another stationary one head on, I have heard that the hitting ball will often stop entirely while the…
joshuaronis
- 3,145
95
votes
8 answers
Does juggling balls reduce the total weight of the juggler and balls?
A friend offered me a brain teaser to which the solution involves a $195$ pound man juggling two $3$-pound balls to traverse a bridge having a maximum capacity of only $200$ pounds. He explained that since the man only ever holds one $3$-pound…
adamdport
- 1,157
95
votes
1 answer
If we had a "perfectly efficient" computer and all the energy in the Milky-way available, what number could it count to?
The idea for this question comes from an example in cryptography, where supposedly 256-bit symmetric keys will be enough for all time to come (brute-forcing a 256-bit key is sort-of equivalent to counting to $2^{255}$, with some constant in front of…
cooky451
- 1,069
95
votes
5 answers
Does a gun exert enough gravity on the bullet it fired to stop it?
My question is set in the following situation:
You have a completely empty universe without boundaries.
In this universe is a single gun which holds one bullet.
The gun fires the bullet and the recoil sends both flying in opposite directions.…
JadaLovelace
- 1,449
94
votes
10 answers
Why do scientists think that all the laws of physics that apply in our galaxy apply in other galaxies?
I like watching different videos about space. I keep seeing all these videos saying scientists found so and so at 200 billion light years away or this happened 13 billion years ago.
My question is why do scientists think that all the physics that…
andre chancellor
- 845
94
votes
4 answers
Why does a yellow object turn white under a yellow light? Shouldn't it turn yellow instead?
Recently I was eating a yellow rice for lunch in a restaurant with only yellow lights. But the rice looked white! I was intrigued by this because I always thought it should look yellow since the yellow pigment reflects only yellow light, but the…
David A.
- 1,039
94
votes
7 answers
Why do metals only glow red, yellow and white and not through the full range of the spectrum?
Why don't metals glow from red to yellow to green to blue etc.? Why only red, then yellow and then white? Shouldn't all wavelengths be emitted one by one as the temperature of the metal increases?
If some metals do glow at with different colours,…
Dieblitzen
- 1,687
94
votes
9 answers
Why does matter exist in 3 states (liquids, solid, gas)?
Why does matter on the earth exist in three states? Why cannot all matter exist in only one state (i.e. solid/liquid/gas)?
Kiran Kumar
- 1,017
94
votes
7 answers
If dark matter only interacts with gravity, why doesn't it all clump together in a single point?
I'm a complete layperson. As I understand, dark matter theoretically only interacts with the gravitational force, and doesn't interact with the other three fundamental forces: weak nuclear force, strong nuclear force, and electromagnetism.
Those are…
user151841
- 1,649
94
votes
15 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
- 9,691
93
votes
9 answers
When I walk down the stairs where does my potential energy go?
When I leave my room I walk down three flights of stairs releasing about $7\,\text{kJ}$ of potential energy. Where does it go? Is it all getting dispersed into heat and sound? Is that heat being generated at the point of impact between my feet and…
A. Kriegman
- 1,303
93
votes
10 answers
Why can't the Schrödinger equation be derived?
Honestly, I don't get it. People say it's because it's a postulate. But, I mean, I see people deriving the Schrödinger equation with the help of the wave function, $T+U$ and partial differentials in three space coordinates and one time coordinate.…
anon
93
votes
3 answers
Why does the LIGO observation disprove higher dimensions?
I recently read this article which claims that last year’s LIGO observation of gravitational waves is proof that, at least on massive scales, there cannot be more than three spatial dimensions.
I don’t understand the physics fully, so could someone…
DonielF
- 972
93
votes
7 answers
Why is it "bad taste" to have a dimensional quantity in the argument of a logarithm or exponential function?
I've been told it is never seen in physics, and "bad taste" to have it in cases of being the argument of a logarithmic function or the function raised to $e$. I can't seem to understand why, although I suppose it would be weird to raise a…
sangstar
- 3,280
93
votes
1 answer
What happened to David John Candlin?
This is an ultra-soft question about relatively recent history. While reading some of Mandelstam's papers, I noticed that he cites David John Candlin consistenly whenever he does anything with Grassman path-integral. Everyone else cites Berezin.
So…
