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1500 questions
809
votes
24 answers
Cooling a cup of coffee with help of a spoon
During breakfast with my colleagues, a question popped into my head:
What is the fastest method to cool a cup of coffee, if your only available instrument is a spoon?
A qualitative answer would be nice, but if we could find a mathematical model or…
fortran
- 7,807
531
votes
8 answers
Can I compute the mass of a coin based on the sound of its fall?
The other day, I bumped my bookshelf and a coin fell down. This gave me an idea. Is it possible to compute the mass of a coin, based on the sound emitted when it falls?
I think that there should be a way to do it. But how?
Vinicius L. Beserra
- 5,207
495
votes
6 answers
How does light bend around my finger tip?
When I close one eye and put the tip of my finger near my open eye, it seems as if the light from the background image bends around my finger slightly, warping the image near the edges of my blurry fingertip.
What causes this? Is it the heat from my…
Daniel A.A. Pelsmaeker
- 4,709
490
votes
21 answers
How does gravity escape a black hole?
My understanding is that light can not escape from within a black hole (within the event horizon). I've also heard that information cannot propagate faster than the speed of light. I assume that the gravitational attraction caused by a black hole…
Nogwater
- 5,019
397
votes
8 answers
Did the Big Bang happen at a point?
TV documentaries invariably show the Big Bang as an exploding ball of fire expanding outwards. Did the Big Bang really explode outwards from a point like this? If not, what did happen?
John Rennie
- 355,118
372
votes
29 answers
Why are mirror images flipped horizontally but not vertically?
Why is it that when you look in the mirror left and right directions appear flipped, but not the up and down?
Arlen
- 3,907
342
votes
34 answers
Do we know why there is a speed limit in our universe?
This question is about why we have a universal speed limit (the speed of light in vacuum). Is there a more fundamental law that tells us why this is?
I'm not asking why the speed limit is equal to $c$ and not something else, but why there is a limit…
TheQuantumMan
- 7,664
323
votes
11 answers
What experiment would disprove string theory?
I know that there's big controversy between two groups of physicists:
those who support string theory (most of them, I think)
and those who oppose it.
One of the arguments of the second group is that there's no way to disprove the correctness of…
Albert
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311
votes
2 answers
What is Chirped Pulse Amplification, and why is it important enough to warrant a Nobel Prize?
The 2018 Nobel Prize in Physics was awarded recently, with half going to Arthur Ashkin for his work on optical tweezers and half going to Gérard Mourou and Donna Strickland for developing a technique called "Chirped Pulse Amplification".
In general,…
Emilio Pisanty
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306
votes
18 answers
Why does kinetic energy increase quadratically, not linearly, with speed?
As Wikipedia says:
[...] the kinetic energy of a non-rotating object of mass $m$ traveling at a speed $v$ is $\frac{1}{2}mv^2$.
Why does this not increase linearly with speed? Why does it take so much more energy to go from $1\ \mathrm{m/s}$ to…
Generic Error
- 3,187
304
votes
2 answers
Why do ballpoint pens write better on pages that have pages below them?
If I write on the starting page of a notebook, it will write well. But when there are few or no pages below the page where I am writing, the pen will not write well. Why does this happen?
Ram Keswani
- 2,151
304
votes
1 answer
Resource recommendations
Every once in a while, we get a question asking for a book or other educational reference on a particular topic at a particular level. This is a meta-question that collects all those links together. If you're looking for book recommendations, this…
David Z
- 76,371
302
votes
10 answers
Why don't metals bond when touched together?
It is my understanding that metals are a crystal lattice of ions, held together by delocalized electrons, which move freely through the lattice (and conduct electricity, heat, etc.).
If two pieces of the same metal are touched together, why don't…
jcw
- 2,681
288
votes
17 answers
What really allows airplanes to fly?
What aerodynamic effects actually contribute to producing the lift on an airplane?
I know there's a common belief that lift comes from the Bernoulli effect, where air moving over the wings is at reduced pressure because it's forced to travel further…
David Z
- 76,371
276
votes
12 answers
Why are four-legged chairs so common?
Four-legged chairs are by far the most common form of chair. However, only three legs are necessary to maintain stability whilst sitting on the chair. If the chair were to tilt, then with both a four-legged and three-legged chair, there is only one…
Karnivaurus
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