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691
votes
21answers
236k views

Cooling a cup of coffee with help of a spoon

During the 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 ...
440
votes
4answers
53k views

Can I compute the mass of a coin based on the sound of its fall?

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 ...
405
votes
6answers
47k views

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 ...
389
votes
16answers
39k views

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. It would seem to ...
316
votes
25answers
111k views

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?
313
votes
1answer
76k views

Book recommendations [closed]

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. ...
284
votes
31answers
41k views

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 $c$...
276
votes
6answers
37k views

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?
271
votes
10answers
52k views

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 ...
271
votes
11answers
38k views

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 ...
268
votes
2answers
24k views

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?
267
votes
2answers
22k views

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 ...
245
votes
14answers
69k views

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 ...
239
votes
5answers
29k views

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 ...
235
votes
11answers
40k views

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-...
231
votes
11answers
53k views

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 ...
228
votes
8answers
35k views

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 ...
223
votes
16answers
57k views

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 ...
201
votes
19answers
32k views

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, "...
199
votes
9answers
19k views

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 ...
197
votes
1answer
16k views

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 ...
197
votes
3answers
48k views

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 ...
191
votes
8answers
32k views

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 and ...
175
votes
8answers
13k views

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 ...
172
votes
16answers
16k views

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 ...
172
votes
1answer
85k views

Why does NASA use gold foil on equipment and gold-coated visors?

I've read several websites about equipment covered with gold foil and astronaut helmet visors are coated with gold. However, their explanations are devoid of almost all physics content. Can someone ...
171
votes
3answers
65k views

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 ...
168
votes
2answers
17k views

When separating an Oreo cookie, why does the cream stick to just one side only?

There is probably some reason for this, but I can't figure out what it is. I agree that it probably doesn't happen 100% of the time, but most all of the time, the cream is clinging to just one of the ...
164
votes
10answers
223k views

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 ...
163
votes
3answers
36k views

How does a knife cut things at the atomic level?

As the title says. It is common sense that sharp things cut, but how do they work at the atomical level?
163
votes
9answers
20k views

Could Legolas actually see that far?

The video “How Far Can Legolas See?” by MinutePhysics recently went viral. The video states that although Legolas would in principle be able to count $105$ horsemen $24\text{ km}$ away, he shouldn't ...
158
votes
7answers
12k views

Why do we actually see the sun?

I haven't yet gotten a good answer to this: If you have two rays of light of the same wavelength and polarization (just to make it simple for now, but it easily generalizes to any range and all ...
156
votes
6answers
23k views

Why do shadows from the sun join each other when near enough?

I was laying on my bed, reading a book when the sun shone through the windows on my left. I happened to look at the wall on my right and noticed this very strange effect. The shadow of my elbow, when ...
154
votes
11answers
68k views

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 ...
154
votes
13answers
23k views

Why does space expansion not expand matter?

I have looked at other questions on this site (e.g. "why does space expansion affect matter") but can't find the answer I am looking for. So here is my question: One often hears talk of space ...
153
votes
15answers
56k views

How do I explain to a six year old why people on the other side of the Earth don't fall off? [closed]

Today a friend's six year old sister asked me the question "why don't people on the other side of the earth fall off?". I tried to explain that the Earth is a huge sphere and there's a special force ...
151
votes
3answers
32k views

Why does Stephen Hawking say black holes don't exist?

Recently, I read in the journal Nature that Stephen Hawking wrote a paper claiming that black holes do not exist. How is this possible? Please explain it to me because I didn't understand what he ...
150
votes
2answers
28k views

Why do sunbeams diverge even though the sun is much more than a few kilometers away?

Consider this picture of sun beams streaming onto the valley through the clouds. Given that the valley is only (at a guess) 3km wide, with simple trigonometry and the angles of the beams, this gives ...
148
votes
25answers
21k views

Simple check for the global shape of the Earth

I have been on a date recently, and everything went fine until the moment the girl has told me that the Earth is flat. After realizing she was not trolling me, and trying to provide her with a couple ...
145
votes
9answers
23k views

Why are the wet patches on these floor tiles circular?

My friend's 3-year-old daughter asked "Why are there circles there?" It had either rained the night before or frost had thawed. What explains the circles? Follow-up question: Ideally, are these ...
144
votes
10answers
43k views

Why is my hand not burned by the air in an oven at 200 °C?

I have this problem from University Physics with Modern Physics (13th Edition): The inside of an oven is at a temperature of 200 °C (392 °F). You can put your hand in the oven without injury as ...
144
votes
2answers
20k views

Why do we not have spin greater than 2?

It is commonly asserted that no consistent, interacting quantum field theory can be constructed with fields that have spin greater than 2 (possibly with some allusion to renormalization). I've also ...
143
votes
3answers
11k views

Why are the harmonics of a piano tone not multiples of the base frequency?

I was trying to figure out which piano keys were being played in an audio recording using spectral analysis, and I noticed that the harmonics are not integer multiple of the base note. What is the ...
142
votes
8answers
24k views

Why is nuclear waste more dangerous than the original nuclear fuel?

I know the spent fuel is still radioactive. But it has to be more stable than what was put in and thus safer than the uranium that we started with. That is to say, is storage of the waste such a big ...
141
votes
10answers
14k views

What makes a theory “Quantum”?

Say you cook up a model about a physical system. Such a model consists of, say, a system of differential equations. What criterion decides whether the model is classical or quantum-mechanical? None ...
140
votes
2answers
15k views

Why does a mirror split my laser beam?

Last night my daughter was asking why a mirror "always does that" (referring to reflecting a spot of light). To help her figure it out, I grabbed my green laser pointer so she could see the light ...
140
votes
8answers
16k views

Is $\pi^2 \approx g$ a coincidence?

In spite of their different dimensions, the numerical values of $\pi^2$ and $g$ in SI units are surprisingly similar, $$\frac{\pi^2}{g}\approx 1.00642$$ After some searching, I thought that this ...
139
votes
3answers
21k views

Why are the windows of bridges of ships always inclined?

What is the reason that the windows of ships' bridges are always inclined as shown in the above picture?
137
votes
7answers
176k views

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 ...
137
votes
5answers
16k views

Gauge symmetry is not a symmetry?

I have read before in one of Seiberg's articles something like, that gauge symmetry is not a symmetry but a redundancy in our description, by introducing fake degrees of freedom to facilitate ...

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