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54
votes
8answers
13k views

Why does holding something up cost energy while no work is being done?

I read the definition of work as $$W ~=~ \vec{F} \cdot \vec{d}$$ $$\text{ Work = (Force) $\cdot$ (Distance)}.$$ If a book is there on the table, no work is done as no distance is covered. If I ...
17
votes
3answers
1k views

Why does light of high frequency appear violet?

When people are asked to match monchromatic violet light with an additive mix of basic colours, they (paradoxically) mix in red. In fact, the CIE 1931 color space chromaticity diagram shows this ...
34
votes
3answers
11k views

Why is the sky not purple?

I realise the question of why this sky is blue is considered reasonably often here, one way or another. You can take that knowledge as given. What I'm wondering is, given that the spectrum of ...
18
votes
8answers
3k views

What's the difference between running up a hill and running up an inclined treadmill?

Clearly there will be differences like air resistance; I'm not interested in that. It seems like you're working against gravity when you're actually running in a way that you're not if you're on a ...
17
votes
2answers
4k views

How does space affect the human body (no space suit, no space craft)

How does "outer space" affect the human body? Some movies show it as the body exploding, imploding or even freezing solid. I know space is essentially a vacuum with 0 pressure and the dispersion of ...
43
votes
7answers
6k views

Cyclist's electrical tingling under power lines

It's been happening to me for years. I finally decided to ask users who are better with "practical physics" when I was told that my experience – that I am going to describe momentarily – prove that I ...
8
votes
1answer
3k views

Can microwaves affect WiFi?

I listen to the radio via my iPad with wifi. When I switch the microwave oven on, the radio cuts out. When the microwave oven is finished, the radio comes back on. (This is 100% reproducible!) So - ...
10
votes
4answers
3k views

Do eyeballs exhibit chromatic aberration?

Fairly straightforward question. If not, why not? I suspect that if they do, it is not perceived due to the regions of highest dispersion being in one's region of lowest visual acuity.
12
votes
2answers
29k views

Effects of a very large magnetic field on the human body

Ever since reading about the NHMFL I have always wondered about this and asked several people without getting a good satisfactory answer. My question is, considering the simplest case let's say a ...
14
votes
2answers
3k views

How strong of magnetic field would noticibly attract a person?

There is Iron in blood. Iron is magnetic. Roughly how strong would a magnet have to be to induce a noticeable attraction? It would be nice to know this for several distances. Also, do electromagnets ...
2
votes
4answers
540 views

What was the motivation behind suggesting the trichromatic theory of vision?

Background In this thread, I asked whether it is true that the colors red, green and blue, through additive mixture, can make up any color. Turns out they can't. However, when reading about the ...
13
votes
1answer
514 views

Is colour, as represented using primary colours, accurate only to humans?

Slightly biological, hopefully physical enough to be answered. Suppose a magenta hue is represented by a mix of red and blue pigment. This is all very well for a creature with red and blue ...
9
votes
3answers
10k views

Does the human body have a resonant frequency? If so, how strong is it?

Inspired by this question on Music beta SE, I'm wondering if the human body has a strong resonant frequency. I guess the fact that it's largely a bag of jelly would add a lot of damping to the system, ...
16
votes
6answers
11k views

Google interview riddle and scaling arguments

I am puzzled by a riddle to which I have been told the answer and I have loads of difficulties to believe in the result. The riddle goes as follows: "imagine you are shrunk to the size of a coin ...
15
votes
4answers
3k views

What does Peter Parkers formula represent?

Okay, so the trailer for the new Spider Man movie is out and appearently our friendly physicist from the neightborhood came up with something. However I can't find out what this is. Transcription: ...
4
votes
2answers
588 views

Human perception of distance

When we see things around us, distant objects look smaller to our eyes than nearby objects do. Is there any physics-related reason why our eyes or brain perceive things like this? Or if this is ...
7
votes
2answers
860 views

What physical forces pull/press water upwards in vegetation?

Each spring enormous amounts of water rise up in trees and other vegetation. What causes this stream upwards? Edit: I was under the impression that capillary action is a key factor: the original ...
1
vote
2answers
594 views

What happens to body chemistry at the speed of light? [duplicate]

Assume that I'm traveling at the speed of light in one direction. My brain is also traveling at the speed of light in that direction. Presumably there is at least one receptor site in my brain that is ...
7
votes
1answer
2k views

What are the characteristics of the magnetic field surrounding a human brain?

The human brain is said to produce a magnetic field resulting from the action potentials released inside the brain. What's the nature of such a field in terms of size and strength, and what is the ...
3
votes
4answers
4k views

Is it only red, green and blue that can make up any color through additive mixture?

I'm reading about color vision and have some trouble understanding the motivation for why the trichromatic theory was suggested in the first place. The book I'm reading ("Psychgology: The science of ...
0
votes
2answers
2k views

Why the pressure of atmosphere doesn't crush you when you e.g. walk outside?

Why the pressure of atmosphere doesn't crush you when you e.g. walk outside? I mean the density of air is $1.26 kg/m^3$, so with $100 km$ above us, it exerts much pressure on you when you walk ...
56
votes
3answers
6k views

How can ants carry items much heavier than themselves?

This morning I saw an ant and suddenly a question came to my mind: how do ants actually carry items much heavier than themselves? What's the difference (in physics) between us and them?
26
votes
4answers
2k views

The physical definition of work seems paradoxical

So this is possibly a misunderstanding of the meaning of work, but all the Physics texts, sites, and wiki that I've read don't clear this up for me: In the simplest case with the simplest statement, ...
16
votes
2answers
1k views

Why can't Humans run any faster?

If you wanted to at least semi-realistically model the key components of Human running, what are the factors that determine the top running speed of an individual? The primary things to consider would ...
13
votes
5answers
999 views

Octave equivalence: biological or more?

I'm a graduate student in mathematics doing a bit of research in signal processing and Fourier analysis and I've come across a question that could probably be better answered by a physicist: Is the ...
8
votes
3answers
2k views

How are the calories in food calculated?

This is intended to be a fun question. Calorimetry used for calculating the heat generated from chemical changes has been around for centuries, however, I suspect the process for calculating food ...
6
votes
2answers
425 views

Quantum Computing and Animal Navigation

Someone sent me this link to a talk by Prof. Klaus Schulten from the University of Illinois: (my emphasis) Quantum Computing and Animal Navigation Quantum computing is all the rage nowadays. ...
4
votes
2answers
16k views

How efficient is the human body?

This question sort of comes to mind when hearing how efficient an internal combustion engine is turning chemical energy in mechanical energy (something like 20-40%) with lots of excess heat. As an ...
18
votes
1answer
1k views

How long would it take for a smelly object to evaporate?

This question is a follow on from this deleted one: http://physics.stackexchange.com/q/177894/26076 as I was writing what I thought to be a valid physics answer to it. Version 1 of this question ...
6
votes
1answer
491 views

Do human bodies give off a consistent but unique radiation/electromagnetic/energy signature?

Is there any facet of the energy emitted by a human body that is consistent and unique - like a fingerprint, but a signal that could be detected by a remote device?
4
votes
2answers
1k views

Why does adding red light with blue light give purple light?

Our eyes contain 3 photoreceptor cells (cones) to perceive three wavelength ranges of light. Here is a visual representation of the wavelengths by these receptors (S, M and L). So if we have light ...
3
votes
1answer
602 views

Is it dangerous if one elementary particle with high energy penetrates our brain?

We might be killed if a bullet penetrates our brain. How about an elementary particle moving with high energy penetrates our brain? Assume that we can have exactly a single elementary particle for ...
2
votes
2answers
147 views

Order of magnitude estimation for some intriguing questions

The physics TA showed us a few examples in which one can estimate many things from first principles, sound logic, and scaling arguments. This led usually to understanding of why some numbers have the ...
8
votes
2answers
5k views

How can an an ant lift 50 times its own weight and pull 30 times its weight?

According to many sites like this one, an ant can apparently lift 50 times its own weight and pull 30 times its weight. Is it true? Can it be proved using physics? Though most sites agree that an ...
6
votes
2answers
191 views

While holding an object, no work done but costs energy (in response to a similar question)

I read the answer to Why does holding something up cost energy while no work is being done? and wanting to know more, I asked my teacher about it without telling him what I read here. Instead of ...
6
votes
1answer
4k views

Adverse Health Effects of Strong Radio Waves

A while ago, I was hiking near the Hollywood Sign in Los Angeles. When I got to the sign, I found out it was fenced off; with several signs and a security camera promising prompt enforcement. As I was ...
2
votes
1answer
298 views

Would there be EMF induced in our body due to electromagnetic radiations?

The experiments of innovative Faraday and Joseph Henry in USA, conducted around 1830, demonstrated conclusively that electric currents were induced in closed coils when subjected to changing magnetic ...
1
vote
1answer
3k views

How does the height of a jump change if the body is scaled down?

This is a physics question that appeared in the movie 'The Internship.' If you are reduced to the size of a coin and put into a blender what would you do? Apparently the answer was that with a ...
2
votes
1answer
3k views

How to calculate calorie expidenture from lifting a weight?

Am I correct to say that when a human lifts a dumbbell from, say, 2 feet off the ground to 6 feet off the ground, he will have increased the potential energy of the weight and thus will have burned ...
1
vote
2answers
430 views

Was the early Earth radioactive?

I've been reading of the (surprising) fact we are uncertain on whether there is nuclear fission in the center of the Earth or not (yet we know so much detail on structures at the other end of the ...
1
vote
1answer
2k views

What is physics of a normal jump?

What is the physics behind a normal human jump ? when a normal human wants to jump. First, some energy is stored in their thighs and The elastic tendons Just like a spring. In mechanics and ...
1
vote
2answers
354 views

Running: Determine how much more energy is needed per extra kilogram of weight

(I recently asked this on maths but was directed here) I have recently become a runner and having a keen interest in kinematics I'm very interested in the maths/physics of my running. Can someone ...
0
votes
3answers
248 views

How to generate Red light from a Blue-Dominated Spectrum [closed]

I have a white LED light source, the spectrum of which looks as follows --- I want to filter out red light from this white LED source. But as you can see the spectrum dwindles in the red zone. And ...