Hot answers tagged

91

Yes gravity can kill you because as you approach something super dense like a black hole, the gravity will change with the square of the distance which means that eventually the gravity at your feet would become significantly larger than at your head. This gravitational gradient is referred to as tidal forces and is the same effect that keeps the same side ...


66

While you do spend some body energy to keep the book lifted, it's important to differentiate it from physical effort. They are connected but are not the same. Physical effort depends not only on how much energy is spent, but also on how energy is spent. Holding a book in a stretched arm requires a lot of physical effort, but it doesn't take that much ...


65

Quantum mechanics has almost no bearing on the operation of the brain, except insofar as it explains the existence of matter. You say that signals are carried by electrons, but this is very imprecise. Rather, they are carried by various kinds of chemical signals, including ions. Those signals are released into a warm environment that they interact with over ...


64

Strength is proportional to surface area divided by volume, but since volume is directly proportional with mass and i can't get an accurate density (i am guessing approximately both for mass and size.), i will use mass instead. According to Wolfram Alpha, the average mass of human body is 70 kilograms. The surface area of a person weighing 70 kg with a ...


47

Sort of, yes. Ice water is, in fact, a negative-calorie foodstuff and could be used to lose some weight. Fats contain about 37 kJ/gram of energy, drinking one glass of ice water will burn about 37 kJ or up to three times more if you eat some crushed ice as part of drinking the water: so that's 1 gram of fat burned per drink, up to 2-3 if you eat ice. The ...


41

While they work on the same principles, the detonation of an atomic bomb and the meltdown of a nuclear plant are two very different processes. An atomic bomb is based on the idea of releasing as much energy from a runaway nuclear fission reaction as possible in the shortest amount of time. The idea being to create as much devastating damage as possible ...


41

I think the answer has less to do with their construction and more to do with their smaller size For more information lookup Scaling Laws. Basicly the mass of a object scales as it's size cubed so a ant 10 times the size will be 1000 times heavier. But the strength of an organism depends on the cross sectional area of muscle (I've heard this somewhere, not ...


40

If you're pushing a 10-ton truck and it's not moving, you are not doing any work on the truck because the distance $ds=0$ and the nonzero force $F$ isn't enough for the product $F\cdot ds$ to be nonzero. Your muscles may get tired so you feel that you're "doing something" and "spending energy" but it's not the work done on truck. You're just burning the ...


39

This is from the Physics FAQ article that I wrote 15 years ago: If shorter wavelengths are scattered most strongly, then there is a puzzle as to why the sky does not appear violet, the colour with the shortest visible wavelength. The spectrum of light emission from the sun is not constant at all wavelengths, and additionally is absorbed by the high ...


35

It's fair to say that the "spaghettification" problem occurs where there's a strong enough gravitational gradient across a typical body-distance - but your question seems to ask about cases where the gravity is great - extreme, but uniform (at least in terms of human proportions/distances). Einstein’s principle of equivalence states that, as long as there's ...


31

There have actually been cases of (accidental!) exposure to near-vacuum conditions. Real life does not conform to what you see in the movies. (Well, it depends on the movie; Dave Bowman's exposure to vacuum in 2001 was pretty accurate.) Long-term exposure, of course, is deadly, but you could recover from an exposure of, say, 15-30 seconds. You don't ...


29

From the wiki article on color vision as an illustration of how photons are absorbed: Perception of color begins with specialized retinal cells containing pigments with different spectral sensitivities, known as cone cells. In humans, there are three types of cones sensitive to three different spectra, resulting in trichromatic color vision. Each ...


26

This is about how your muscles work -- the're an ensemble of small elements that, triggered by a signal from nerves, use chemical energy to go from less energetical long state to more energetical short one. Yet, this obviously is not permanent and there is spontaneous come back, that must be compensated by another trigger. This way there are numerous ...


23

First, Field strength. This calculation is strictly an electric potential calculation; radiation and induction are safely ignored at 50Hz.* For a 200kV transmission line 20m above ground, the max electric field at ground level is about 1.2 kV/m.** This number is reduced from the naive 200kV/20m=10 kV/m calculation by two effects: 1) The ~1/r variation ...


23

Yes - but only in the sense that all macroscopic processes depend on underlying quantum mechanics at the microscopic scale. No - quantum mechanics is not the best model for describing what happens in the brain. In one sense, the behaviour of a neuron is similar to a quantum process, such as (for example) the decay of an electrically excited or radioactive ...


22

Humans are weakly diamagnetic. Rather than being attracted by a magnetic field we would tend to repel the lines of force. Look at the work of the High Field Magnet Laboratory http://www.ru.nl/HFML/, in particular http://www.ru.nl/hfml/research/levitation/diamagnetic/ where they demonstrate levitation of a living frog. It took about 16T to levitate the ...


21

For me it is axiomatic that machine miles are easier than real miles, but let's analyze the situation. Assume the runner maintains a constant velocity up the hill, or remains stationary in the frame of the gym on the treadmill. In both cases the runner's acceleration is zero, so we know that her legs must provide a constant force with upward magnitude $mg$, ...


20

You aren't creating a vacuum, but you are reducing the pressure in your lungs when you inhale. In effect your lungs are working as a diaphragm pump. When you pull your diaphragm down, and/or expand your chest, this increases the volume inside your lungs. Boyle's law tells us: $$ P_0V_0 = P_{\rm inhale}V_{\rm inhale} ,$$ where $P_0$ and $V_0$ are ambient ...


20

Strength / weight is a funny thing. The stress on a long thin rod (like an ant's leg) is limited by the Buckling strength which is given (for rod that can freely rotate at each end) by $$F = \frac{\pi^2EI}{L^2}$$ where $I$ is the second moment of area which scales with $r^4$ - so $$F \propto \frac{r^4}{L^2}$$ So when you make an object 2x smaller, the ...


20

The cross-section for neutrino interactions is energy dependent. For solar neutrinos at $\sim 0.4$ MeV, which would likely dominate any neutrinos likely to interact (the cosmic background neutrinos have way low energies) , the cross-sections are $\sigma \sim 10^{-48}$ m$^2$, for both leptonic processes (elastic scattering from electrons) and ...


20

The human voice box produces a fundamental frequency and its harmonics because the mechanism is like that of a relaxation oscillator. However, we have limited control over the relative amplitude of the harmonics (we do have some - that is how we change the "color" of a tone we sing, and the sound of vowels). In order to produce the Shepard scale, you need ...


19

This interference is unfortunately quite typical as David pointed out in his comment. A typical household microwave oven operates at 2.45 GHz, the 802.11g wireless spectrum lies in the range of 2.412 to 2.472 GHz. This by itself is not a big problem, as the WiFi algorithms use sophisticated algorithms to operate even with noise at the same frequencies. The ...


18

Mammalian sense of smell is in general exquisitely keen: even though we think of ourselves as an animal having a dull smell sense comapared to that of, say, a dog, a pig or a rat, receptors for certain scents are still triggered by molecules counted in the tens. So the outgassing of volatile wood oils from, say, a table, can still be miniscule and well ...


18

Photons can be created and destroyed freely, since they don't have charge or mass. Turn on a light, and you create many photons. Any body (made of atoms) not at absolute zero temperature will spontaneously emit photons. They are consumed just as easily. Most any bit of bulk matter will absorb a photon in the electrons on the surface, transforming the energy ...


17

No, they will not appear the same. Humans have three color receptors so any possible color for us is just three numbers in RGB space. However, electromagnetic spectrum is continuous and there is an infinite number of spectra that would produce the same RGB stimulus. That is why you perceive the this page as white although it is in fact a combination of R,G,B ...


17

You are looking at this incorrectly. Pale skin allows the UV to penetrate more deeply than dark skin (that has the melanin in the dead skin cells). Since dark skin individuals absorb the UV in the dead skin layer, it make no difference if it causes DNA damage.


16

Perhaps an analogy is in order. Lets hold up the book by using an electromagnet (say we put a piece if steel under it ). If the coils were made of superconducting material it would take no energy input to maintain the position/field strength. But if we use ordinary wire, ohmic loses within the coil must be made up for by externally supplied electrical ...


16

This exact problem was rose also by Feynman in his lectures. In particular at the beginning of chapter 14 in the first volume. It is a fact that when one holds a weight he has to do “physiological” work. Why should he sweat? Why should he need to consume food to hold the weight up? Why is the machinery inside him operating at full throttle, just to hold ...


15

In the 19th century, the physicists Young and Helmholtz proposed a trichromatic theory of color, in which the eye was modeled as three filters with overlapping ranges. This is essentially a physical model of the pigments in the eye, and it predicts the response of the nerve cells at the retina. Helmholtz did related work on sound and timbre. Ca. 1950, ...


14

Moore's law from the Wikipedia article: Moore's law is the observation that the number of transistors in a dense integrated circuit doubles approximately every two years (since the invention of integrated circuits). His prediction concerned the rate at which the density of transistors in circuits increased with time. So I'm not sure it makes sense to ...



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