# Tag Info

29

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 ...

26

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 ...

24

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 ...

23

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 ...

17

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 ...

17

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 in ...

16

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 ...

13

Yes, it's very much physics related: The perceived smallness of distant objects is a direct function of how many space dimensions we live in. Here's an example: For a one-dimensional or "string land" creature, what would be the apparent difference in size between a dot nearby and a dot many miles away? If you think about it a bit, the answer is "none" -- ...

11

Water is transported through xylem tissue, which reassemble just a passive bundle of pipes. They are narrow enough to provide quite a huge capillary effect, but this is not a process of transport because it converges fast to equilibrium water levels and stops. The flow is powered by two other processes; first and most important is evaporation of water from ...

11

The horizontal component of running is believed to be fairly negligible for humans. Some research suggests that the limit isn't strength related at all, but design --- in particular, based solely on power, humans could theoretically run up to almost 40 mph. The issue is two fold: first, our limbs are actually too heavy, for big strength (e.g. climbing in ...

10

Here is one reason: a note with a fundamental frequency of 100 Hz will have harmonics at 100 HZ, 200 Hz, 300 Hz, 400 Hz, 500 Hz, 600 Hz, etc., while a fundamental of 200 Hz has harmonics of 200 Hz, 400 Hz, 600 Hz, etc. These are a subset of the harmonics of the 100 Hz note an octave below. The human auditory system detects the pitch of the fundamental ...

9

Here is a video of the film's science advisor explaining what the equation is and how he came up with it: http://www.youtube.com/watch?v=WjfT6MqTCqQ It is based on the Gompertz equation, which is a model of mortality rates, with some added "mathematical glitter."

9

This is an example of "scaling laws". Have a look at http://hep.ucsb.edu/courses/ph6b_99/0111299sci-scaling.html - for once Wikipedia doesn't have a good article on the subject. The strength of a muscle is roughly proportional to the area of a cross section through the muscle, so strength is roughly proportional to size squared. That's why I'm a lot ...

9

If the power line is 20m high, and has the voltage of 1MV , then the electric field (near ground), very roughly, is on order of 1000/30 kv ~ 30 000 v/m (the numbers are very approximate and the field is complicated because it is a wire near a plate scenario, and wire diameter is unknown but not too small else the air would break down, i.e. spark over, near ...

9

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, ...

9

When I was an undergrad, I was routinely subjected to magnetic fields thousands of times more powerful than Earth's because I was a research subject for MRI experiments. I noticed no powerful emotions induced by the magnetism, and I haven't heard of other subjects experiencing them, either. (People do experience claustrophobia in the MRI, but probably not ...

9

You can get an upper limit by simply treating the case of The radioisotope structured as a point source. The whole dose still present, corrected for half-life. No shielding. A simple $\frac{\text{presented area}}{4\pi r^2}$ for the acceptance (in this case the radiation is emitted in all directions, acceptance represents the fraction that hits the target, ...

9

Your question seems to be about human body heat rather than other human activities that contribute to global warming. Humans body heat doesn't actually add any energy to the whole-Earth system (see below) but for a moment, I will assume that it does. Instead of looking at the mean temperature of humans, it's easier to look at the amount of energy our ...

8

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 ...

8

The reason is that you need to spend energy to keep muscle stretched. The first thing you need know is that the work $W=F \Delta x$ is the energy transfer between objects. Hence, there are no work done on the book when it is put on the table because there are no movement. When your arm muscle is stretched, however, it consumes energy continuously to keep ...

8

The human vision has 3 types of cones. (that is why all perception-based color spaces are 3 dimensional: LAB, XYZ, HSV ). Each cone type has a different sensitivity curve in the color spectrum (think of them as color filters). It gets complicated because these curves overlap: there isn't a single wavelength of light that triggers just one cone type. So, in ...

7

They heat it, by different degrees depending on the polarization of molecules in the tissues and liquids. The molecules try to re-align after the radio-wave field and the movement dissipates as general heat. Think microwaves.. a consumer-grade microwave oven operates at the same radio wave spectrum as your home WiFi network (2.4 GHz) but much stronger. The ...

6

I think the issue is we need to separate the 'expected'/obvious quantum effects, from the unexpected ones. For instance, some of your questions refer to quantum mechanics in molecular structure. In the most trivial sense, we couldn't even have molecular structure or even stable atoms if $\hbar \rightarrow 0$ such that there are no quantum effects. So in a ...

6

Short answer: A nuclear power plant contains a lot more nuclear material than an atomic bomb. The "Little Boy" bomb was detonated at 1968 feet (600m) over Hiroshima with the nuclear material dispersed quickly in the air; the Chernobyl meltdown contaminated its environment for decades. Long answer: http://en.wikipedia.org/wiki/Background_radiation Total ...

6

When it comes to modeling the human body and its movements it becomes a lot more complex, because you actually have to model the different components of your body and how they attribute to the system. Plus you are not traveling at a perfectly constant velocity when walking at a steady pace. If you were to model the work done by the human body while ...

6

There is some evidence of polarization perception. Many people are able to perceive polarization of light. It may be seen as a yellowish horizontal bar or bow-tie shape (with "fuzzy" ends, hence the name "brush") visible in the center of the visual field against the blue sky viewed while facing away from the sun, or on any bright ...

5

Current in any acid electrolyte (eg a lead accumulator) is carried predominantely by hydronium ions (real protons do not exist in water). Due to the about tenfold mobility of hydronium ions compared to all other cations, in acid solution charge transport is almost by hydronium alone. Georg

5

a great question. First, a detail: a person needs something like 2,000 kilocalories a day. At least that's how the unit is used today; in the past, the modern kilocalorie used to be called a calorie. Today, 1 kcal equals 4.182 kilojoules. Ideally, one could measure the kilocalories by direct calorimetry, i.e. in a bomb calorimeter. However, that would also ...

5

There is some heating that takes place, but the amount is pretty trivial, because there just isn't that much light reaching the back of your eye. A back-of-the-envelope sort of estimate would be to say that the light of the Sun reaching the Earth's surface amounts to about a kilowatt of radiation per square meter. Your pupils have a radius of maybe a ...

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