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34

The following fact lies at the heart of this and many similar issues with sizes of things: Not all physical quantities scale with the same power of linear size. Some quantities, like mass, go as the cube of your scaling - double every dimension of an animal, and it will weigh eight times as much. Other quantities only go as the square of the scaling. ...


9

You can grow arbitrarily large as long as you are essentially flat. For example, one fungus covers several thousand acres; there's a grove of clonal aspen trees that may have higher mass. Scaling in three dimensions is much harder, though. The pressure on the bottom is proportional to the height--eventually that pressure is too great for tissue to ...


6

The basic answer is that mass scales with the cube of linear dimension and strength of things like legs scales with the square of the linear dimension. Note that large animals have therefore evolved comparatively thicker legs than smaller ones. Linearly scale up a dog to elephant size, and its legs would snap. Even more extreme, think of scaling a ant to ...


6

Potentially yes it could. There are no noise-cancelling headphones to stop the U.S. Navy's 235-decibel pressure waves of unbearable pinging and metallic shrieking. At 200 Db, the vibrations can rupture your lungs, and above 210 Db, the lethal noise can bore straight through your brain until it hemorrhages that delicate tissue. If you're not deaf after ...


5

Looks like neutrons can cause visual perception: Visual phenomena noted by human subjects on exposure to neutrons of energies less than 25 million electron volts, Science. 1971 May 21;172(3985):868-70, "Six subjects reported multiple starlike flashes and short streaks on exposure to neutrons of energies up to 25 million electron volts. The probable mechanism ...


5

This is obviously a very broad question, but here are a few thoughts that may be helpful. As dmckee points out in a comment, it's difficult to define consciousness. However, consciousness clearly requires computation, and computation is something that physics can address. There is a psychological arrow of time: we can remember the past but not the future. ...


5

The answer is that your eyes would be focusing not at the concrete distance where the mechanism is placed, but at a virtual image which appears farther away. This is just basic optics. It is (one aspect of) what happens when you use a magnifying glass, for instance. The virtual image of some point viewed through an optical instrument is that location in ...


4

I'm only going to try to address the question of DC fields. Medical MRI uses uniform fields of about 0.5 to 3.0 T. In a head MRI, the Lorentz force on ions in the brain can cause neurological effects such as vertigo. I've heard that this shows up in particular when the patient moves his head. Here is a famous picture of a frog being levitated by a 16 T ...


4

One simple approximation that you could make is to assume that the human body is made of water. Then you can reduce your question to: what happens to water molecules in a magnetic field. Consequently, you would have to ask how you can break the Van der Waals Bond in water with a magnetic field. I think here you would have to differentiate between a static ...


4

Yes, it will be harmful, because the isotope will travel through your bloodstream and deliver radiation damage to cells all over your body. If you want to know how harmful it is, check the recent case of Alexander Litvinenko, who was assassinated with polonium-210.


4

In addition to the deposition location when EnergyNumbers mentions in the comment the types of radiation and the length of the decay chain are an issue. Radon sits at the top of a long sequence of decays many of which are alpha emitting (quality factor $\approx 10$--$20$) including Po-210 (5.34 MeV alpha, yikes!). Also the radon has a non-zero fission ...


3

The RC circuit that approximates a myelinated fiber looks like a series of resistors along the length of the fiber with capacitors connected to ground at the gaps between the Schwann cells. The time to reach discharge voltage at the next junction after depolarization at the prior junctions will be determined by the time it takes to raise the capacitor ...


2

[A] report published in the current issue of the Proceedings of the National Academy of Sciences shows that the size of a landmass limits the maximal body size of its top animal. Scientific American This is the report that is referred to: "Dinosaurs, dragons, and dwarfs: The evolution of maximal body size". (You may click on "Full Text (PDF)" on the ...


2

It looks like Bi-209 is almost stable, with an extremely long half life, and its toxicity is low (http://en.wikipedia.org/wiki/Bismuth ). In general, however, alpha-radioactive materials are quite deadly when ingested (http://en.wikipedia.org/wiki/Polonium#Acute_effects )


2

The answer to the valve-question,1 according Pilotfriend, seems to be the "floppy walled Eustachian tubes". During ascent the gas (air) in the middle ear cavity expands and a small amount of pressure builds up against the ear drum causing them to bulge outwards ever so slightly (that ‘fullness’ you feel in your ears just before they ‘Pop’). This pressure ...


2

The straight physics answer to this question - namely "Not all physical quantities scale with the same power of linear size." - is perfectly put by Chris White. This essentially answers your question about giant robots - there are no hard limits, but the scaling power issues simply mean that it gets harder and harder to build bigger and bigger. Modern ...


2

Perhaps let me try to address this question based on a discussion with Prof. Frank Wilczek. This post is not going to be complete or anything. The punch line question I discussed with him: is there a set of mathematical equations from physical principles to distinguish life and lifeless beings? (say, hand in a system as an input, one can check its live or ...


2

Some kinds of mutation provide an example of this kind of indeterminacy. UV light can be bad for our health. One of the reasons is that, when we are exposed to sunlight, UVB photons are absorbed by double bonds in pyrimidines, which break open, become reactive, and dimerize (photo-dimerization). This damages the DNA in the same way that it would damage a ...


2

We need to explain a discrepancy amounting to a factor of $10^6$, so clearly the explanation can't lie in things like quality factors for alphas versus betas. Your calculation assumes that exposure due to radon is only due to the air that's in the person's lungs at any given time. But this article says: [...] charged radon particles can easily bind to ...


2

This article is very good in describing the dangers of electricity, which I suggest you read. http://www.allaboutcircuits.com/vol_1/chpt_3/4.html To summarise the article, $20\text{ mA}$ for $60\text{ Hz}$ causes "severe pain, difficulty breathing, loss of voluntary muscle control", whereas $20\text{ mA}$ for $10\text{ kHz}$ is in between "threshold of ...


1

The universe is made up of one hundred billion galaxies each with between tens of millions of stars to hundreds of trillions of stars. So we have quite a few stars. It used to be somewhat unknown whether or not stars had planets and if so how many. Recently a satellite called Kepler was designed to look for evidence of planets and found that many stars have ...


1

The repelling is another way of saying that owing to the strength of the hydrogen bonding between water molecules, the water molecules are better off with themselves alone as compared to with non-interacting non-polar molecules within. A substance dissolves only in a solvent, where the solvent-solute interaction is as strong (or stronger) than the ...


1

The fault in this reasoning comes from the fact that you are inspecting a small part of the system (the biosphere) and ignoring the total entropy in the system. All biological processes, from those present in bacteria (or for multicellular organisms, mitochondria) to firing of neurons in human brains, increase the total entropy of system.


1

I'm reading some excellent answers here. One aspect has not received any attention though: survival by agility. Mice are agile, elephants less so. Size definitely plays a role here. Considering animals utilizing legged locomotion in an environment with gravitational acceleration $g$. The leg height $h$ in combination with the gravitational acceleration ...



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