Hot answers tagged biophysics
22
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 ...
20
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 ...
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 ...
16
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 ...
15
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 ...
12
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
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
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 ...
10
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 ...
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
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, ...
7
The ratcheting in photosynthesis is due to energy loss at each step. If you go from a high energy to a lower energy state, where the difference exceeds kT by several multliples, you cannot go back, because the reverse process requires a thermodynamic conspiracy to take the energy out of many modes and transfer it back to one electron, and that's just not ...
7
The atoms in the enamel of the teeth are not exchanged with the environment. Indeed, long after your death their isotopic composition can be used to tell where you lived while your teeth were growing.
6
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 ...
6
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
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 ...
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
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 ...
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 ...
5
The physics is being explored in the rapidly growing field of biophysics. The short answer is that plants are able to move, in general, via hydraulic or osmotic processes.
Osmotic process describe the tendency of water to move into a solution via osmosis, where osmosis is "the movement of solvent molecules through a selectively permeable membrane into a ...
5
The facts that
there is a sum over $i$ but the product doesn't involve $i$;
the product is a product of exponentials, which as a major result (boxed and marked "DO NOT ERASE") would typically be written as a single exponential;
given that the first equation is a differential equation, one should expect the second equation that gives $\Phi$ to be either an ...
5
Yes, the cone will produce sound waves if you move it to and fro by hand. The speaker produces sound by moving the cone, and it doesn't matter whether it's the moved by current in the coil or you moving it by hand. The sound waves will have whatever frequency you're moving the cone at, and I doubt you could manually move the cone at more than a few Hz, so ...
5
One place this issue is discussed is here. The key paragraph is
Whales can withstand this pressure because their bodies are more
flexible. Their ribs are bound by loose, bendable cartilage, which
allows the rib cage to collapse to some degree under high pressure
that would easily snap our bones. A whale's lungs can also collapse
safely under ...
5
Yes (probably).
Since neutrinos interact so rarely with matter (ie you) you would need an awful lot of them to have any significant effect. A supernova conveniently emits about 99% of its energy as neutrinos. So if you were standing quite close to a supernova when it went off then there could possibly be enough interactions to harm you.
If you stay a ...
5
Under regular circumstances, neutrinos are absolutely harmless due to their small interaction cross section. If you hold your thumb up to the sun, about 60 billion (60 · 109) neutrinos will pass through your thumbnail per second, but you will rarely ever have any neutrino interaction in your body.
That is for 'regular' neutrino fluxes which you have from ...
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