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3
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1answer
76 views

Simple estimation of the critical temperature of water

I'm trying to develop fermi estimation skills and I came up with a question for which I don't even know where to start from. Here goes: Is it possible to estimate the critical temperature (say in ...
2
votes
2answers
111 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 ...
0
votes
0answers
40 views

Algorithm for calculating lunar crescent visibility

Is there a simple algorithm for predicting lunar crescent visibility. Simple enough to do with a calculator at least. Those offered in Meeus' Astronomical Algorithms are a bit too involving to do by ...
1
vote
1answer
49 views

How to calculate the Impact of a meteor

I am trying to add a dose of realism to a story I'm writing: imagine an invulnerable superman-like being that won't disintegrate at any point, weighing in at 349lbs, travelling at 86,000mph through ...
2
votes
2answers
221 views

Temperature on the surface of the sun calculated with the Stefan-Boltzmann-rule

In a German Wikipedia page, the following calculation for the temperature on the surface of the Sun is made: $\sigma=5.67*10^{-8}\frac{W}{m^2K^4}$ (Stefan-Boltzmann constant) $S = 1367\frac{W}{m^2}$ ...
0
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0answers
35 views
3
votes
1answer
20 views

Can physicist estimate is it safe to walk/drive on ice? [closed]

I'm curious about the following: Suppose that I know the the weather forecasts of a given place. How can I compute from that data whether or not it is secure to walk or drive by a car to an island.
3
votes
1answer
56 views

Mammal Population Estimates as a Fermi problem [closed]

Can We estimate the order of magnitude of number of mammals on earth? Can this be treated as Fermi problem or not?
2
votes
1answer
56 views

Is there a Fermi estimations toolbox?

I am a theoretical physics student and am a little ashamed at my inability to estimate any measurable quantity. I would like to develop my skills at Fermi estimations. Although it is hopeless to ...
2
votes
3answers
141 views

Will my car consume more if I plug in my laptop?

I drive a Volvo v40 with a 110hp diesel engine. It usually use 4.6L/100km. If I plug my laptop which is 85W, will this change the fuel need? If so, by how much?
2
votes
0answers
25 views

Quantum Efficiency Estimation

Might there be a way to do a rough estimate of the quantum efficiency of a photo-detector like a CCD or CMOS sensor based only on a picture taken with it? I've read papers and guides (like this one: ...
0
votes
1answer
180 views

How thick (in molecules) would a piece of paper be if it was folded to the moon? [closed]

Apparently you can fold a piece of paper 42 times and reach the moon! 42 is the answer the everything after all! Oh, Douglas Adams, you clever son of a bitch. The reasoning behind this is that as you ...
4
votes
0answers
73 views

Intuitively, why does removing solutes cost $k_B T$ of free energy per molecule?

I can calculate that if you want to, for example, desalinate water, you will have to pay a free energy cost of $k_B T$ for each ion you remove. In other words, removing an ion from a volume of water ...
0
votes
2answers
81 views

How does one estimate the electrical power of a power plant?

I have two related questions that I would like help on: When a power plant creates power like the Hoover Dam, it can provide 2.07 GW of electrical power. My question is what does this mean? I assume ...
1
vote
1answer
49 views

Ultra Deep Field 3D camera speed

I guess most of you saw Ultra Deep Field 3D video. In the end it shows camera moving in 3D perspective through all the galaxies. My question is, how fast is the camera moving in that video? I couldn't ...
0
votes
2answers
167 views

How to estimate a person's mass from a sensor? [closed]

I'm trying to estimate the person's weight from some available sensors and I have an accelerometer, a gyrometer and a magnetometer. The triaxial accelerometer is fixed in a band in the person's ...
1
vote
5answers
3k views

Estimate number of hairs on human head [closed]

A technique of vital importance at all levels in physics is estimation. This is obvious from the first chapter in any introductory physics textbook, but is also related to the working physicist. ...
18
votes
3answers
4k views

Roughly how many atoms thick is the layer of graphite left by a pencil writing on paper?

Actually I can't expand much as the question pretty much explains the query. I would be interested in the method of estimating an answer as well as a potential way to measure it experimentally. ...
1
vote
1answer
178 views

How much lead is there in the universe?

I know there is around 10^80 atoms in the observable universe, but is there any estimate the amount of lead in the universe (within a couple magnitudes of 10)?
0
votes
0answers
74 views

How much energy does it take to simply run forward?

I'm interested in tracking as much data about my runs as I can in an effort to get faster, and while I can easily estimate energy expenditure during an uphill run due to the change in elevation, I ...
18
votes
2answers
2k views

How long until we fall into the Sun?

As a planet moves through the solar system, a bow shock is formed as the solar wind is decelerated by the magnetic field of the planet. Presumably the creation of this shock wave would cause drag on ...
0
votes
0answers
21 views

Estimating the fatigue life of elastomer diaphragm [duplicate]

Disclaimer: I have asked this question before, but it got put on hold because it was judged as off-topic. I happened to disagree and explained my reasons in the comments, but they have been ignored, ...
1
vote
0answers
874 views

What is the maximum non-fatal force withstandable by a human being for a short period?

Assuming any (optimal) body orientation/position relative to the direction of force and mechanism of death. I'd be interested in a rough estimate of threshold force and a suggested mechanism of ...
29
votes
3answers
2k views

Floating a ping pong ball in the air just using a pen!

Caution: Apparently this problem is harder than it seems! There is a well known phenomena, which I first learnt about when I was a 10 years old kid. You can levitate a ping pong(or whatever ...
9
votes
2answers
1k views

Why do mirages only appear on hot days?

A previous question asked why the road sometimes appears wet on hot days. The reason is that when there's a temperature gradient in the air, it causes a gradient in the index of refraction, causing ...
2
votes
2answers
2k views

When do I apply Significant figures in physics calculations?

I'm a little confused as to when to use significant figures for my physics class. For example, I'm asked to find the average speed of a race car that travels around a circular track with a radius of ...
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 ...
2
votes
1answer
236 views

Why do some liquid metals have anomalously-high surface tension/heat of vaporization ratios?

In a didactic article, Victor Weisskopf estimated the size of molecules in a liquid from measurements of their surface tension and heat of vaporization. If atoms are exceedingly small, then only a ...
8
votes
5answers
268 views

What are the chances that a deadly asteroid will hit Earth in the next decade?

What are the chances that an asteroid that will kill multiple people will hit Earth in the next decade?
8
votes
3answers
659 views

How can we make an order-of-magnitude estimate of the strength of Earth's magnetic field?

The source of Earth's magnetic field is a dynamo driven by convection current in the molten core. Using some basic physics principles (Maxwell's equations, fluid mechanics equations), properties of ...
2
votes
0answers
60 views

Are there any connections between James–Stein estimator and quantum mechanics?

Very nice statement from wiki: When three or more unrelated parameters are measured, their total MSE can be reduced by using a combined estimator such as the James–Stein estimator; whereas when ...
0
votes
2answers
40 views

How can I estimate the power between the guide and the sky diver?

I would like to estimate the Maximum power/ tension that can be, between the sky diving guider and the "tourist". In this picture what the max tenstion will be, I think it is maximum when the ...
2
votes
3answers
963 views

How much energy does a super nova generate?

For a scene in a SciFi book, I want to know: Is it possible to estimate how much energy per m² an object would receive that hides behind an in-system planet when the sun goes nova?
23
votes
6answers
3k views

How fast a (relatively) small black hole will consume the Earth?

This question appeared quite a time ago and was inspired, of course, by all the fuss around "LHC will destroy the Earth". Consider a small black hole, that is somehow got inside the Earth. Under ...
3
votes
1answer
194 views

What is an approximation of the average number of supernovae every century in the Milky Way?

Besides giving out a number and quoted source I would appreciate a short derivation of this number/formula based on our current data and knowledge of supernovae and the Milky Way. Show some traceable ...
5
votes
1answer
449 views

How to estimate condensation from air?

How to estimate the amount of water condensing from air on a surface, given the air's temperature and relative humidity and how they change over time, the surface temperature, material's thermal ...