# Search results

1. ### NASA NASA's Ephemerides

I was wondering, how does NASA actually calculate their super exact Ephemerides?
2. ### I Vector addition and Newton's law

I know that n-body problem can be complicated, but that's for the dynamics. What about a static case: e.g. if I have the distances of several bodies A, B and C etc. and their distance to a reference mass m, can I just use the vector addition of the Newton's gravitational force to add up all of...
3. ### I Foucault pendulum and a round globe

cool. then all pendula must be installed at the same altitude to guarantee smoothness. :smile:
4. ### I Foucault pendulum and a round globe

Imagine we are all cavemen without satellite technology and just discovered the Foucault pendulum! As we know the angle of presession depends on the latitude. This can be used to prove that the earth is rotating, right? Now by putting several Foucault pendula around the Earth, at equidistant...
5. ### Same physics, two theories?

true. Thanks. Now I have to think more... o_O
6. ### Same physics, two theories?

Was just thinking, is it ever possible that the same phenomenon can be fully explained by two theories that are not subset of each other? :rolleyes:
7. ### Understanding quadrature sampling

ok, the concept of DDC is explained here nicely by T. Schilcher starting from page 249: http://cdsweb.cern.ch/record/1003726/files/full_document.pdf [Broken] :oldwink:
8. ### Understanding quadrature sampling

Thanks so far for replies. After checking the above mentioned document written by Lyons, I also found another doc from him here http://www.ece.ucsb.edu/Faculty/rodwell/Classes/ece218b/tutorials_etc/sampling.pdf'][/PLAIN] [Broken]...
9. ### Understanding quadrature sampling

thanks for the explanation. Yes, I know the nice article by R. Lyons. But I checked it again. There he shows the ADCs to be placed after the quadrature oscillators. i.e. you already have 2 ADCs sampling at a fixed frequency fs, signals which come from the I branch and Q branch. All is good here...
10. ### Understanding quadrature sampling

Hey everyone! Can anyone explain, why quadrature sampling works the way it does? i.e. taking 2 samples for the I and 2 samples for the Q. I mean I can understand if one tries to sample one mono frequency signal, say 40 Hz sine wave, on 0, 90, 180, 270 degrees, that is a sampling frequency of...
11. ### Name of a geometrical shape

Found it! I think it is called a superellipse. I found a formula and an octave program on Wikipedia. The one I was looking for is the one with parameters "3 6 6 6", the table on the right side. The plot in octave also works by using the following command: sf2d([3 6 6 6],[1 1]) anyway...
12. ### Name of a geometrical shape

What is the geometrical shape called, when combining 3 ellipses together, such that each two share one focal point and together form a kind of 60 degree triangular shape? It will look similar to Reuleaux triangle, but it is not formed out of a triangle, but 3 ellipses, as if you are gluing...
13. ### PF is getting a face lift!

Oh, cool. I like the new design. :-)
14. ### Energy of light vs energy of photon

this has become a wonderful thread thanks to you all, I love it! :approve: In fact Darwin123 made a wonderful description, which I can now relate to what I have read in a book. Maybe the answer to Crazymechanic is also here. Near field effects seem to be the result of the interaction of 2...
15. ### Energy of light vs energy of photon

Great! thanks. o:)
16. ### Energy of light vs energy of photon

How is energy of electromagnetic waves (classical electrodynamics → Poynting vector) related to the photon energy (E=hv) ? e.g. what is exactly the difference between photons from two radio stations or two laser devices with the same frequency, but different power? thanks! :cool:
17. ### Question on Coulomb law

Great answer! thanks. I was also checking some books on this. Found also the Gauss's law applied to the energy conservation, which states that the sum of mechanical and field energy in a volume V is reduced as energy is radiated away from that volume. Now is it correct to think like this: If...
18. ### Is unification really needed?

just came to my mind to ask: Is gravitation really a fundamental force? As far as I recall from my GR lecture, gravitation is a property of space-time. Other forces seem to be well described by the grand unified theories (GUT). So why is there still a need to include gravitation? cheers...
19. ### Question on Coulomb law

It is known from the Coulomb's law (F = q E) that if an electric field is applied on a charge, it will accelerate it, i.e. the position of the particle changes macroscopically. But why mechanical displacement? why not a change in particles internal energy, say for example excitation of an...
20. ### Observation time and its relation to the quantum Zeno effect

Hi. I have a problem to understand the following situation regarding observation of a quantum system: Imagine we have an unstable particle in a box, together with many sensors where each is connected to a lamp. The sensors continuously monitor the particle, the lamp turns on whenever the...
21. ### Energy exchange between a bell and a hammer

thanks for the replies so far. Now things are becoming more clear. I also like the notion of mechanical impedance which is similar to the electrical case. An ideal elastic collision would resemble a perfect impedance matching, such that all of the power of the impulse is transferred into the...
22. ### Energy exchange between a bell and a hammer

so considering the pure mathematical model, is it ok to say that the portion of the energy that does not fall on one of the ideal eigenfrequencies ω1, ω2 or ω3 is turned to heat? I am confused whether this portion of energy turns into heat, or it is not absorbed at all, i.e. it is reflected back.
23. ### Energy exchange between a bell and a hammer

I mean really ideal. we can also do it more abstract, imagine there is a system whose frequency response consists of 3 Dirac deltas, say at ω1, ω2 and ω3. Now I am exciting it with a Dirac delta in time domain with a certain energy E. By definition, this corresponds to a flat spectrum that...
24. ### Fortran Fortran FFT library

great! that is what I need.
25. ### Fortran Fortran FFT library

I am new to Fortran. Is there an ultimate best library for FFT for Fortran (95)? I found this one FFTPACK. I am not sure if this is the best one. Is there anywhere a simple example how to use it? I have huge time files (260 MB each) that I like to read in Fortran and perform FFT and...
26. ### Fortran Fortran complex array assignment

Yes. works perfectly now. :biggrin: many thanks.
27. ### Fortran Fortran complex array assignment

sorry for late reply. But unfortunately it doesn't work. Even in the following simple code: program test implicit none integer::t complex::in(4) do t = 1, 4 in(t) = ( 2*t, 3*3 ) end do end program test gfortran gives the following result: test.f95:6.18...
28. ### Confused about autocorrelation and PSD

I am confused a bit :confused:. I read in a paper that the following property holds, but can't find where it comes from. \mathsf{E}\left[X(\omega)X^*(\omega')\right]=2\pi\delta(\omega-\omega')S_{XX}(\omega) it says that the expected value of the Fourier transformed signal is proportional...
29. ### Fortran Fortran complex array assignment

I have a an array of type complex. I am trying to assign a value to it in a loop, but I get a strange error "Unclassifiable statement at (1)". I really don't understand why. do t = 1, count real(in(t)) = cos(2 * pi * f0 * t) aimag(in(t))=sin(2 * pi * f0 * t) end do thanks.
30. ### Energy exchange between a bell and a hammer

Imagine a bell that has only 3 ideal resonant frequencies (no damping) and I am hitting it with a point like ideal hammer, does this mean that all of the energy of the hammer is distributed among those 3 resonances? I think not, i.e. the energy stored in the resonant modes are less than the...