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Posts posted by SionR25

  1. The above expressions are for particles moving with speed v. E=mc2 is the special of these expressions when a particle is at rest, i.e., when v = 0.

    For a particle moving with speed V don't you need to add on the kinetic energy (1/2 mv^2) as well. I know this probably won't make a massive difference at low speeds but I thought I read it somewhere.

  2. 2. Heat always moves to a cooler area.

    Just a little thing, don't get heat and temperature mixed up. Temperature flows to a cooler area via the process of heat. Heat is the transfer of energy via temperature. If your thinking of doing physics at a higher level in school, don't let this catch you out :).

  3. Think first of all what goes on outside the box. A vibrating string pushes air, hence creates a sound wave. The musical pitch depends on the frequency of the wave. The frequency depends on the length, tension, and mass per unit length of the string. If you make the string shorter (by stopping a fret) the frequency is higher and you hear a higher note: if you half the length the note goes up by an octave, and other musical intervals such as a fourth or fifth arise from simple ratios (as the ancient Greeks discovered). Increase the tension (by turning a peg) and you likewise increase the frequency of vibration, hence raise the pitch.

    The vibrations cause the whole instrument to vibrate. Sound waves inside a cavity can create constructive interference (resonance), hence sound louder. This is what the soundbox does.

    For an instrument to sound nice you want it to approximate to a simple harmonic oscillator (so that the overtones on the note are natural harmonics). Instruments of irregular shape (e.g. bells) produce overtones that are not in the harmonic series. In the case of something like a guitar or piano this means you want the strings to be as effectively one-dimensional as possible, i.e. long, thin and regular.

    Thanks for the answer :). Seems nice and simple.

  4. Hi all,

    I have an interview with Lancaster University and have been tipped off that because I have put in my personal statement that I play guitar I will very likely be asked to explain what the physics are behind an acoustic guitar, but to be honest I'm not completely sure what it is. I have an idea it is to do with resonance and the natural frequency of the sound waves but could anyone help explain what goes on inside the box?

    Many thanks,


  5. Lose energy, yes. Lose velocity, no. How so? the wavelength changes from, say a blue, to a red. so, light that was blue but is now red, still travels at "C", but with less energy.

    I have wondered about this for years and years. Also about the big bang??? Can't get my head round it . :confused:

    I think it is because photons are mass-less, the energy they contain is not needed to move it as a photon/wave is a way of transferring energy. The definition of a wave is a transfer of energy. A photon is one little part of a wave. Imagine it as water, a water wave when pushed will keep going until the energy you gave it in the push is dissipated by the water hitting a physical barrier. With light that energy is dissipated in the same way, transferring the energy to an electron.

    Don't know how clear that explanation is but hope it helps.


  6. I think you also have to consider things that have cropped up from CERN as well. Without CERN there would be no internet. Here is a list of things that have come about because of particle accelerators.

    Accelerators [*]

    • semiconductor industry
    • sterilisation - food, medical, sewage
    • radiation processing
    • non-destructive testing
    • cancer therapy
    • incineration of nuclear waste
    • power generation (energy amplifier)?
    • source of synchrotron radiation (biology, condensed matter physics...)
    • source of neutrons (biology, condensed matter physics...)

    Particle detectors

    • Crystal Detectors

      • medical imaging
      • security
      • non-destructive testing
      • research

      [*]Multiwire Proportional Chambers

      • container inspection
      • research

      [*]Semi-conductor Detectors

      • many applications at the development stage

    As you can see there have been quite a few benefits from funding programs like this.


  7. Thanks for the explanation :)

    I think what you're talking about there is "Crookes Radiometer". Mind-blowing thing to see if no-one has explained how it's believed to work. Actually, I think it's a mind-blowing thing anyhow :)

    For those interested in why E=mc^2 and why it doesn't quite apply, I recommend Brian Cox and Jeff Forshaw's "Why does E=mc^2?" book. It's not for the maths-phobic, but they do try to walk you through it as easily as possible and to be honest it's not really hard maths in the first place, though it might require the occasional pause and reality-check.


    I should probably finishing reading this before posting anymore questions :p .


  8. Taking the square root of both sides of this equation gives E = cp. So, massless particles are possible as long as they have energy and momentum related by this equation. If massive particles have energy, they must have momentum. Massless particles must move, they cannot be at rest! A little more work shows that massless particles must move at the speed of light.

    Sorry to be awkward here :p, but isn't the definition of momentum; mass x speed. So if you are taking photons to not have any mass then surely p=0?

    We just started this at school before summer, looking at (can't remember the name :p) the thing with one black side and one silver that will spin in a vacuum when light shines on it. As it was the last week no one really felt like doing any theory on it :).


  9. Each element is defined by he number of protons it has within its nucleus. Hydrogen is the most basic element and only has one proton. In extreme pressures and heat of a star, two hydrogens (two protons) are forced together to make helium, which has a proton number of two. This process is the same for all other elements, just greater number of protons being forced together.

    It isn't a chemical process hence not every element is shown to contain hydrogen.


    • Like 1
  10. Thats a very good image, can't wait for the skies to clear up here so I can try some widefield shots.

    Hope you dont mind but I tried some editing on your pic to help remove the colour cast. I reduced the reds in levels in photoshop then reselected Sagittarius and Scorpius with lasso tool and boosted their reds back to roughly how they were.


    If you want any help with photoshop just send me a message :).


    EDIT: Colour in Sagittarius and Scorpius hasn't come up to well in uploading for some reason :huh:

  11. For questions 1+2 the answer is just to release the pressure or heat it up.... a lot.

    For question 3 it depends on the metal, some go transparent and some have other changes. A bit more complicated some experience changes in their electron orbitals, E.G. p + s subshells change for transition metals which causes a rearrangement of the electron structure.

    I don't think it would be viable to use metal hydrogen as a material, unless it was found to have superconducting properties, in which case it might be worthwhile.


  12. I studied Physics and Astrophysics and left after 4 years with a masters. I realised that the life of an academic was not one for me during the masters year when the reality of astrophysical research set in (I prefer just to admire the beauty now) though a PhD was being suggested to me.

    I will echo what has been written on this thread, mathematical ability is key, and to be honest further maths at A-Level would be beneficial. By the time I started my masters the physics department was forced to run remedial maths classes for the 1st year intake despite them all walking in with As and Bs at A-level, and this was because some items from pure maths A-Level had been bumped into Further Maths.

    I thoroughly enjoyed my time studying, especially astrophysics, cosmology and particle physics. I was very lucky that I was able to spend a week at a professional observatory as part of my course, and during my masters conducted a photometry project of T-Tauri type stars using the university's telescopes.

    Thanks for the information. Out of interest which Uni did you go to and how did you find it?

    I usually don't have problems with maths, one of my strong subjects and I take part in Maths challenge every year and have won a silver award the last four times.

    Thank again,


  13. I read in one book that when the universe "ends" it will collapse in on it's self and a second big bang will occur creating a new universe with completely different physics!

    This is still a possibility but recently it has been discovered the expansion of the universe is accelerating, due to dark energy, and not being slowed down by gravity as we thought it might. Because of this we think it is more unlikely the universe will collapse back in.


  14. Reading further into E=mc2is this because everything moves through spacetime at the same speed? If you are stationary, to achieve the same speed as someone moving your clock needs to run faster. Vice Versa, if someone is moving their clock has to run slower because they are moving through space and the time needs to be slower. Simplified a bit (I know the real equation is a bit different but don't have book on me atm) movement in spacetime= movement through time + movement through space. So as one gets smaller the other gets larger and vice versa.


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