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    (Original post by OL1V3R)


    I now realise why I couldn't approach the question; because I hadn't remembered that formula! Cheers for reminding me!
    That isnt in my CGP book! or the formula sheet... what physics principle is it based on?

    Also, if someone could explain this parsec/arcsecond business to me I would be very grateful!
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    (Original post by lefneosan)
    That isnt in my CGP book! or the formula sheet... what physics principle is it based on?

    Also, if someone could explain this parsec/arcsecond business to me I would be very grateful!
    We know pV = NkT, right?
    Hence pV/T = Nk.
    Or p1V1/T1 = Nk.
    And p2V2/T2 = Nk, because Nk is constant (k itself is a constant, and N - the number of molecules inside the balloon is constant).

    1 parsec = 1AU/tan(1 arcsec), where AU = astronomical unit (radius of Earth's orbit around the Sun) and arcsec = 1°/3600 (1 minute of arc = 1°/60).
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    I'm thinking of doing another Astrophysics revision summary online if anyone is interested...
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    (Original post by OL1V3R)
    I'm thinking of doing another Astrophysics revision summary online if anyone is interested...
    :top:
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    (Original post by OL1V3R)
    I'm thinking of doing another Astrophysics revision summary online if anyone is interested...
    Absolutely. I tried to join a couple of the others but the ropey internet meant I had to leave.
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    When were you thinking about doing it?

    Also, people wanting a few more of the test papers I piled together from those documents helpfully put on the windows live area, I've got two more.

    Sit it like an exam paper, with an hour and a half time set aside, and assess the percentage you get
    Attached Files
  1. File Type: docx madeuptest2.docx (64.9 KB, 425 views)
  2. File Type: docx madeuptest2_answers.docx (29.3 KB, 102 views)
  3. File Type: docx madeuptest3.docx (53.4 KB, 422 views)
  4. File Type: docx madeuptest3_answers.docx (25.1 KB, 105 views)
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    (Original post by yadas)
    We know pV = NkT, right?
    Hence pV/T = Nk.
    Or p1V1/T1 = Nk.
    And p2V2/T2 = Nk, because Nk is constant (k itself is a constant, and N - the number of molecules inside the balloon is constant).

    1 parsec = 1AU/tan(1 arcsec), where AU = astronomical unit (radius of Earth's orbit around the Sun) and arcsec = 1°/3600 (1 minute of arc = 1°/60).
    ah many thanks! I'm still not sure on the arcsec business... COuld you perhaps explain it on a more theoretical level? Is it based on 1 degree equalling a 360th of a circle? thanks
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    From different people, I have collected a couple of different defintions of standard candles!
    Which are correct?
    I think it means that there are objects which have luminosities that you know
    CGP: you can directly calculate their luminosity
    A standard candle is an object whose luminosity is constant
    lol
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    (Original post by lefneosan)
    I'm still not sure on the arcsec business... Could you perhaps explain it on a more theoretical level? Is it based on 1 degree equalling a 360th of a circle? thanks
    Here are two pages from Miles Hudson's book, might help.
    187, 188.

    You can see angle θ there.
    This angle is really small. Like 10^-4. So it's 0.0001°.
    Instead of using that, we express these small angles in minutes and seconds of arc.
    So like 1° = 1 hour of arc, there are 60 minutes of arc in 1°, or 3600 seconds of arc (arcsec).
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    (Original post by lefneosan)
    From different people, I have collected a couple of different defintions of standard candles!
    Which are correct?
    I think it means that there are objects which have luminosities that you know
    CGP: you can directly calculate their luminosity
    A standard candle is an object whose luminosity is constant
    lol
    ...standard candle, which is an astronomical object that has a known luminosity.
    From here.
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    In one of my revision books they've got a formula for potential energy in a simple harmonic oscillator that I don't understand at all.

    Can anyone tell me how/why,
    E = 1/2 k X^2 ?

    Where k is the spring constant and x is displacement.
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    The weaker part of Astrophysics for me is to do with circular movement of stars and planets. I'm fine with the circular motion equations but I don't understand things like why stars wobble due to gravitational forces, why geostationary orbits remain in the same position above the Earth and also applying Doppler shift. These types of exam question seem to cost me a lot of marks.
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    (Original post by OL1V3R)
    The weaker part of Astrophysics for me is to do with circular movement of stars and planets. I'm fine with the circular motion equations but I don't understand things like why stars wobble due to gravitational forces, why geostationary orbits remain in the same position above the Earth and also applying Doppler shift. These types of exam question seem to cost me a lot of marks.
    Some satellites remain in position because they go around the earth in cicular motion at the same speed at which the earth rotates. So over 24h the earth is rotating, and the satellite rotates with it.
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    (Original post by yadas)
    ...standard candle, which is an astronomical object that has a known luminosity.
    From here.
    so the other definitions are wrong?
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    (Original post by The Magnificent KoloToure)
    In one of my revision books they've got a formula for potential energy in a simple harmonic oscillator that I don't understand at all.

    Can anyone tell me how/why,
    E = 1/2 k X^2 ?

    Where k is the spring constant and x is displacement.
    That's just the elastic potential energy of the spring when displaced from its mean position. As you either stretch or compress it, you're doing work, so you give it energy.
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    (Original post by OL1V3R)
    I'm thinking of doing another Astrophysics revision summary online if anyone is interested...
    And yeah, I would be, not sure how free I'll be this weekend though. Forcing myself to spend most of it locked up in a library without the internet/distractions.
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    (Original post by OL1V3R)
    That's just the elastic potential energy of the spring when displaced from its mean position. As you either stretch or compress it, you're doing work, so you give it energy.
    But where has it come from?

    Potential energy is mgh and Elastic Strain Energy is 1/2Fx, which is why I (still) don't get the equation. Don't think we need to learn it though, it's not even in the CGP book.
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    Okay, how about I do 2 revision sessions on Saturday:

    1:00pm to 2:30pm - Astrophysics
    5:00pm to 6:30pm - General 6PH05 FAQ (i.e. problem session)
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    (Original post by yadas)
    Here are two pages from Miles Hudson's book, might help.
    187, 188.

    You can see angle θ there.
    This angle is really small. Like 10^-4. So it's 0.0001°.
    Instead of using that, we express these small angles in minutes and seconds of arc.
    So like 1° = 1 hour of arc, there are 60 minutes of arc in 1°, or 3600 seconds of arc (arcsec).
    Thanks! That book finally shows its use! I have two questions though: Why do you need to use two different positions of the earth over a six month period and also in the third worked example given, they say that the the angle measured after a six month period was 1.52 arcsenconds different from that originally measured. so doesnt that mean that the angle isnt 1.52 divided by 2 but actually 1.52 as the questions suggests that there was originally 1.52 and then another 1.52 was measured?
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    Also yadas do you agree that the miles hudson book disagrees with this wiki definition http://en.wikipedia.org/wiki/Arcsecond because wiki gives arc measurements as part of a circle but the hudson book as part of a degree. Are they consistent?
 
 
 
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