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Teaser - no relativity required (promise!)

Posted July 01, 2006 1:38 PM

Using just their 1g-acceleration engine, Virtual Galactic plans to hover their spaceship at 1% (by radius) outside of a non-rotating black hole's event horizon. How many solar masses must this black hole have, given that: (i) a 15 million solar mass black hole would demand a 1 million g-acceleration engine, and: (ii) this acceleration is inversely proportional to the mass of the black hole?

The theory behind this question can be found here: What is gravity?.
But, as promised, relativity is not required to find the answer!

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Associate

Join Date: Jun 2006
Posts: 27
#1

Hmmm

07/03/2006 9:01 AM

Based on your information and not bothering to look up the math. acceleration inversely proportional to black hole mass a = k *1/m k = a * m = 15 million mass * 1 million g acceleration = 15 million million mass * g acc m = k / a = 15 million million mass * g acc / 1 g acc = 15 million million solar mass.

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Associate

Join Date: Jun 2006
Posts: 27
#2
In reply to #1

Revised answer

07/03/2006 10:58 AM

This answer does not make sense.

Now what I don't understand is your claim that it is 'inversely proportional'. I would think that it should be 'directly proportional'.

F = m * a

  • F_blackhole = F_space_craft
  • M_blackhole = 15 M Sol
  • a_blackhold = a_bh
  • M_spacecraft = M_sc
  • a_spacecraft = 1 M g
  • F_bh = M_bh * a_bh = 15 M Sol * a_bh
  • F_sc = M_sc * a_sc = M_sc * 1 M g

Solve for M_sc

  • M_sc = 15 M Sol * a_bh / 1 M g = 15 Sol/g * a_bh
  • Solve for Force on space craft

    a_sc = 1 g

  • F = M_sc * a_sc = 15 Sol/g * a_bh * 1g = 15 Sol * a_bh
  • Solve for Mass of black hold.

    M_bh = F / a = 15 Sol * a_bh / a_bh = 15 Sol

    This answer seems reasonable. Therefore they must be directly proportional and not inversely proportional!

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    Guru
    Engineering Fields - Aerospace Engineering - Retired South Africa - Member - The Rainbow-nation Engineering Fields - Engineering Physics - Relativity & Cosmology Popular Science - Cosmology - The Big Picture!

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    #3
    In reply to #2

    Re:Revised answer

    07/03/2006 2:15 PM

    The trick lies in the "... 1% (by radius) outside of a non-rotating black hole's event horizon." of the question. This event horizon radius (r_e) grows linearly with the mass (M) of the black hole:

    r_e = 2GM/c^2, where G is Newton's gravitational constant and c is the speed of light.

    So direct proportionality for the acceleration against the mass cannot work...

    __________________
    "Perplexity is the beginning of knowledge." -- Kahlil Gibran
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    Guru
    Engineering Fields - Aerospace Engineering - Retired South Africa - Member - The Rainbow-nation Engineering Fields - Engineering Physics - Relativity & Cosmology Popular Science - Cosmology - The Big Picture!

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    #4
    In reply to #2

    Re:Revised answer

    07/07/2006 7:25 AM

    Greetings Nick:

    To prevent this dangling as a loose end, your original answer: "... 15 million million solar mass" is correct! It is a simple proportionality, provided that you stay a at constant fractional distance outside of the event horizon radius – the more massive the hole, the farther you are from it's center…

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    Associate

    Join Date: Jun 2006
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    #5
    In reply to #4

    Re:Revised answer

    07/07/2006 2:19 PM

    What is the "Score" after my name for?

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    Guru
    Engineering Fields - Aerospace Engineering - Retired South Africa - Member - The Rainbow-nation Engineering Fields - Engineering Physics - Relativity & Cosmology Popular Science - Cosmology - The Big Picture!

    Join Date: May 2006
    Location: Pretoria, South Africa
    Posts: 3804
    Good Answers: 69
    #6
    In reply to #5

    Re:Revised answer

    07/07/2006 2:32 PM

    You have to direct this question at the administrator/moderators of this forum. I guess it comes from an algorithm working on the number of accepted threads that you started... or whatever!

    __________________
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