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Science Forum

  1. Standard member sonhouse
    Fast and Curious
    28 Aug '12 18:40
    Wiki says it is the product of G and M, G=6.7E-11, and for Earth, M=~6E24 Kg.
    That multiplied together is about 4E14. The thing I don't understand is why they list that as being a bit under 400,000 for Earth. The moon is 4902 and change.

    How do they derive that number? It is a billion times lower than just multiplying G and M directly.
  2. Subscriber AThousandYoung
    Do ya think?
    28 Aug '12 21:52 / 1 edit
    Originally posted by sonhouse
    Wiki says it is the product of G and M, G=6.7E-11, and for Earth, M=~6E24 Kg.
    That multiplied together is about 4E14. The thing I don't understand is why they list that as being a bit under 400,000 for Earth. The moon is 4902 and change.

    How do they derive that number? It is a billion times lower than just multiplying G and M directly.
    Probably a unit conversion problem. Change the units of G to cgs and you get E-8 instead of E-11. Using hours instead of seconds you get 8.650E-1.

    From the wiki article:

    http://en.wikipedia.org/wiki/Standard_gravitational_parameter

    The SI units of the standard gravitational parameter are m^3/s^2...The value for the Earth is called the geocentric gravitational constant and equals [~400,000] km^3/s^2
  3. Standard member sonhouse
    Fast and Curious
    29 Aug '12 00:17
    Originally posted by AThousandYoung
    Probably a unit conversion problem. Change the units of G to cgs and you get E-8 instead of E-11. Using hours instead of seconds you get 8.650E-1.

    From the wiki article:

    http://en.wikipedia.org/wiki/Standard_gravitational_parameter

    The SI units of the standard gravitational parameter are m^3/s^2...The value for the Earth is called the geocentric gravitational constant and equals [~400,000] km^3/s^2
    6.67 E-11 is the # I always use for G and the mass of Earth is in kilograms, the units are correct. I don't get it.
  4. Subscriber AThousandYoung
    Do ya think?
    29 Aug '12 00:42 / 3 edits
    Originally posted by sonhouse
    6.67 E-11 is the # I always use for G and the mass of Earth is in kilograms, the units are correct. I don't get it.
    Your calculated answer uses meters. Wiki expresses the same answer using kilometers. km^3 is 1 billion times larger than m^3.
  5. Standard member sonhouse
    Fast and Curious
    29 Aug '12 01:43
    Originally posted by AThousandYoung
    Your calculated answer uses meters. Wiki expresses the same answer using kilometers. km^3 is 1 billion times larger than m^3.
    If I used Km instead of meters that would only be 3 orders of magnitude difference, a thousand to one. Instead I have a thousand MILLION to one difference.

    But you are right, G can be expressed as 6.67 E-8 in Km. That would not change the mass which is in Kg.
  6. Subscriber AThousandYoung
    Do ya think?
    29 Aug '12 03:48
    The km is cubed, giving 1000x1000x1000 = 1 billion
  7. Standard member sonhouse
    Fast and Curious
    29 Aug '12 18:15
    Originally posted by AThousandYoung
    The km is cubed, giving 1000x1000x1000 = 1 billion
    Yeah, forgot about that. Thanks.