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Question reguarding gravitational pull

  1. Mar 28, 2009 #1
    Hey there, firstly i'll just say to start i've not studied general relativity at all, and the answer to my question may lie there, but i'll ask anyway;

    Well I was thinking about the gravitational equation;

    F (1,2) = m1m2G/d²

    However putting the earth & a person in there gives an absolutely ridiculously high result, and yet we know for a fact that the pull of gravity on us is constant (10 ms ish), by just using really rough internet figures i get

    (70 (mass of avg person) * 5.9 * 10^24 (earth mass) * G)/ 6377000^2 (dist from core - surface of the earth)

    = 583 ish ms? A ridiculous answer

    Any reason for this? I don't think i've gone much wrong, not enough to cause that :eek:

    Thanks in advance for any help

    Edit :JEEZ MEGA FAIL ON MY PART - F = MA, LAWLLLLL, diving by 70 gives a very good answer for gravity -

    does this then mean that 9.8 isn't constant? And it is indeed dependant on mass, but it makes almost no difference whatsoever - sorry for failthread
  2. jcsd
  3. Mar 28, 2009 #2
    A person with a mass of 70 kg has a weight close to 700 newtons on earth. That's the gravitational force exerted by the earth on the person. Your method should give you a number close to that expected answer.
  4. Mar 28, 2009 #3
    G m_earth / (r_earth)^2

    = (6.67 X 10^-11 N m^2/kg^2) ( 5.98 X 10^24 kg) / (6.37 X 10^6 m)^2

    = 9.83 kg m/s^2
  5. Mar 28, 2009 #4
    So....it's not constant? :P Didnt really get the 2nd post
  6. Mar 29, 2009 #5


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    Gold Member

    Where are you incorporating G, which equals 6.67*10^-11?
  7. Mar 29, 2009 #6
    Yeah sorry i forgot to edit the original post - i did get it in the end, just forgot to divide by mass xP, got 9.75 ish which is awesome considering the mega-rounding i did with the wiki figures, it's just really nice to see these awesome equations work using them yourself! Thanks guyz
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