1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Approximate value of g

  1. May 30, 2007 #1
    1. The problem statement, all variables and given/known data
    2. Relevant equations
    So I know that there's an inverse relationship between the value of g and r; the farther I get away from earth, the weaker the gravitational force -> weaker acceleration....

    BUT I'm not sure how the (1-x)^-2 thing is related, need help with that. Also, how do I set up for Newton's universal law of gravitation?

    3. The attempt at a solution
    Uh... In process. :eek:)

    Thanks in advance!
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. May 31, 2007 #2


    User Avatar
    Homework Helper

    From Newton's universal gravitational law we have:

    [tex]W = mg = GMm\ \frac{1}{r^2}[/tex]


    [tex]g = GM\ \frac{1}{r^2}[/tex]

    now use the hint ...
  4. Jun 7, 2007 #3
    I still don't really understand... :grumpy:
  5. Jun 7, 2007 #4
    in newtons equation, put distance=(re+delta r)

    then write it as d= re(1+ (delta r/re))

    d has a power of -2

    and at 100 km. "delta r/re" is between -1 and 1 since the radius of the earth is greater than 100 km

    does this help or do you need more?
  6. Jun 7, 2007 #5
    I think I almost have it. The only thing that I'm still wondering is about the relation part, what does the minus sign mean?
  7. Jun 7, 2007 #6
    because you are given an equation that is stating the change in g (delta g), that minus indicates a decrease.
    think about what happens to g as the distance from the center of the earth increases by considering newton's equation
  8. Jun 8, 2007 #7


    User Avatar
    Homework Helper

    the minus sign means that for a positive [tex]\Delta r[/tex] (going further up) the change in the gravitational acceleration decreases (is negative). So that the new gravitational acceleration is given by

    [tex]g_{new} = g_{old} + \Delta g[/tex]
  9. Jun 13, 2007 #8
    Ok, sorry to bring this up again.

    Talking to some of my classmates, some of them think that this is supposed to be a proof using Taylor series stuff, and that at the end that's where the -2 comes from. Any thoughts on that?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Approximate value Date
Approximation to square root Jan 31, 2018
Approximation for a slipped pendulum Dec 31, 2017
Approximate Values of Kinetic Energy and Potential Energy Nov 4, 2015