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Earth gravity equation

  1. Jul 7, 2011 #1
    I'm learning physics and I am a bit confused by this equation which gives me the acceleration of gravity on earth. I already know the answer to the eqaution,but would like to learn who the equation works
    Code (Text):

    a =[U] 6.674e*-11 * 5.9736e+24[/U]  = 9.81
    now I calculate this equation buy doing 6.674e-11 * 5.9736+24 / 6.375e+6 * 6.375e+6
    but I don't get the acceleration due to earths gravity but I get this number 2.4138016743981442679973588367884e-13 which is no where near 9.81.What am I doing wrong am I calculating the equation wrong?
  2. jcsd
  3. Jul 7, 2011 #2
    the way you need to think about it is m1*a=G*m1*m2/r^2, assuming you are finding the acceleration of the person or thing and not the earth you just divide m1 (what ever the persons mass is) and all you have is G*m2/r^(2)=a, gravitational constant times mass of the earth divided by the radius of the earth squared
  4. Jul 7, 2011 #3

    Doc Al

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    Staff: Mentor

    I don't quite know what you're doing wrong, but try again. For a check, solve for the exponents by hand and use the calculator for the precise number.
  5. Jul 7, 2011 #4
    the m1*a=G*m1*m2/r^2 because F=G*m1*m2/r^2 and F=ma
  6. Jul 7, 2011 #5

    Doc Al

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    Staff: Mentor

    Just to be clear: The expression is correct, but your calculation is not. (When you do the arithmetic, you should get close to 9.8 m/s^2, as expected.)
  7. Jul 7, 2011 #6
    Hi I figured out my problem the equation is correct but my widows calculator seems to be returning the wrong calculation, but my old casio calculator seems to give me the correct answer, this is a bit off the topic but do I need to us a certain setting for my windows calculator?
  8. Jul 7, 2011 #7
    While I am helping, I will be learning to use this forum's word processor. Sorry if I make a mistake because of that.

    Let's try it on the Window's calculator as follows:

    6.375 x 106 times 6.375 x 106 = 40640625000000 for the denominator. Did you get this? Save this in the memory.

    I guess I could have typed it in as (6.375 x 106)2 = 40640625000000 for the denominator. Did you get this? Save this in the memory (Press MC to clear the memory or put 0 in the contents of the memory, then M+ to add the value stored in the display to the contents of the memory).

    6.674e-11 * 5.9736+24 = 398678064000000 for the numerator. Did you get that? It is stored in the calculator's display and can be seen in the display.

    Now divide that by this: With that still in display, press the sequence [divide] [memory recall] [=]

    Comes out fine. Ignoring sig.figs., on paper you might write the final step of the calculation as follows:

    398678064000000 / 40640625000000 = 9.8098408673587081891580161476355 for the final step, or as

    6.674e-11 * 5.9736+24
    (6.375 x 106)2

    = 9.8098408673587081891580161476355

    Got to go. At library and time up.
    Edit: I am back now and have time to review and make some minor changes.
    Last edited: Jul 8, 2011
  9. Jul 7, 2011 #8


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    Staff Emeritus
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    Homework Helper

    I don't know about the Windows calculator. You could try evaluating the numerator (6.674e*-11 * 5.9736e+24) and denominator ((6.375e+6)2) separately, and compare each calculation with what your casio says.
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