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Homework Help: Gravity Pull on an object

  1. Jun 5, 2009 #1
    Planet Z is 13700 km in diameter. The free-fall acceleration on Planet Z is 8.79 m/s2.
    What is the mass of Planet Z?

    2. Relevant equations


    3. The attempt at a solution

    I am not sure how this problem is suppose to be worked at all, any of these kind I am having a problem connecting things together. I thought I would use potential energy which would be -G*M1*M2/r but that won't work because I am not given another weight. So my questions are.......

    Where do I start?

    What equations do I have to use?
  2. jcsd
  3. Jun 5, 2009 #2


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    Homework Helper

    You have the equation. Simply recognize that

    G*M*m/r2 = F

    But you know that F = m*a

    ... so the acceleration part of that expression should be ...

    a = G*M/r2

    They have given you a and r and you can look up G, so ... solve for M.
  4. Jun 5, 2009 #3
    FML. You are a genious I was missing the relationship that my equation was equal to force. Now that brings up another question when dealing with gravity will that equation always equal the force?
  5. Jun 5, 2009 #4
    so i solved for M...and got M=aR^2/g with numbers is 8.79 m/s^2*1.37x10^7 m/ 6.67x10^-11 which comes out to be 2.47x 10^25 correct? It is saying my anwser is worng. I have done it 5 times and it keeps coming up with the same anwser on my calculator.

    EDIT: I was rushing and forgot to make it radius instead of Diameter.
  6. Jun 5, 2009 #5


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    Homework Helper

    You didn't square the radius either in your post, a typo or a genuine mistake?
  7. Jun 5, 2009 #6
    Typo as well as forgetting to turn it into the radius