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Homework Help: Planet density -- no idea what to do

  1. Apr 7, 2015 #1
    1. The problem statement, all variables and given/known data

    Consider a spherical planet of uniform density ρ. The distance from the planet's center to its surface (i.e., the planet's radius) is Rp. An object is located a distance Rfrom the center of the planet, where R<Rp. (The object is located inside of the planet.)

    Find a numerical value for ρearth, the average density of the earth in kilograms per cubic meter. Use 6378km for the radius of the earth, G=6.67×10−11m3/(kg⋅s2), and a value of g at the surface of 9.80m/s2.
    Express your answer to three significant figures.

    2. Relevant equations

    I answered the questions before this, and they go like this:

    Find an expression for the magnitude of the acceleration due to gravity, g(R), inside the planet.
    Express the acceleration due to gravity in terms of ρ, R, π, and G, the universal gravitational constant.


    Rewrite your result for g(R) in terms of gp, the gravitational acceleration at the surface of the planet, times a function of R.
    Express your answer in terms of gp, R, and Rp.


    3. The attempt at a solution

    To be quite honest, I do not know what to do. I need step by step instructions and explanations of why I had to do things that I should have.
  2. jcsd
  3. Apr 8, 2015 #2
    Just a simples hints:
    -What's the formula for density ?
    -What is the formula of acceleration due to gravity ? I wonder if it's possible to take a "mass" from here?
    Note: that R<Rp so the result may be bounded by 2 number(less or more,)
    Hope That Help :p
  4. Apr 8, 2015 #3


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    Science Advisor
    Homework Helper
    2017 Award

    Dear physicsquestion,

    Please read your post before posting. It is unclear what your exercise is and it is unclear what your question is.
    There are no relevant equations in the section by that name and there is no attempt at solution under the section by that name.

    Read the guidelines, follow them (or at least a fair percentage :smile: ) and you'll get better assistance. You will also learn how to order your thoughts, which is a also very useful skill.
  5. Apr 8, 2015 #4


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    Staff: Mentor

    Given the problem statement, I don't see a need for the object inside the Earth. You are given the acceleration due to gravity at the surface, and the radius of the Earth. What is the mass of the Earth? What is the volume?
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