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Homework Help: Consider a planet in some solar system which has a mass double

  1. Nov 9, 2011 #1
    1. The problem statement, all variables and given/known data
    Consider a planet in some solar system which has a mass double the mass of earth and same density as of earth . What is the weight of object on the planet in terms of 'W' , where 'W' is weight of object on Earth

    2. Relevant equations
    W=mg ... density = M / V . .... g=GM/R2

    3. The attempt at a solution
    since they both have same density..
    i equated their M/V ratios and it gets me the ratio of their radii ..
    but it still gets me nowhere
  2. jcsd
  3. Nov 9, 2011 #2


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    Gold Member

    Re: Gravitation

    Letting 'D' indicate the information you know for the double sized planet, you know

    [itex]M_{D} = 2M_{Earth}[/itex]

    and you know [itex]R_{D} = X R_{Earth}[/itex] where X is whatever you determined the ratio between the radii are. So simply find what the gravitational acceleration on this new planet will be with [itex]g = {{GM}\over{R^2}}[/itex]. The trick will be being able to write 'g' as [itex]g = Y \times {{GM_{Earth}}\over{R_{Earth}^2}}[/itex] where you have some constant, 'Y', multiplying the original values known for the Earth. Those known values you know gives [itex]g_{Earth} = 9.8 m/s[/itex].

    You'll have some multiplicative value in front that will tell you how many times stronger or weaker the gravity is.
  4. Nov 9, 2011 #3
    Re: Gravitation

    thanks , got it
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